The RV world is abuzz with articles and presentations, including a number I have done in the past, about the significant benefits derived from replacing your current lead-acid house batteries with lithium-ion batteries.

You are promised far greater capacity per unit of weight and size, five to ten times longer life, higher voltages to extend the useful life of your 120VAC appliances powered through an inverter, and the ability to fully discharge the lithium batteries without long-term damage, along with many other benefits.

The issue for many of you is that these benefits come at a very high price. The upfront investment for lithium-ion batteries can be thousands of dollars and you give up whatever life remains in your existing lead-acid batteries, but you don’t need to do that!

In this article, I will show how you can enjoy these many lithium battery benefits, pay half as much upfront, and continue to enjoy the life remaining in your present lead-acid batteries - a real win-win! Even better, you will learn how you can easily fine-tune this new house's electrical system to fit the way you live in your bus. And, you do not need to do any complex calculations - you will fine-tune your system based on actual real-world experience living in your bus. The key is to create two different house battery banks.

One of these new house battery banks will use about half of your existing lead-acid batteries. It will only be used for the low AMP draw 12VDC applications like lighting (LEDs for most of us now), running vent fan(s), running the motor in the furnace or hydronic heating system, the water pump, and perhaps a circulation pump or two, and a refrigerator if yours runs on 12V.

Lead-acid batteries are ideally suited to these low amp draw applications and can last a long time when relieved of also trying to power the very high AMP draw applications of running your 120VAC appliances and wall outlets through an inverter - tasks for which they are not well suited.

Lithium-ion batteries, on the other hand, are ideally suited for running an inverter to power 120VAC appliances and to power outlets since they operate at a higher voltage for longer and do not suffer the voltage sag like lead-acid batteries do under heavy loads.

For the second half of your two-bank house battery system, you will install just enough lithium-ion batteries to provide your 120VAC needs.

Now you will have the “Best of Both Worlds”, because your existing lead-acid batteries will continue to provide power for all your 12VDC needs, you won’t need as many new lithium batteries, about half as many as the drop-in replacement lithium battery industry would suggest, saving you about half of the upfront investment.

It turns out, that accomplishing this dual-house battery bank setup is way easier than you might imagine and only requires adding a couple of new items.

In a nutshell, these are the steps you will perform to not only expand your battery capacity but to run your bus on a combination of both lead-acid and lithium-ion batteries.  More details will be discussed in Part 2 of this article.

Step 1 - Determine where you want to mount your new lithium-ion batteries.  Leave enough room to add one, or even two or more later if fine-tuning shows you need more capacity for the 120VAC loads.

Wire these lithium batteries in parallel adding

a 300 to 400-amp fuse holder to the outgoing positive cable and adding a shunt for a battery monitor to the outgoing negative cable. Do not yet install the fuse so you know the outgoing cables are not “hot” and cannot spark if they touch anything metal.

Step 2 - Remove the cables going from your lead-acid batteries to the positive and negative terminals on your existing inverter/charger. Good practice says to remove any lead-acid battery cables that go to chassis ground first to prevent any chance of sparking if a positive cable touches metal. Also, ensure you get all of them as some may be hidden from your view.

Once you have removed the positive and negative cables going from the lead-acid battery bank to the inverter/charger and safely insulated or removed them altogether, connect the positive cable coming from the lithium-ion battery bank to the positive connection on the inverter/charger(s).

Some of you with all-electric buses may have two inverter/chargers. Treat them the same as they will act in parallel, one providing power to the appliances and outlets on one side of your bus, and the other will do the same on the other side of the bus.

Run a new black cable from the chassis ground to the negative connection on the inverter/charger(s) and connect the negative cable coming from the lithium-ion batteries (or the shunt installed to monitor battery performance) to the chassis ground. Insert the fuse in the fuse holder at the lithium batteries so your inverter/charger(s) are powered ONLY by the new lithium batteries.

The inverter/chargers(s) will also keep the lithium batteries charged while onshore or generator power. They need to be charged differently than your lead-acid batteries. Follow the manual for your inverter/charger(s) to set them for the charging profile required of your lithium batteries. The 120VAC side of your new house battery bank is now complete and fully functioning.

Step 3 - Decide which half of your current

lead-acid batteries you want to remove and one by one remove and tape the ends of the interconnecting cables that were used to wire them in parallel to protect them from shorting out.

Be careful not to allow any of the positive cable ends, or the tools you are using, to touch anything metal in case you somehow missed one connection to chassis ground. Carefully mark any cables other than interconnect cables going to either the positive or the negative posts on the batteries you intend to remove.

Once you have removed those batteries and the interconnect cables, move any other cables that were attached to positive posts on the batteries you removed over to the positive post on one of the lead-acid batteries that remain.

Do the same for any other cables that were connected to the negative post on one of the lead-acid batteries you removed over to the negative post on one of the remaining lead-acid batteries.

Finally, reconnect any cables that previously ran from a negative post on the lead-acid batteries (or the battery monitor shunt) to chassis ground and your lead-acid batteries alone will now be powering your 12VDC loads.

Step 4 - Install a stand-alone battery charger to the remaining lead-acid battery bank by running a red wire from a positive post on the remaining lead-acid battery bank to the positive post on the new battery charger, and run a black wire from a negative post on the new battery charger to chassis ground.

Plug the new battery charger into a convenient 120VAC wall outlet that is powered ONLY when on shore or generator power. DO NOT plug into a wall outlet that is also inverter-powered.

At this point, the lead-acid batteries power only the 12VDC loads and will recharge any time you are plugged into shore power or are running the generator.  The lithium-ion battery bank now only powers the 120VAC loads through the inverter/charger(s) and will recharge any time you are plugged into shore power or are running your generator.

Step 5 - Install a Sterling Power BB1260 (12V to 12V, 60A) Battery-to-Battery charger or equivalent. This will allow your existing engine alternator to continue to charge your lead-acid batteries as always. In addition, it will take the alternator’s output and convert it internally to the charge profile required to charge your lithium-ion batteries at the same time.

Of great importance, it also internally prevents any possibility of the higher voltage from the lithium batteries back flowing into the lower voltage lead-acid batteries.

You are done!

That is all it takes to gain the many advantages of lithium batteries to do the heavy lifting of powering your 120VAC appliances and wall outlets through your existing inverter/charger(s) while enjoying the lighter weight of half as many of your lead-acid batteries now powering only the 12VDC bus loads. You get the “Best of Both Worlds” at half the investment of changing your whole bus over to lithium batteries while at the same time enjoying whatever life is remaining in your existing lead-acid batteries.

Before we launch into the details “how-to” part of this article let me be clear about a few things. I am going to talk about what I did to accomplish this “Best of Both Worlds” dual house battery bank in my coach, a 1997 Prevost XL40 Country Coach. Only you can decide if you have the skill and knowledge to do something similar in your bus.

The way your bus’s house and chassis electrical systems are now set up may be similar to or very different from how my bus is set up. What you decide to do and how you decide to do it is up to you. I make no guarantees or representations that what you do will work properly for you, but I will show you what I did and share with you the test results I have observed since.

Another thing that is important up front: My bus has a 24VDC chassis battery bank and a separate 12VDC house battery bank. Those two are never connected together anywhere.  I have two different engine-driven alternators, each handling only one of those battery banks. The only battery isolators or combiners in my setup are between the lead-acid 12VDC house batteries and the lead-acid generator starting battery. There are no shared battery chargers when on shore or generator power.

Your bus may be quite different as some buses employ a 24VDC house battery bank and may have only one alternator that charges both the house and 24VDC chassis batteries at the same time and/or may also share one shore/generator-powered battery charger.

What you would do with that kind of setup might be very different so I do not recommend you start down this “Best of Both Worlds” approach unless you really understand what you’re are doing or can have a competent shop do the work for you.

For most of you, the setup on your bus will be similar to what I have, so what I did may be a good guide for you.

The key to making this “Best of Both Worlds” approach work is to completely separate the lead-acid portion from the new lithium-ion portion of the new house battery bank.  These two must never be directly connected together because they operate at very different voltage regimes and could cause the lithium battery bank to dump large amounts of energy into the lead-acid batteries trying to bring them up to the same voltage. That could, and likely would be dangerous.

Making this conversion is a simple five-step process summarized in part 1 of this series and involves only buying two additional items to go along with what you likely already have in your bus - a common converter/charger to recharge your remaining lead-acid batteries while on shore or generator power and a suitable battery-to-bat-tery charger to keep both banks recharged at the same time from the existing alternator while driving.

The first thing you need to do is to decide where to put the new lithium-ion phosphate batteries that will make up one-half of your new house battery bank.

A good starting point is to remove half of your existing lead-acid batteries and buy that same number of new 105-amp hour lithium-ion batteries.

Most large buses came with four, six or eight 4D, or, 8D lead-acid batteries that were rated around 200-250-amp hours when new. It doesn’t matter whether they are called “wet cell” or “AGM” or

“gel cell” or “SLA”, they are all lead-acid batteries and you will do the same things for this implementation.

You likely know if you read my previous article: The Advantages of Lithium Batteries https://www.busconversionmagazine.com/the-advan-tages-of-lithium-batteries-for-bus-conversions/ that you can only draw half that capacity without damaging a lead-acid battery so they really were rated at more like 100 to 125 available amp-hours when new.

Since those lead-acid batteries are no longer new and have been hammered for years trying to power inverters running your 120VAC appliances and wall outlets, they may have less than that rated capacity now. That is why a good starting point is to remove half your current lead-acid batteries and buy the same number of new 105-amp hour lithium-ion batteries from the vendor of your choice.  I chose Lion Energy UT-1300 batteries.

Leave room in the area where you locate your lithium batteries to add one or more later if you find you need more 120VAC capacity when you go through your fine-tuning.

Similarly, you can always put back one of your existing lead-acid batteries later if you find you need more 12VDC capacity so don’t yet use all that newfound space in your battery bay. But, for most buses, the “Best of Both Worlds” new dual house battery bank will wind up being the same number of lead-acid and lithium-ion batteries, as you have lead-acid batteries now. If you have a residential refrigerator, you likely will need one more lithium-ion battery than lead-acid batteries.

I initially installed four lithium-ion batteries and retained four of the six original 8D AGM lead-acid batteries in my bus so I could do fine-tuning and testing more quickly. As it turns out, because we have a residential refrigerator, I do need that extra lithium battery and appear to have way more lead-acid capacity than I need so long as we don’t frequently park off grid when it is quite cold outside. In a later part of this article called “Planning Assumptions” I will explain why.

While the lead-acid batteries need to be installed in a bay compartment vented to the outside, the lithium batteries do not need to be vented. They do not put off gas, do not smell, do not leak anything, are very safe, and can be installed in any orientation in any location. It is best to put them in a place inside your living areas where they will not get overly cold or overly hot if you live or travel in climate regions.

They are less susceptible to damage from being too hot or too cold than their lead-acid counterparts, but the built-in battery management system will prevent them from being charged if the cell temperatures go much below freezing. That low temperature shut off point is lithium battery manufacturer dependent but is usually around 25F.  The lithium batteries will continue to discharge safely well below zero degrees.

My coach has a clothes closet with a bank of drawers directly over the outside battery bay to keep the cable lengths to a minimum. The bottom drawer was an ideal size to hold four Lion Energy UT-1300 105-amp hour lithium-iron phosphate batteries. While some Lithium batteries are much heavier, four combined Lion Energy batteries weigh under 100 pounds so it was not hard to reinforce that drawer and install larger capacity ball-bearing drawer slides to carry the weight.

Other lithium batteries of the same capacity would have been too large to fit there, but the compact size of these Lion Energy batteries fit like a glove with room left to mount a 400-amp fuse on the outgoing positive cable and a battery monitor shunt on the outgoing negative cable.

I used 2/0 marine cable to interconnect all four batteries in parallel and for the short cables needed to pass through two holes drilled through the floor to put the lithium battery power into the existing external battery bay, where my two Mag-num 2800 inverter/chargers are located. My bus originally used 2/0 wiring from the factory which is why I used cables of that size instead of larger 4/0 cables.

Since all I needed to power with the lithium batteries are these two inverters/chargers the wiring is really simple.  Use short interconnect cables to wire the lithium-ion batteries in parallel (positive post to positive post and negative post to negative post).

Next, run a short red cable from the positive post on the lithium batteries to the fuse holder (do not install the fuse yet) and a short black cable from the negative post on the battery to the battery negative post on the battery monitor shunt if you plan to install a battery monitor.

From the load side of the shunt, run a black cable to the chassis ground, this way everything the inverter/charger(s) uses in the way of power will return through the shunt to properly measure Amp-hours used. Amp hours put back while recharging, and therefore Amp hours remain.

You now need to remove the existing lead-acid battery positive and negative cables from your inverter/charger(s) and replace them with the positive cable coming from the new lithium battery bank and a negative cable running to the chassis ground. It is good practice to make sure the bus is not plugged into shore power, turn off the inverter/charger(s), and remove the cable(s) that connect the negative side of your existing lead-acid battery bank to the chassis ground.

You do not want to accidentally touch the positive side of the existing lead-acid battery bank to the ground which would throw off a large spark and could be a fire hazard due to the hydrogen gassing off from those lead-acid batteries.  There may be more than one lead-acid battery negative cable running to the chassis ground, which could be hidden from view, so be sure you remove all of them.

Now carefully remove the positive cable that now runs from your lead-acid batteries to the inverter/charger(s). Make sure that cable or any tools you use DO NOT TOUCH any metal on your bus. Tape the end fully to insulate it. Trace that cable (or both positive cables if you have two in-verter/chargers as I do) back to where it/they connect to the positive side of your existing lead-acid battery bank and remove that or those cables from your bus altogether.

Now connect the positive cable coming from your new lithium battery bank to the positive lead on your inverter/charger(s).  Next, connect the negative cable coming from the new lithium battery bank to chassis ground and run a new cable from chassis ground to the negative side of your inverter/charger(s).

The final task in this step is to insert the fuse on the positive side of your lithium battery bank. Now your inverter/charger(s) and all of your 120VAC plugs and appliances are powered ONLY by your new lithium-ion battery bank. The lead-acid battery bank is no longer involved in any way power-ing 120VAC loads so those batteries will now last a lot longer than you might imagine.

Next, you will remove any unwanted lead-acid batteries from the existing lead battery bank. Since we are talking here only about buses with a 12VDC house battery system made up of 4D or 8D lead-acid batteries, all those lead-acid batteries will be wired in parallel with large cables, most likely 2/0 in size.

Decide which half of those lead-acid batteries you will remove. Check again to make sure they are not connected to chassis ground so you avoid any possibility of a spark. Remove the interconnect cables one at a time. Make a special note and/or take a photo of where any cables other than the interconnect cables are located.

If they are now attached to a positive post, you will want to reattach them to a positive post on one of the remaining lead-acid batteries. If they are now attached to a negative post, you will want to reattach them to a negative post on one of the remaining lead-acid batteries. All you are doing at this step is removing one half of your existing lead-acid batteries without changing anything about how your bus is wired.

Once you have removed half your lead-acid batteries and the interconnect cables that wired them in parallel, reconnect all the remaining positive cables to a positive post on one of the remaining lead-acid batteries. Do the same for any cables that went from the negative side of your lead-acid batteries to chassis ground. At this point, all your 12VDC lights and appliances should work just as they always have. And, your alternator will recharge the lead-acid batteries just as it always has.

You now want to make sure both the lithium batteries and the lead-acid batteries that make up your new house battery bank will be properly re-charged from your existing alternator, from shore or generator power, and from solar power if you have that now or plan to add it in the future.

Keep firmly in mind that the lead-acid and lithium battery banks must NEVER be connected directly together. 

Here is what my external battery compartment looks like after the removal of three of the 8D AGM lead-acid batteries and the installation of the new charging equipment on the second level which I will describe in detail as we go along. The silver panel on the lower level covers the three remaining lead-acid batteries.

The two Magnum 2800 inverter/chargers are still mounted in the same place on the first level but now are powered only by the Lion Energy lithium batteries mounted in the closet above this bay.

Charging both the lead-acid and the Lithium portions of the new house battery bank while driving. 

To recharge both the lead-acid and lithium batteries from the alternator you need to buy a device called a battery to battery charger. The one I used, is made by Sterling Power. It is a 12-volt to 12-volt, 60 Amp charger (Sterling Model BB1260). This unit will automatically turn on when it senses charge current coming from your alternator when you start your engine. Internally it will convert the alternator output to the charge profile required by your lithium-ion batteries so they will be properly recharged at the same time the existing lead-acid batteries are getting recharged. Importantly, they also internally block any possibility of the higher voltages on the lithium battery bank from flowing back into the lower voltage lead-acid battery bank.

Installation is really simple. Run one wire from a positive terminal on your lead-acid battery bank to the input side of the BB1260. Run another wire from the output side of the BB1260 to a positive post on your inverter/charger(s) that is now fed only by the lithium-ion side of your new house battery bank. That is all there is to it.

When the charge current coming from the alternator stops, the unit will turn itself off to isolate the lead-acid and lithium parts of the new house battery bank. There is no operator intervention required. It does this seeming feat of magic automatically. The lead-acid batteries will be charged by the alternator as they always have been. The lithium batteries will get charged with the correct charge profile from the BB1260 charger.

This set-up mitigates one of the major issues associated with charging lithium-ion batteries from an existing vehicle alternator. When the Battery Management System built into most lithium batteries senses they are fully charged, it will abruptly shut off the charge current from the alternator.

Since the alternator is still spinning and putting out a current, that sudden shut-off of demand could cause a huge voltage spike potentially be ruinous to the alternator and everything attached to the house batteries. The lead-acid batteries in this setup have so much internal resistance they will absorb any such voltage spike so everything is protected automatically.

If you were to do a full replacement of your existing lead-acid batteries with new lithium batteries you would need to change the charge profile coming from your existing alternator to match the requirements of the lithium batteries.

You would have to add a new device to your system to absorb that potential voltage spike.

Keeping at least one lead-acid battery in the system and using an appropriate one of these BtoB chargers solves all these issues.

These BtoB chargers also serve another useful purpose in this setup. One problem with trying to charge lithium batteries from an alternator designed for charging lead-acid batteries is that the lithium batteries are perfectly happy taking far more amperage than lead-acid batteries.

There is so much internal resistance inside a lead-acid battery that it usually can’t take enough charge current to damage the alternator. Not so with lithium batteries which will happily absorb all the charge current the alternator can put out and will do so for as long as the lithium battery takes to fully charge. Many engine alternators are not designed to output that much charge current for that long so can easily burn themselves up trying.

In the typical drop-in replacement lithium installation, it is necessary to install a device that will limit the charge current going from the alternator to the lithium batteries. That is most often accomplished by turning the charge current on for ten or fifteen minutes and then off for fifteen to twenty minutes to give the alternator time to cool down. The BtoB charger has an internal limit to the amount of charge current it will allow to pass. If this is below what the alternator can put out continuously, it will be protected.

In bus conversions and most large diesel motorhomes, the alternator is designed for continuous output of large amounts of charge current.  In my situation, the engine has a Delco 50DN series oil-cooled alternator to charge the house batteries that are rated for continuous duty at the 270-amp output.

That is way more than the 60-amps of charge current the Sterling BB1260 will allow to pass through. The only issue is there is less charge current available to the lithium portion of our new house battery bank so it may take a bit longer to recharge while driving. In any event, the lithium batteries will likely recharge faster than the lead-acid batteries so you will likely not notice the somewhat longer recharge time.

This Sterling Power BB1260 unit is designed so you can install two of them in parallel if you want to double the charge rate so long as your alternator can handle that amperage draw load. A good rule of thumb is to draw no more than about half the rated capacity of the alternator and to not exceed two-thirds the rated capacity.

The amount of heat an alternator produces is a function of the amperage it is putting out and most alternators are fan-cooled. When running slower, they may get overly hot as they near half their rated output. Best to check with the manufacturer of your alternator to be sure.

While a bit expensive, adding two appropriate BtoB chargers also provides you with running redundancy. If one were to fail, you would still be charging both the lead-acid and the lithium portions of your new house battery bank, just at a slower rate.

I mounted my Sterling BB1260 charger in the vented bay where the remaining lead-acid 8D AGMs live so it would have lots of airflow to keep it cool while in operation. I did leave room for a second Sterling unit if I ever want to increase the charge rates while driving.

Charging both the lead-acid and lithium-ion portions of the new house battery bank from shore or generator power.

In this scenario, the major assignment for the lithium portion of the new house battery bank is to power the inverter/charger(s) which supply 120VAC power to your outlets and appliances as noted previously. As part of our fine-tuning, we will see later that it can also be used to serve some of the 12VDC needs as well. For now, let’s just concentrate on powering the inverter/chargers.

My two Magnum 2800 Pure Sine wave inverter/chargers were already installed near where both sides of the new house battery bank reside (lead-acid in the battery bay and lithium-ion in a drawer in the closet right above that bay) so I did not need to move or alter them in any way.

I did need to change the settings from the inverter/charger control panel to output a lithium battery profile since they now will be used to recharge just the lithium side of the new house battery bank. They are still tied into the same 120VAC power and output lines as they always have been so there is nothing more I needed to do to have them keep the lithium-ion batteries at full charge while plugged into shore power or running the generator.

These two Magnum 2800-watt pure sine wave inverter chargers can put out up to 120-amps of charge each so they will quickly and easily fully recharge the 400-amp hour lithium battery bank.

Each of the Lion Energy UT1300 lithum-iron-phosphate batteries can take up to a 100-amp charge rate, but 50 to 60 each is ideal so the 240-amps of charge going into four of these UT1300 batteries it is fine. Check with your lithium battery manufacturer you use to make sure the output of your inverter/charger(s) is correct for your lithium batteries to charge without damage.

While the inverter/chargers will keep the lithium battery side of the new house battery bank fully charged while you are plugged into shore power or any time you are running your generator, we also need to keep the lead-acid side charged as well but we never want the lead-acid and the lithium sides to ever be connected directly together.

The simplest way to do that is to install a completely separate charger for the lead-acid batteries. I used a Progressive Dynamics PD9270A shown on the right end of the second level in the picture of the remodeled battery bay below. This provides 70-amps of charge current to the lead-acid batteries and it plugs into a standard 15-amp 120VAC wall plug.

They also make them in different amperage outputs from 40 to 80-amps. If your coach has a limited electrical system, you might be able

to use one with an amperage output lower than 70-amps, but most will find the 70-amp unit will recharge the lead-acid batteries quickly enough while you are plugged into shore power or are running your generator.

These units have been widely used in RV applications for a long time and are well proven to provide a reliable multi-stage charging profile proper for your lead-acid batteries. A number of manufacturers make similar units so the choice is yours. You might even already have one if your setup uses separate inverters instead of inverter/chargers like mine does.

To provide power to this unit while plugged into shore power or while running the generator, I searched for and found a 15-amp 120VAC plug that is ONLY powered from a generator or shore power and NOT from inverter power. There were several of those outlets from the factory in various bays in my coach.

Use a simple outlet tester that plugs into the outlet to tell you when it is powered and if it is wired correctly to make sure the outlet you choose is ONLY powered from shore or generator power and not from inverter power.  If you can’t find such an outlet already installed in your bus, add one by wiring it to the output side of your shore/generator transfer switch. If you don’t know how to do that safely, have an electrician do it for you.

If the cord on your converter/charger is not long enough to reach from the plug to where you have mounted the converter/charger (you can use a 12-gauge grounded extension cord), you want to mount your converter/charger as close as practical to your lead-acid batteries. DC cables will suffer a loss of voltage with distance so keep them as short and as large of gauge as you can. On the AC side, the losses are minimal so an extension cord won’t hurt anything.

What you have now accomplished is having independent ways of recharging the lithium batteries and the lead-acid batteries in your new house battery bank. The inverter/charger(s) will take care of the lithium side and the converter/charger will take care of the lead-acid side. Those two banks should never be connected directly together.

I opted to install an ON/OFF switch between the lead-acid batteries and the input side of the Sterling Power BB1260. In cases where we were staying for a month or two on shore power, I did not want that unit to be turned on when plugged into shore or generator power and the lead-acid batteries were being charged by the Progressive Dynamics converter/charger. I am not sure it is necessary, but it seemed like a cost-effective, simple thing to do.

Solar charging

The simplest way to add solar charging is to use two MPPT solar controllers and two independent sets of solar panels. These are shown at the left side of the second level of the remodeled battery bay pictured above.

One set of solar panels will be connected to one of the MPPT controllers (the blue and white unit in my case) which will be connected to the positive and negative terminals on the lithium bank. The other set of solar panels will be connected to the other MPPT controller (the black one in my case) which in turn will be connected to the lead-acid portion of the new house battery bank.

There are ways of allowing all the solar panels to charge both the lithium and the lead-acid batteries, but you have to be very careful not to create any situation where the lead-acid and lithium batteries can be connected directly together so there can be no sudden voltage surge from the lithium battery side to the lead-acid side. I do not recommend doing this unless you are very experienced and sure of the performance of every part of your solar set-up. 

It is way easier and safer to divide your panels and use two MPPT controllers so you are sure the lithium batteries and lead-acid batteries remain completely isolated one from another. Using two MPPT controllers also provides you with built-in redundancy if one ever fails.

If you find one side of your new house battery bank consistently recharges faster than the other, you may be able to move one of the solar panels from the fast charge side to the slow charge side as part of your fine-tuning. For example, if your lead-acid batteries are charging twice as fast as your lithium batteries, you may want to rewire your panels such that you may want to move the wires from the solar panels charging your lead-acid batteries to your lithium battery controller. Or, if you have room, you can add additional panels to one or both sides to enjoy faster solar recharging.

One final note on solar charging, manufacturers are beginning to make all-in-one charge units that could be applicable to your situation. Renogy, for example, offers a BtoB charger that also can accept solar charging input. I have not tested one of these so can’t say how well it might work in this “Best of Both Worlds” set-up but they may be worth looking into.

It appears to work similarly to the Sterling BtoB charger when connecting lithium and lead-acid battery banks together and has a stated rating of 50A.  If you add solar charging to this Renogy unit, it reconfigures itself to only 25-amps for solar charging and only 25-amps for BtoB charging which would really slow down the recharging of the lithium batteries while driving.

I do not know whether two or more of these units can be installed in parallel to allow for more charge current. The product literature says all solar charging will go into the lithium bank until it is fully recharged and then the solar will be used to trickle charge the lead-acid side. It doesn’t say how those two are kept completely isolated one from the other as I strongly urge be done to safely implement the “Best of Both Worlds” concept outlined here.

Adding Bluetooth battery monitor shunts to both the lithium battery side and the lead-acid battery side of this new house battery bank so you can monitor each side independently.

A Bluetooth battery monitor shunt is easy to mount. Just connect the B-side of the shunt to the negative side of the lead-acid portion of the new bank and connect the load side of the shunt to the chassis ground where all 12VDC loads are grounded in your bus.  You don’t want any current to flow into or out of this portion of the battery bank without going through that shunt.

Do the same for the lithium side. Connect the B-side of the second shunt to the negative side of the lithium portion of the bank, and connect the load side of the shunt to the chassis ground. You do not want any current to flow into or out of either the lead-acid or the lithium side of the new house battery bank without going through one of those two shunts.

Both shunts will also require a small positive wire to power the Bluetooth communications board attached to the shunt.  Follow the manufacturer’s instructions.  I used Victron Bluetooth shunts and they work very well from inside or outside the bus so long as you are less than 30 feet from each shunt. With these Bluetooth shunts, by using an app on your cell phone, you can see exactly how each bank is performing given how you use your bus.

You will see the amp hours remaining in the bank, the instantaneous AMPS going in or coming out of your batteries, voltage, an estimate of how long the batteries will last under the current draw conditions, and other parameters.

These also do data logging so you can see graphs of all this info over time.  Shunts come in various AMP capacities.  Most of you will use shunts rated at 500 amps for each side of your new house battery bank. However, if you have an unusually large lithium or lead-acid side to your new house battery bank, you may need 1000-amp shunts.

If you prefer, you can install battery monitors that include both the shunt and the monitor display instead of using a Bluetooth connection to your phone. Whichever you choose to use, a battery monitor will keep you well informed.

Planning Considerations

My underlying planning consideration was that roughly half the battery power needs in your bus is to run 120VAC loads through one or two inverters and the other half of the power needs are to run 12VDC lights, fans, pumps, and motors like the ones in your furnace or hydronic heating system. That is why we started by removing half of our lead-acid batteries and installing the same number of new lithium-iron phosphate batteries.

However, no two buses and no two bus owners have anywhere near the same power requirements. The range can be all the way from one-half to three-quarters 120VAC depending on whether you do or don’t have a residential refrigerator, where you are parked while off the grid, how much solar you might employ, and many other factors.

That residential refrigerator is the biggest power draw in your bus because it is relentlessly re-quiring 100 to 150 AMPS per day just to keep your food cold. That is one or two 100-amp hour lithium-ion batteries all by itself. Or, it will mean running your generator for a longer period of time each day just to make up for that draw.

On the 12VDC side, the biggest power draw is running your furnace when outside temps are low. A typical bus heating system can draw 7 to 10 AMPS while running. How much of the day/night it needs to run is a function of how cold it is, how well-insulated your bus is, and how warm is comfortable for you and your family.

If that heating system runs 10 hours over a 24-hour day, it will consume 70 to 100 amps.  That is one full 8D AGM battery all by itself. If you seldom are parked offshore power when it is cold outside, then you can basically save one full lead-acid battery.

The bus manufacturer or bus converter made a lot of assumptions when originally designing your electrical system including the size of the battery bank, the size of the inverter/charger(s), what kind of appliances were installed, etc.

They also knew no two owners would live the same way in that bus so for some their assump-tions would be right on while for others those same assumptions could be way off, one way or the other.

All of this adds up to the need for the fine-tuning made possible by this “Best of Both Worlds” approach.

The final step is fine-tuning

Here is where the fun begins! With this “Best of Both Worlds” setup, you have many ways to fine-tune your system to match how you uniquely live in your bus. You can do any or all of these as you need to continue fine-tuning.

Since no two people would live exactly the same way in the same bus, the objective is to get the capacity and load on both sides balanced to perform about the same way day in and day out to satisfy your unique requirements:

- You can add or subtract batteries from either the lithium side or the lead-acid side if you feel you need more or less total capacity on one side or the other. Normally you cannot incrementally add new lead-acid batteries to an existing lead-acid bank as the new and old batteries will exhibit different internal resistance and different voltage profiles.

In this case, the batteries you would add back in if you needed them would be the very ones you took out so they are already a matched set so long as any additions are made within a year of the time you took them out.

You would consider adding one battery to one side or the other if you consistently found you were running out of power on that side prematurely. Add to the lead-acid side if you consistently find yourself short of power for the 12VDC loads. Add to the lithium side if you consistently find yourself short of power for the 120VAC appliances and wall outlets.

- If you consistently find it takes longer to fully recharge either the lithium side or the lead-acid side while you are driving your normal travel day, then you can add a larger Amp output BtoB charger, or add a second one in parallel if the BtoB charger manufacturer says it is okay to do so, as it is with the Sterling Power BB1260 unit I used.

- You can add, subtract, or move one solar panel at a time if you consistently find one side of your new house battery bank gets recharged during the day on solar while the other does not. You can add more solar panels to either or both sides if neither side regularly recharges in conditions where you think it should.

If your lead-acid side depletes faster than you would like, you can always move one or more 12VDC loads over to the lithium side. For example, if you find the furnace regularly struggles to make it through the night but you seem to always have ample 120VAC power available, you could move the furnace B+ wire from the lead-acid side over to the lithium B+ side. To make this easier, I recommend you consider installing a fuse panel to the B+ side of the lithium bank.

- If your lead-acid batteries are not getting fully recharged fast enough while on generator or shore power, you can add a second converter/charger in parallel with the first to the lead-acid side.  Just be sure not to plug it into the same 120VAC outlet where you plugged in the first one. You would have to find a second outlet that only operates on generator or shore power, and not on inverter power, to plug the second converter/charger into.

The 70-amp Progressive Dynamics unit (PD9280) I used can be plugged into a 15-amp outlet that is serviced by 14 gauge or larger wire and a 15-amp breaker. Trying to plug two such units into one 15-amp receptacle would trip the breaker so neither would continue to work.

How to know when you are in need of fine-tuning

Measure the state of charge on both the lead-acid and the lithium-ion battery banks first thing in the morning when you get up while boondocking.  What you are looking for is to find both banks to be at about the same state of charge each day.

It isn’t so critical what that state of charge is first thing in the morning so long as it is above fully depleted on either side, but you want the two states of charge to be similar. Measure the state of charge again after you run the generator for a certain amount of time, (two to three hours normally).

Ideally, you want both sides of the bank to reach about the same state of recharge at the end of the generator run period.  And, you want that state of recharge to be enough so you can get through the day and the next night with about the same state of charge remaining in both sides of your new house battery bank when you measure it again the next morning.

If one side of your dual house battery bank or the other is just not big enough to make it day in and day out given how you live in your bus, the simplest and cheapest way is to increase the length of time you run your generator each day.

If you don’t like the idea of running your generator more, you can always increase capacity on one side or the other by adding one more battery to either or both sides and/or employ any of the other fine-tuning techniques.

The best strategy is actually to run the generator during the time you are cooking breakfast, heating water for showers, and warming the bus up from a chilly night, and again while cooking dinner.

Most generators are large enough to both supply the power needed for cooking and to recharge batteries at the same time. That way you avoid drawing power out for cooking which will likely mean shorter overall generator run times each day.

Over time, with all these fine-tuning tools at your disposal, you will figure out what is the best and most cost effective set up for how you actually live in your bus. 

It is possible that your final fine-tuned set-up might be quite different from what you started with.  That is one of the really great benefits of this “Best of Both Worlds” concept. No speculation is needed here at all. Live in your bus as you normally do and slowly fine-tune until you reach that magic spot where you have all the power you want day in and day out.

Conclusions

In this approach, you start out spending half what the traditional drop-in replacement lithium vendors would ask you to spend and you get to use up the capacity left in your existing lead-acid batteries at no cost.

The lithium batteries get used for what they are best at providing - 120VC power for your appliances and wall outlets which is where one of the major benefits of lithium-ion batteries comes into play.

The existing lead-acid batteries get a new lease on life free of trying to power inverters which they are not very good at doing so they can live longer while only supplying the low Amp draw 12VDC applications.

Because you are only swapping out some of your lead-acid batteries for the lighter-weight lithium batteries, you will not get as much weight savings, but you will still get significant savings. Lithium batteries weigh 20 to 30 pounds each where lead-acid batteries of the same accessible capacity will weigh close to 150 pounds each.

Your lithium batteries will likely serve longer than you will own your bus. When it comes time to consider replacing the last of your lead-acid batteries, you will have gained real-world experience in how many lithium batteries you really need and whether it is more cost-effective for you to replace the old lead-acid batteries with new ones or go all lithium.

If your bus is now set up with a 12VDC lead-acid chassis battery bank and a 12VDC lead-acid generator battery that is also charged by the alternator via a battery isolator or combiner, then keep one or more lead-acid batteries as part of your house battery bank will make a lot of sense.  You don’t need to change anything there.

This setup will handle keeping all the lead-acid batteries charged while driving, on shore power, or while running the generator. Consequently, there is a long-term gain by keeping one or two lead-acid batteries as part of your house battery bank even if you moved most of the 12VDC loads over to the lithium side.

Enjoy your new-found energy freedom while saving half of the up-front cost and gaining all the life remaining in your existing lead-acid batteries. You will find it really is the “Best of Both Worlds!”