Important Note: There are always risks associated with being around and/or handling electricity. If you have any questions about your ability to handle these risks safely, please do not attempt to follow any of what I have outlined below and instead, hire a licensed professional to do this for you. I am only sharing what I have done. Neither I nor Bus Conversion Magazine assumes any liability for what you do or how you do it.

In Part 1 of this final article in the series on how to safely, inexpensively, and correctly convert your motorhome or bus from lead-acid (AGM, sealed lead-acid, wet cell, etc.) house batteries to LiFePO4 batteries I discussed why you don’t need to do this conversion all at once.  Rather, you can, as I did, “ooze” into the process as your needs and budget allow.  I did it in three steps over two years which allowed me to use up some of the remaining life in the six 8D AGM batteries that were in my coach when I purchased it while enjoying the many benefits of LiFePO4 batteries from day one to power the 120vac appliances in my coach while off the grid.

In part two, I want to talk about why I created this extensible topology the way I did and also cover what I feel are the critical aspects of great build quality for today’s high-capacity LiFePO4 batteries.

The first thing that determined this simple topology is, you are sure you never allow a lead-acid and a Li battery to be directly connected one with another.  These two battery chemistries operate at very different voltage levels - a Li battery when 80% discharged still outputs the same voltage as a lead-acid battery does when it is fully charged.  

If you were to connect them directly together, the higher voltage Li battery would send a current surge into the lead-acid battery trying to bring it to the same voltage level.  That could damage one or both batteries and could cause the lead-acid battery to explode under some circumstances.  Here, either the BtoB/MPPT charge controller or the Inverter/charger prevents the two different battery chemistries from ever being directly connected one to the other.

The second consideration is that it is not a good idea to mix Li batteries from different manufacturers, or different capacities or even Li batteries of the same capacity from the same manufacturer purchased more than 3-6 months apart because they may have different battery management systems which might not function well together.  In this topology all the battery banks are isolated one from another, so you don’t need to be concerned about that.  

Some authors and YouTube videos will suggest that it is okay to mix them in some circumstances.  I think that is a risk my readers should not take.  The increased cost of keeping them isolated in my mind is well justified and will make sure there is no issue ever, not initially and not over time.

Saving your alternator is the third consideration for why I designed this topology the way I did.  Unless your motorhome or bus is only a year or two old, your existing alternator and voltage regulator were designed ONLY to charge lead-acid batteries.  Lead-acid batteries have so much internal resistance that they severely limit how many amps the batteries can take out of the alternator.  If you try to run your existing alternator at more than about 60% of its rated capacity, you run the very real risk of overheating it which can cause permanent damage.  The alternators used in large motorhomes and bus conversions can be verrrry expensive to replace.    

Li batteries have very low internal resistance so they will happily take all the amps your alternator can put out.  To protect your alternator from overheating you need a way to limit the amps the alternator supplies to recharge your Li battery bank(s).  That is one thing the Battery-to-Battery charger does.  It takes as input the charge current going to a lead-acid battery and internally converts that charge current to the profile required to fully recharge the Li bank while at the same time limiting the Amperage draw to a specified value - in the case of these Renogy combination BtoB/MPPT charges that limit is 50 amps.  

So, even if you are charging three separate Li banks as I do, the maximum amp draw on the alternator is only 150 amps (50x3), about 60% of the 270 amps my oil-cooled alternator was designed to put out all day long without damage.  In most cases, not all three Li banks will require all 50 amps at the same time anyway since they will be at different levels of discharge most of the time while you are boondocking.  As a result, there is almost no way I can harm my existing 20+-year-old expensive alternator using this design.

For the alternator to work it needs to see voltage on the line running from the alternator to the battery it is trying to charge.  That is why you cannot directly connect the output from the alternator to the input of a Battery-to-Battery charger.  The BtoB charger does not allow voltage to flow backward from the battery through the BtoB charger so the alternator would never see voltage on that line.  Instead, leave one lead-acid battery in the system as shown.  Connect the output line from the existing alternator to that lead-acid battery.  Run a separate line from the B+ terminal on the lead-acid battery to the B+ input on the BtoB charger.  

When the engine is running, the alternator will see the voltage from the lead-acid battery and turn itself on to recharge it.  At the same time, the BtoB charger will see the elevated voltage put out by the alternator which will allow it to turn itself on to charge the Li battery with the proper charge profile.  No user intervention is required.

The final reason to keep one lead-acid battery in the system as shown is that each Li battery has its own built-in battery management system which protects the Li battery from overcharging, undercharging, charging while too cold, over-discharging, etc.  If all the BMSs in all the Li battery banks were to shut down charging at the same time with the alternator still spinning, it could result in a large voltage spike which could damage some or all of the electronics in the system.  By keeping one lead-acid battery in the system, that lead-acid battery will absorb any such voltage spike and the alternator would just see itself still charging a lead-acid battery as it always has.

The fourth consideration is that by isolating the lead-acid from the Li batteries and isolating the Li battery banks from one another by using separate BtoB chargers (one for each Li bank), you limit the recharge rate to the output rating of the BtoB charger(s).  In this case, each Renogy combined BtoB/MPPT charger can output a maximum of 50 amps to recharge each Li battery bank.  That is a much lower recharge rate than a 300 or 400-amp-hour Li battery can safely handle.  

It does no harm to charge them more slowly so the only thing you “lose” is fast charging from the alternator while driving.  It has no effect on the time to recharge while running the generator or while on shore power as the recharge rate, under those circumstances, is determined by the output of the inverter/charger connected to each Li battery bank - and they usually are designed to output 100 or more amps.

If you use these Renogy units to recharge both from the alternator and from solar at the same time, each charging mode will be limited to 25 amps.  If you really want faster charging while driving, you can always add one or more of these Renogy units in parallel for each Li battery bank.  The effective charge rates will double to about the same rate as charging off the generator or shore power as long as the total recharge amps do not exceed the safe working output of your alternator which will be about 60% of the rated output of the alternator.

The final consideration that caused me to design this conversion system the way I did was to allow you full flexibility to add or subtract battery capacity over time to match what you actually use - the way you live in your coach.  I recommend you record the state of charge on each of your Li battery banks first thing each morning when you get up while boondocking.  That will show you how much capacity you used the day before for whatever each battery bank is powering.  

For the vast majority of you, the 1100-amp hours of Li battery capacity shown in this series of articles will be more than you will ever need unless you are trying to live fully off the grid with only occasional running of the generator or you are wanting to power air conditioning units off your batteries (seldom a good idea).  If you do need more power, you can easily add it.  And, if you have been measuring the battery state of charge each morning as you have incrementally added more capacity in the past, you will be fine-tuning your system to just what you need/want now

Now let’s turn our attention to what makes for great build quality in a LiFePO4 battery.

Let’s use as an example the new EEZ RV Products 400-amp hour ruggedized Evergreen brand LiFePO4 battery.  It has among the best build quality I have seen to date.  The stainless-steel case is made from non-magnetic (the higher grades), with fully welded corners all around.  The top of the sides are rolled and recessed enough that the lid sits flush with the sides.  There are no raw edges anywhere.  

The screws that hold the top in place are stainless-steel machine screws going into threaded inserts, not sheet metal screws, so there are no sharp edges on the inside of the case either.  The cells are properly separated with fiberglass separator sheets and the cells are connected with busbars.  The whole battery pack is wrapped with fiberglass insulating sheets and it sits on top of a custom-made heating pad that covers the whole bottom of the pack.  

A dense foam insulator layer protects the battery cells from vibration and insulates them electrically from the stainless-steel case at the bottom and around all four sides.  The case is larger than an 8D AGM case to provide lots of open-air space for heat dissipation while holding the cells and internal components securely in place.  A stainless-steel bulkhead keeps the battery cells from shifting front to back and the dense vibration-insulating foam prevents them from shifting side to side.  

Stainless steel straps hold the battery cells in place even when this battery is turned upside down.  The openings for the included 1-amp and 2.1-amp USB power ports, the 12vdc non-regulated cigarette lighter power port, and the standard solar charge controller input port are all sealed with rubber caps to provide water protection.

All of the internal electrical connections are made via flag connectors with a nut and bolt so the connections can be properly torqued without fear of stripping.  Same with the external battery cable connections.  They are a nut and bolt to a flag-like connector for the same reason.  Many, even high-capacity batteries, are using threaded holes in the battery posts and 6 to 8-mm bolts to hold the battery cables to the battery.  You need to be careful not to strip the threads in those holes or you run the risk of ruining the battery before you start.

Because it is so easy to remove the top, this case design lends itself to easy replacement of internal components if ever needed.  A simple bead of caulk on the inside of the top flanges will make the case more water-resistant.  I wouldn’t call it waterproof, but for most applications, this case design looks like it would likely survive even moderately heavy water splash.  However, it is not meant to be immersed in water.

The DALY battery management system and all the internal wiring are rated to handle the 300-amp continuous discharge rate the battery is designed to output.  This high-quality BMS looks to be similar to, if not the same as, the high-quality, fan-cooled DALY unit that retails in the USA for $300 or more all by itself.  All the high-power wiring inside the battery is double runs of 4 AWG high strand count tinned solid copper marine grade wire with 200 degree C insulation. 

They will easily carry 2 x 150 = 300 amps over these short runs.  Your cables connecting to this battery should be AWG 00 to 000 depending on the length and wire type.  You want to be sure your wire can handle up to 300 amps of continuous discharge.  Charge current should be limited to 150 amps for the longest battery life.  Note that my use of the Renogy BtoB/MPPT charge controllers l the charge rate while driving to 50 amps per battery.  While on shore or generator power the charge rate is right at 100 amps per battery.

This battery has a Bluetooth battery monitor built in which connects to an app on your phone.  As mentioned above, it also has automatic heating built in.  When the battery cell temperature reaches freezing, the BMS will prevent charge current from entering the battery and will automatically turn on the battery’s internally powered heating blanket.  Once the cells reach a safe-for-charging temperature, the heater will turn off and the BMS will once again allow the charge current to enter the battery.  No action on your part is required.  

Keep in mind that with this much capacity along with all these features and this build quality, this battery is quite heavy.  It weighs just under 100 lbs. so mount it accordingly.  It can go anywhere in your coach since the stainless-steel case and internally powered heater will keep the battery safe in most mounting locations.  I put mine in the rack where the original AGMs were located in an external bay.

Here in list form are the things I think you should look for when assessing Li battery build quality:

1) The battery case.  I am fond of high-quality (non-magnetic) stainless steel for a Li battery case.  Many people like the glued-top ABS plastic cases because they can be waterproof if properly sealed and they are inexpensive.  I have seen too many situations where the top was not properly glued in place to form a water-tight seal, yet that is not usually visible from outside examination, so you never really know for sure.  

The welded stainless-steel cases usually have the top screwed in place so you can open them up for inspection or component replacement if needed.  Yes, they are usually more expensive than a plastic case, but I like to be able to see inside without damaging the battery.  The top can be sealed with the proper application of caulking if some level of waterproofing is important to you but that is seldom required in an RV application, in motorhomes, or in bus conversions.

2) Switches or buttons instead of a phone app to turn the battery on/off.  The phone apps are okay for seeing the batteries’ state of charge and cell voltages but not nearly as convenient for turning the battery on/off as mechanical switches or buttons.

3) Flag tyle battery connections.  Flag style means hard metal plates with holes in them for connecting battery cables instead of clamp-style battery terminals or a threaded hole in the battery post area.  The flag lets you connect with a bolt and a nut so you can properly torque these important connections.  The regular clamp-style battery terminals can loosen over time and build up corrosion which can lead to increased resistance which effectively downrates the capacity and performance of your battery.  

I really don’t like the threaded hole-style connectors.  Often the threaded hole only takes a short 6mm or 8mm bolt which can easily be stripped so they are harder to properly torque down.  Another advantage of the flag-style connector is you can easily add more battery cables if necessary.  All it takes is a longer bolt.

4) Proper internal interconnect cables.  The interconnect cable(s) must be rated to carry the full discharge current capacity of the battery.  Copper is copper so it doesn’t matter if the manufacturer uses one large or two or more smaller interconnect cables as long as the sum of those cables can carry the full discharge capacity the battery is capable of producing.  The discharge capacity is not the amp hour rating.  It is how many amps the battery can discharge continuously.  

This is determined by the BMS and the wiring, not the number or organization of the cells.  Genuine high strand-count tinned copper marine wire with 200-degree C insulation can carry significantly more amps than the same wire gauge welding wire or standard car-stranded wire.  High-quality Li batteries will only use wires rated for the max discharge rate the battery is rated for.

5) Continuous discharge rate of more than 50% of amp hour capacity.  All modern 100-amp hour Li batteries should be able to continuously discharge 100 Amps in use.  Even inexpensive BMSs can handle that.  The high capacity (300 to 400-amp hour rating) Li battery should use a much more expensive BMS capable of handling a discharge of 200 to 300 Amps.  Unfortunately, many on the market now use cheaper BMSs in their 300-400-amp hour Li batteries so they can only discharge 100-150 amps.  

Powering an inverter, a 150-amp discharge would provide you with less than 1800 watts, more like 1200-1500 watts depending on the efficiency of the inverter/charger.   This will allow you to use only one kitchen appliance at a time.  Look for 300 discharge amps if you want to power two or more appliances at the same time as you should be able to do with these high-capacity Li batteries.

6) Internal automatic heaters instead of just low temp disconnect.  All Li batteries can be damaged if charged while the cell temperature is much below freezing.  The lowest quality of the Li batteries will have no real low-temperature charge protection even if they say they do.  The medium-quality Li batteries will have low temp charge protection that really works.  

High build quality Li batteries will not only turn off charging if the cell temperature is too low, but they will also feature internal heaters that will automatically turn on if the cells are too cold to properly charge.  Once they heat the cells to a safe temperature the BMS should automatically turn charging back on and turn off the internal heaters without any user interaction.  This process should repeat, again and again without any user interaction.

7) Vibration protection.  The best build quality Li batteries will feature dense foam vibration isolation to add longevity to a Li battery used in RV service where vibration is a real concern.  Cells will be well secured so they neither move around, swell or shrink in use, even if turned upside down.

8) Well-secured internal components.  All of the internal components - the cells, the BMS, a Bluetooth battery monitor, load balancing wire leads, etc. will be mechanically secured in the best Li batteries.  Lesser build-quality Li batteries will use globs of glue or tape for this function.  While globs of glue or tape work when the battery is new, over time heat and vibration may cause the adhesives to become less reliable.

9) Cable protectors for all internal wires.  The best build quality Li batteries will protect all internal wires with hard plastic cable protectors, so the insulation is not compromised by rubbing or abrasion.

10) Grade A new cells.  Quality Li batteries will use nothing but new, tested, grade-A cells.  Lesser quality Li batteries will employ lower grade cells or even used cells since a Li cell is considered at the end of its useful life when it still has 80% of its rated capacity remaining.  

11) Expect an expensive, high-quality BMS.  BMSs range in price from less than $10 to more than $300.  You really should get what you pay for, so look for the very best BMS made to be inside today’s quality Li batteries.  The manufacturer should be upfront about where and how they source the BMS used in their battery.

12) Continuous improvement.  The best of manufacturers will be constantly making improvements in build quality whether they raise prices or keep them the same from batch to batch.  Many times, the marketing name on the Li battery will be determined by the USA/Canadian distributor or retailer so you have few ways to know which batch/build quality Li battery you are getting unless you open them up and look for yourself.  Doing so may void any warranty so don’t do that just out of curiosity.

A good example is shown in the two pictures of these batteries. The first picture (above) is from EEZRV Products batch of 400-amp hour ruggedized Li batteries with the welded stainless-steel cases and very high-quality BMSs featured in part 1 of this article. This was the model that was contemporary when this article was written in the fall of 2022.  

The second picture (below) is of their next generation model. Both look the same from the outside and my understanding is there is no difference in price between these two models.  The changes are all inside where you can’t see them but notice the significant build quality improvements. 

What can you expect in the future?  I don’t see near-term fundamental improvements in the Li cells themselves.  Grade A new cells are already quite good and much more cost-effective than any lead-acid alternative.  A totally different battery chemistry that is suited for RV use takes a long time to get all the way through the development cycle, so I don’t see different technology cells any time soon, either.  What I do expect to see is continuous improvement in build quality across the board like the example from EEZRV Products shown here.  

I also expect the quality Li battery manufacturers will soon begin including additional functions within the same battery case.  I don’t think it will be very long before we see MPPT solar charging and Battery-to-Battery charging embedded inside upcoming Li batteries.  

All you will need to do then is attach the positive and negative battery cables and the battery will take care of all the rest, safely recharging itself from the alternator while driving and/or from solar panels whether driving or boondocking.  Your inverter/charger(s) will keep the battery recharged while connected to shore power or while the generator is running.  Internal heating pads that automatically turn on and off will prevent charging if the cells are too cold.  All this will happen with virtually no intervention from you.

Conclusion.  

I hope you have found this series of articles helpful as you plan your own lead-acid to LiFePO4 house battery conversion in your motorhome or bus conversion.  Safety and longevity are the two most important things since your new Li batteries will most likely outlive your ownership time with your bus.  

The design taxonomy I have outlined in this series of articles will allow you to do this conversion on your timetable to fit your budget.  You can get full value and life out of your existing lead-acid batteries if you do the conversion in stages as shown here.  Unless you want to, there simply is no reason to do this conversion all at once as is often touted by the “drop-in replacement crowd” or by the “cost is no object crowd”.