Whether you have a factory bus conversion as we do, or a bus you or someone else converted, you no doubt have heard a lot about the benefits of using lithium-based batteries for your house batteries. Unfortunately, you likely have also been exposed to a lot of information of dubious value and some downright dangerous information from reading internet posts or by watching You-Tube videos.
In this article, I will try to give you the right information you need to make an informed decision for yourself and along the way will try to dispel the many myths and half-truths that are floating around about this subject.
The first thing is to recognize that there are six, entirely different battery chemistries offered that are all called, “Lithium”. Only one of those, Lithium Iron Phosphate, is appropriate and safe for use as house batteries in our buses.
The other five chemistries also called lithium all have significant issues that make them far less than ideal for use in a bus conversion. The following chart summarizes all six of these chemistries.
This chart shows the capacity (in watt-hours per kilogram of weight) for nine common battery chemistries. The red bar on the left shows the common lead-acid auto/RV battery. Whether it is called a wet cell, gel cell, AGM, or SLA, all of these have a capacity of about 40 watts per kilogram (2.2 pounds) of weight.
The green bars show the familiar Nickel Cadmium (NiCd) and Nickel Metal Hydride (NiMH) type batteries that offer up to twice the energy density of lead-acid and have proven themselves in decades of use in portable power tools, portable audio devices, and other portable applications, but which also have known issues in terms of things like limited life, rapid self-discharge, limited amperage output, and memory issues that make them less than ideal for use as house battery banks for bus conversions.
The orange bars show six different types of lithium batteries. The letters at the bottom of each bar are shorthand for what all is alloyed with lithium for each of these six different battery chemistries.
The three bars at the right employ cobalt, a rare earth mineral that is environmentally difficult to mine, to ship, to handle, and to dispose of. It also makes active battery management, especially active heating and cooling, mandatory to avoid unintended thermal events like the Samsung engineers encountered with such batteries in their phones a few years ago.
These cobalt alloyed battery chemistries result in very high energy densities, up to six times the energy density of lead-acid batteries, but unless you really, really know what you are doing, I recommend you stay away from them as house batteries in your bus. They simply can be too dangerous if improperly deployed.
I really caution you not to get caught up in the idea of cracking open electric vehicle batteries to harvest these cells for use in your bus. The internal voltages can be very high, high enough to be deadly for some. And, safely managing harvested cells is not something for a neophyte!
Lithium Iron Phosphate batteries (labeled LFP in the chart on the next page but most commonly shown as LiFePO4) are right in the sweet spot for our use as house batteries in a bus conversion. They offer three times the energy density of lead-acid batteries, are safe, easily managed, widely available, and can handle the large loads we often require when running household appliances through a large inverter.
They also recharge six times faster than lead-acid batteries of a similarly rated capacity, exhibit very slow self-discharge, and last five to ten times longer than the lead-acid batteries we are now using.
In fact, many manufacturers advertise them as “drop-in” replacements for lead-acid batteries in RV applications. While I don’t agree that they are directly drop-in replacements for our buses for reasons I will detail later, they are the right, the safe, and the economically reasonable chemistry for our use.
You will often hear that while LiFePO4 batteries offer many advantages, they are much more expensive so are of questionable value. I want to put this notion to bed and will offer a number of reasons why as we go along.
Suffice it to say that while they cost more to acquire in the first place, the last five to ten times as long so are a much more economical investment when viewed over the life of your bus. In fact, most of the time lithium iron phosphate batteries will be the last house batteries you will ever have to buy.
One of the least discussed but most important traits of LiFePO4 batteries is that for more than 85% of their discharge capacity they operate at a voltage higher than a lead-acid battery does when fully recharged.
Remembering from your high school science classes that the measure of the amount of work electricity can do for you (Watts) is equal to Volts times Amps, then you know the greater the Volts, the greater the Watts available for your use.
A fully charged lead-acid battery at rest will read between 12.6 and 12.8 Volts DC and voltage will drop rapidly as the battery is used and will drop even more rapidly if it is used hard. A fully charged LiFePO4 battery at rest will read between 14.4 and 13.6 VDC and voltage will stay above 12.6 VDC all the way down to where the LiFePO4 battery has only 18% of its discharge capacity left.
Throughout the time you are using your LiFePO4 house battery bank you will experience more Watts available to your lights, fans, electronics and appliances so they can do more useful work for you than can any lead-acid battery of a similar rated capacity.
In the ‘60s and ‘70s when converting over the road buses to motorhomes first began in earnest, the only way you could use household appliances was if you were plugged into shore power or running a generator. There was no such thing as an inverter to produce 120VAC power from a 12VDC source like the house battery. (For a good article about Shore Power Cords by Wulf Ward in the early days, read this article here (Jorge, add Shore Power Cords from March 2019 issue to Blog)
House electrical needs were simple and few: some interior lights, the power to run a 12VDC furnace or ceiling vents for most of the night, and perhaps to power a propane/12VDC refrigerator while going down the road. TVs were heavy, bulky and power hogs so were not common in the bus conversions of that day. Cooking was relegated to propane stoves or a then not very common low power microwave oven. Because these house electrical needs were simple and the typical loads small, the house battery bank was often just a few 12 Volt lead-acid batteries. Two or maybe up to four were used, wired in parallel to give a little more capacity.
Dial forward to today and the story is very different. Inverters are common to provide 120VAC power when we are not plugged in or not running the generator. Far more efficient residential-style appliances abound to do everything from cooling or cooking our food, recharging batteries in our computers, pads, phones and power tools, providing entertainment via low power flat-screen TVs, DVDs, soundbars and audio systems, running CPAP machines all night, and on and on.
The problem is our old house electrical system likely did not keep up with this increased demand and now is often a limiting factor to our enjoying our time on the road. A lead-acid house battery bank is simply not up to the demands we are placing on them or the devices we use to recharge those over-taxed house batteries.
By upgrading your old lead-acid batteries to LiFePO4 batteries you will go a long way towards solving the house battery capacity and rate of draw issues, but you still need to correctly address recharging those LiFePO4 batteries.
While you are plugged into shore power or running your generator, recharging will be done via the onboard battery charger. While LiFePO4 batteries can survive on the same charging profile now used by your lead-acid batteries, you will get longer life and more capacity from your LiFePO4 batteries by changing the charge profile to one that is optimum for lithium.
Often your current battery charger is integrated with an inverter/converter. Most of those have the ability to be reprogrammed to provide the correct lithium charging profile, or many can be upgraded via an inexpensive add-on module available from the inverter/converter/charger manufacturer. Bottom line, most of the time you can reuse your existing unit.
Charging lithium house batteries from your engine alternator is a different matter.
A lead-acid house battery bank has so much internal resistance that it cannot take a very large charge for very long which is why it seems to take forever to recharge a deeply discharged lead-acid house battery bank. It is also why so few of you have experienced overheating of your existing engine driven house battery bank alternator.
The lead-acid battery bank simply will not take anywhere near the max Amps your alternator can put out for very long so the alternator seldom overheats as it would if driven for long periods of time at its maximum Amperage output.
Most LiFePO4 house batteries have much lower internal resistance so are perfectly happy taking as much charge current as they have the capacity. That means your house lithium battery bank will try to draw a lot more Amps from your engine-driven alternator than it may have been designed for which might well lead to it overheating or failing.
The large, continuous duty, oil-cooled alternators common on large truck engines are designed to deliver these much larger recharging loads so will do just fine but the smaller, air-cooled, non-continuous duty alternators common on smaller or older truck/bus engines will need some help to keep them alive.
There is a device called a Battery Isolation Manager that works by altering the duty cycle of the alternator. Charge current is only allowed to flow to the house battery bank for 15 minutes, and then it is turned off for 20 minutes to allow the alternator to cool down. If you have a non-continuous duty, air-cooled alternator, this may help in your circumstance, but be sure to check with the alternator manufacturer to make sure you can periodically interrupt the alternator output like this without damage to the alternator or the regulator.
If your bus has both a 12 Volt chassis battery bank and a 12 Volt house battery bank and uses one 12 Volt alternator to recharge both, you can easily install one of these Battery Isolation Managers between the chassis battery bank and the house battery bank so the greater load required to recharge the LiFePO4 house battery bank will only flow to that bank for 15 minutes and be off for 20 minutes to keep from overloading the alternator. There will be no change in how the alternator recharges your existing chassis battery bank.
If you have two alternators now, one recharging the chassis bank and one recharging the house bank, you should be able to use your existing set up as is so long as the alternator recharging the house bank is 12VDC and is designed for continuous use. The charge profile output by your Voltage regulator may not be optimum for the lithium batteries but likely is close enough that it won’t significantly harm them. Some Voltage regulators can be reset to a Voltage profile more in line with what would be optimum for the lithium batteries so check with the manufacturer to see if this is possible or if they offer a lithium charge profile replacement.
There are devices widely used in marine applications that can help get the correct charge profile to a new LiFePO4 house battery bank if your current regulator cannot be reprogramed or if no lithium specific regulator is available. These are called, “battery-to-battery chargers”.
They normally are wired between two battery banks where they act both as an isolator, preventing the output bank from discharging the input bank, and they also take power from the input battery bank (eg; a lead-acid chassis battery) where the alternator is attached and change the current flowing through the B-to-B device to a charge profile required for the output battery bank (eg: a LiFePO4 house battery). It may be possible to wire one of these between the house batter alternator and the LiFePO4 battery bank, but check with the manufacturer to make sure it would do this in your specific setup.
One issue that you will likely read about has to do with the behavior of the battery management system (BMS) built into all LiFePO4 batteries suitable for use in our buses. That BMS serves several important purposes.
It protects the battery from a variety of conditions that might otherwise damage the battery by shutting it down if the condition is encountered. For example, it will shut off the battery in an over load condition like trying to draw too many amps out too quickly. It protects the battery from over draw conditions like trying to draw too much of the capacity of the battery before recharging. It prevents the battery from taking additional charge current once it is fully recharged. It protects the battery from trying to recharge when the cells are too cold. And, it rebalances all the cells in the battery so all contribute equally to the load thereby increasing the life of the battery.
The issue is what happens when the battery becomes fully charged and the BMS shuts down further charging. Does this sudden shut down of the charge current coming from the alternator cause a voltage spike that might damage the alternator or some other upstream component? If you have a large house battery bank made up of several LiFePO4 batteries drawing something near the capacity of the alternator, the concern is that a very large voltage spike could theoretically occur.
I spoke with the engineers from one of the leading LiFePO4 “drop-in replacement” battery manufacturers about this issue. They said they had been testing for this condition using battery banks and alternators of various sizes, and while acknowledging that it is theoretically possible, they had yet to actually experience such a voltage spike.
Part of the reason is that it is highly unlikely that all the BMS’s in a multiple LiFePO4 battery bank would shut off at the same time so, even if there was a voltage spike, it would be small enough to not be an issue. Also, many of you have a separate generator starting battery that is recharged through an isolator or combiner off of your house battery recharge circuit. If so, that lead-acid generator start battery might absorb some or all of such a voltage spike if it did occur.
Bus conversion owners generally have quite large lead-acid house battery banks – four to eight lead-acid batteries as large as 8D are not uncommon. That means the total rated capacity could be as large as 1600 Amps. Remembering that we can only draw 50% of that capacity from a lead-acid battery without damaging it, when we make the change over to LiFePO4 batteries – where you can access all or nearly all of the rated capacity – we will likely be dealing with something more in the 400 to 800 Amp hour range. So you can’t rule out the possibility of a large voltage spike if all the BMS’s shut off at the same time, but it is unlikely to be an issue for most. There are devices available in the yachting world to address this issue if it is of concern to you.
There is no reason to change your chassis starting batteries from lead-acid to LiFePO4. Only think in terms of changing the house battery bank. If you have only one battery bank that serves both purposes, then there are LiFePO4 batteries designed to do just that as well.
If you have two battery banks recharged from two alternators then you only need to deal with the house battery bank and alternator and can leave the chassis battery bank and alternator alone.
Now let’s address that there is no easy “one size fits all” answer. Most will tell you to do a 1 for 2 substitution – that is, put in half as much LiFePO4 capacity as you now have lead-acid capacity since you can only access 50% of the lead-acid capacity but almost 100% of the LiFePO4 capacity.
Technically you should only access about 80% of the LiFePO4 capacity for optimum life, but many lithium batteries are now underrated by as much as 20% so the manufacturers most often now tell you it is ok to access all 100% of the rated capacity since the BMS will keep you from over-discharging the LiFePO4 battery anyway.
But, you’re very likely to have made changes to the electrical system since your bus was converted that reduce the amount of electrical power you now need from your house battery bank. Most have changed out the interior lighting to LEDs which consume far less power than what was originally installed.
If the original interior lights included a lot of halogen puck lights, changing those over to LEDs greatly reduced the draw on your house battery bank. Depending on how many there were, those old, hot halogen puck lights could well have consumed a lot of amps for every hour of use. If you ran 120VAC interior lights through an inverter the draw would have been even more. The new LED lights will hardly even register.
You likely have changed to a more modern flat-screen LED TV that consumes far fewer watts than your old one. You may have replaced an old, inefficient 120VAC resistance heat stove with a 120VAC induction unit that uses far less electricity to do the same cooking job. You might have replaced the refrigerator to a new one that is much more energy-efficient, and so on.
On the other hand, you could have greatly increased the electrical demand on your house batteries over the years. For example, if somewhere along the way you installed a large inverter and changed out 12VDC appliances for 120VAC appliances running through that inverter, you might now require more power rather than less.
You might now need to run a CPAP machine all night or maybe you are recharging a lot more electronic devices like phones and tools than you did when your bus was originally converted. Or you might now be using different appliances like one of the Instant Pots that could also increase your electrical needs.
The point is, your electrical requirements have likely changed in the time you have owned your converted bus and that has a direct role in determining what size you now need for a house battery bank. It could be less, it could be more.
Finally, you also need to factor in that lead-acid batteries constantly loose capacity in use so the house bank that once might have been satisfactory for your use is now effectively only a fraction of what it was when new. Since you can only discharge your lead-acid batteries down to 50% of their current capacity, which is likely quite a bit less than what the manufacturer claimed was their capacity when they were first installed, you really don’t know what you have in the way of actual capacity, or actual use, unless you have been diligent about doing accurate battery capacity monitoring on a daily basis over a long period of time.
Most sources will tell you to try to calculate your actual energy use and use that as the guide to how large your house battery bank needs to be. The trouble is, few can do that accurately and your needs likely change over time or with the seasons, so I am going to suggest a simpler approach.
Given all these unknowns, my recommendation is to start from where you are now in terms of the original accessible Amp-hour capacity of your existing house lead-acid battery bank (50% of the battery manufacturer’s rated capacity when new), decrease that by around 20% to take advantage of the more efficient electronics and appliances you have now, and build your new LiFePO4 house battery bank with that beginning capacity. Just be sure you can increase capacity marginally in the future by adding one or more additional LiFePO4 batteries later if you find you need to.
That means using LiFePO4 batteries that can be wired in parallel to provide that additional capacity later. Most, but not all, LiFePO4 batteries can be wired in parallel but there may be limits to how many total Amp-hours you can achieve overall. Check with the battery manufacturer to make sure they can meet your needs.
The most common of the so-called “drop-in replacement” LiFePO4 batteries are rated for 100 Amp-hours each and can be wired in parallel to produce as many amp hours as you are ever likely to need. If you now have, say, six 8D AGM lead-acid house batteries as we have in our Prevost conversion, that would have given you about 700 accessible amp-hours of capacity when new. Decreasing that by 20% means you should start with a LiFePO4 battery bank made up of five or six 100 Amp-hour units.
If you are currently using a house lead-acid battery bank with less capacity than these six 8Ds, you will likely need four 100 Amp-hour LiFePO4 batteries. For a bus conversion, I don’t recommend starting with fewer than four no matter how small your house battery bank is now. You want to be able to live on your bus just as you do in your home and at least four LiFePO4 100 Amp hour batteries will allow you to do that. As noted before, if you find you want or need more capacity later you can always add more LiFePO4 batteries.
Wow! That was a lot of words to say, use four to six 100 Amp-hour LiFePO4 batteries for your new house battery bank no matter when or who did your bus conversion!
The next recommendation has to do with wiring. No matter when your bus conversion was completed, there is a good chance that you will want to change out your existing battery wiring for high quality, multi-stranded, tinned copper marine wire of a size 1AGW or larger – preferably 00AWG.
You will be passing a lot of Amps between batteries wired in parallel and between the battery bank and the inverter/charger/converter so you want to make sure there is as little resistance as possible in the wires and the wire ends. If the wire is too small or there is resistance in the wire ends, it will choke off the flow of current to do useful work for you. There is no need to invest in quality batteries and then hamstring them by using old or inadequate wire.
If yours is an older bus conversion, decades of exposure to the caustic fumes released by the old lead-acid batteries during use or during recharging may well have corroded your existing wires even up under the insulation where you can’t see. In wire, more strands are better than fewer, larger is better than smaller and marine-grade wire is better than automotive grade wire. DO NOT SCIMP HERE!
Since there are a number of different LiFePO4 battery names, whose should I use? There are two basic types of LiFePO4 batteries suitable for our use – those made with cylindrical cells and those made with prismatic cells. Cylindrical cells are normally about 18mm (3/4 in) in diameter by 65mm (2-1/2 in) long. The chemistry dictates that they will output a maximum of close to 3.6 Volts per cell. The amount of amperage in each cell is a function of the packaging of the LIFePO4 in the cell.
To get a nominal 12VDC battery for use in our bus conversions these cells need to be wired in parallel groups to get the needed ampacity, and then four of those groups need to be wired in series to result in a maximum voltage of right around 14.4V when first fully charged. At rest they will be about 13.6 to 13.8 Volts.
Prismatic cells are flat, can be easily molded to different shapes and are made with the total desired ampacity in each cell. Four of those are wired in series to get to the same voltage as the cylindrical cells. Both have their advocates and one type is not necessarily better or worse than the other.
I would not choose simply on the basis of which cells a given battery supplier uses. In both cases, the cells are most commonly made in China, Japan, Taiwan, Thailand, or Indonesia. In some cases, the cells are assembled into a complete battery there and simply marketed here. In other cases, the cells arrive in the US where the manufacturer assembles the cells and the BMS into a suitable case.
One of the most important components in a LiFePO4 battery is who designed the battery management system (BMS), how sophisticated/robust is that design, where the BMS was produced and to what standards was it produced. BMS’s are readily available and sold on places like eBay for prices from a few dollars to over $300 so knowing what is in the battery you select for your bus conversion is really important.
For me, the most important consideration when selecting a LiFePO4 battery is the length of the warranty and how financially strong the manufacturer is who offers that warranty. It is not uncommon in this new field to find warranties offered that are longer than the company has even been in business……hmmm. Since one of the great benefits touted for LiFePO4 batteries is longevity – six or more times the useful life of even the best lead-acid batteries – you want to be confident that the manufacturer/supplier will be there if you have a problem downstream.
Earlier I said LiFePO4 batteries cost more to buy but last so much longer that they represent a good value. Here are some numbers that may help you decide for yourself. Quality LiFePO4 nominal 12VDC batteries in a 100 Amp-hour configuration currently are priced from around $700 to $1200. A quality AGM lead-acid battery of a similar useable rated capacity made by the current market leader is priced at $300 to $400 depending on where you buy it and whether shipping is included or not.
Remembering that the lithium battery will operate at a higher voltage, therefore higher wattage than the lead-acid battery, you will need two to two and a half of those lead-acid AGM batteries to equal the actual output in watts of the one lithium battery. So, while one can argue the lithium batteries cost a little more, the difference is likely far less than you first thought. And, when you consider lithium batteries will last six or more times longer, in my mind the decision is a no-brainer.
Obviously, if you use your bus by going from power pole to power pole, there is no need to spend money on house batteries since you rarely use them anyway. If you have relatively new AGM lead-acid house batteries, there is no need to replace them until you get three or so years to use out of them. And, if you plan to sell your bus within the next year or two there also may be no need to replace your house batteries unless you think it will help sell your bus since many new buyers are conditioned to ask for new batteries as part of the sale.
Under all other conditions, I recommend you look long and hard about going to lithium house batteries sooner rather than later.
Notice that in our discussion about the many benefits from switching to LiFePO4 batteries, I have not even touched on how much lighter they are except on the chart at the beginning that showed they have three times the energy density of lead-acid batteries. That means you will save at least two-thirds of the weight you are now carrying. That can be a really important consideration if your bus load is close to what the axles and/or tires can support. And, it may mean a bit less fuel burn in day to day use.
Now to a question that always comes up. Can you run a roof (A/C) air conditioner off of a lithium house battery bank?
The simple answer is yes, but not for very long – an hour or two or three is about the best you can expect without a lot more engineering than is required for a normal lead-acid to LiFePO4 conversion. However, many people have had success running mini-split A/C units with solar and lithium batteries.
The final thing for your consideration is solar recharging. While that is a subject unto itself, and beyond the scope of this article, suffice it to say that the fact that LiFePO4 batteries recharge six or more times faster than a lead-acid battery, don’t even think about solar recharging unless you are using LiFePO4 batteries. While a few have found success solar charging lead-acid batteries, most have not. Nearly all who have used solar recharging with LiFePO4 batteries have been well satisfied.