RV Batteries on Steroids – DIY lithium

March 12, 2022

Can you imagine running your air conditioner all night, on batteries, and still having over 50% of your battery capacity left? Night-time A/C is our single biggest blocker to boondocking. My wife and I run the A/C in our bedroom all year long and are uncomfortably hot in there when we don’t. My buddy Sean recently built a DIY lithium battery bank and very kindly shared the following article.

Author: Sean McDermott

Disclaimer: If this is your first RV battery or solar project, feel free to read this article and learn more about batteries. But consider a simpler drop-in battery solution to start with. There is a lot to learn before you start wiring up your own battery.

Introduction

After lots of off-grid camping with an inverter, it becomes clear that the more battery you have, the less you need to run a noisy, stinky generator. The more battery storage you have, the more you can take full advantage of the power that your solar panels produce.

Boondocking at a Harvest Host location in Arizona

Doug’s Battle Born batteries installation

For many RVers, a common battery upgrade is a “drop-in” lithium battery such as the popular Battle Born 100Ah 12V battery. A huge advantage of lithium is that it also charges quickly, whether from generator or solar.

Battle Born has in particular established a strong brand in the drop-in 12V 100Ah lithium market and includes a 10-year warranty with their new $950 batteries. Other manufacturers make roughly equivalent drop-ins at cheaper prices, e.g. Lion 105Ah lithium batteries are often available on sale at Costco for as little as $1400 for two. Some drop-ins even have Bluetooth capability.

I briefly explored buying used Tesla modules due to the great price point, but their more dangerous Lithium Nickel Cobalt Aluminum Oxide cells are very tricky to manage safely and there were no cheap off-the-shelf BMSs. I ended up choosing the conservative route and started down the drop-in lithium path in summer 2019 with four Battle Born batteries.

I later added two more and by then had spent almost $5,000 on six 100Ah batteries. I added 1600W of solar panels to the roof during that period, as well as installing a complete Victron electrical system, including two Victron Multiplus 3000VA (2400W) inverters and a Victron BMV-712 shunt (to keep track of battery charge level).

That six battery setup allowed me to use or store my RV off grid with my residential fridge running 24/7. Theoretically, I could run my air conditioning but it would blow through the battery in just a couple of hours so I never did in practice.

In summer 2020, I was chatting with a full-time RV friend who had built his own large DIY battery from imported lithium (LiFePO4) cells. I thought about the possibility of doing this myself. I calculated the basic math and I could sell my six Battle Born batteries and build my own battery system with that money that would be four times larger.

Little did I know the rabbit hole I was diving into…

A lot of learning, selling, buying, waiting, assembly, learning, buying more, waiting more and testing later, I now have a battery bank of over 30kWh or over 22 Battle Borns equivalents. The total cost was under 25% of the BattleBorns so a good payback for the DIY process.

12 lithium cells wired together in series in a Pelican Air box — One of the final built-out 24v battery packs

I can run my air conditioning all night in my large fifth wheel and still have more than 50% of the battery charge left in the morning. To me, it made the whole effort worthwhile.

The five Bs of building a battery

There are a few ways to look at the steps involved in building a DIY Lithium battery. I’ll summarize it as the five Bs. Even the commercial LFP batteries such as Battle Borns are essentially built in this way, just at industrial scale.

Battery cells
Bus bars
BMS
Bits and pieces
Box

When building your own battery, you get to optimize each of these for your own cost and space needs.

1. Battery Cells

The lead acid batteries we have used for years in our cars and RVs aren’t just magically 12 volts (V). They actually contain multiple lead acid cells inside a container to make up the battery. A single lead acid cell has a voltage of roughly 2.1V making a typical six-cell lead acid battery measure at 12.6V. RVs use deep cycle lead acid batteries, where 50-80% of the full capacity of the battery is available. Building a high capacity and long lasting battery system with many large and heavy lead acid batteries becomes impractical quickly which leads us to lithium.

There are a large variety of lithium-metal battery types. While the auto industry uses a variety of battery chemistries, the DIY “build from cells” battery approach is generally using LiFePO4 chemistry, also known as Lithium Iron Phosphate or LFP.

LFP maintains the majority of the valuable attributes of the lithium battery chemistries we see in automobiles, while not being prone to thermal runaway (fire) seen in some of these battery packs. To list a few of my favorite LFP features – no toxic fumes, long life (10-20 years), fast charge, full discharge, and lightweight. A key additional advantage of LFP over other lithium chemistries is the recent availability of new cells to build DIY batteries.

Battery capacity is measured in amp hours (Ah) at a particular voltage (V). Or if you multiply them together, you get watt hours or Wh. The picture opposite shows four 280Ah EVE cells making up a 4×3.2v battery, a little bigger than a typical lead acid battery but three times the storage capacity and about five times the usable capacity. The full capacity of these 4 cells would be 4 cells x 3.2 volts x 280 amp hours = 3,584 Wh = 3.6 kWh. This is about three times the capacity of a single Battle Born 100Ah battery (1.2KWh).

The four cells weigh about 50lbs. In my case, I already have a 24V Victron electrical system set up so I needed a 24V battery bank with 8 cells in each pack (and weighing 100lbs).

Shopping for 3.2V LFP cells isn’t quite as simple as buying them on Amazon. You can find cells on Amazon and on US websites but given that the cells are the most expensive part of the battery, this is the most important area to get the best price. My friend pointed me at the best website to learn and discuss DIY LFP batteries – Will Prowse’s forum site, in the group dedicated to LiFePO4 battery builds.

I read about an LiFePO4 battery cell “group buy” of large cells that I just missed out on. I dropped a note to the person organizing the group buy (Michael Caro) wondering if there were any cells still available. Luckily for me, he was ordering so many cells for the group buy from the supplier in China, he was able to continue ordering at this discounted price. There were 280Ah cells from EVE Energy and 272Ah cells from Lishen Battery. I decided to order 36 EVE cells total – that’s enough for four 8 cell batteries plus 4 spares should anything happen to individual cells down the road.

(Note, the night before publishing this article, Doug with LearnToRV just bought a set of 34 cells from PowerWholesale.net. Help support the site by reaching out to Amy Zheng via Facebook or on WhatsApp and mention that you got your contact information from LearnToRV.)

The total cost including Fedex shipping to my door in California would be $3,430. That’s $95 per 280Ah cell or $380 for a 4-cell series that would make a 12V battery. So almost three times the capacity of a Battle Born for half the price, using quick math (and ignoring the BMS and other costs to come). Yes, this was getting interesting.

It is worth noting that these cells did not come with a 10-year warranty like a Battle Born battery, but at this price I could build the whole system again in a few years and still end up saving money. However if any cells that arrived were duds out of the gate, it was easy to get a free and immediate replacement, shipped from the US.

I placed my battery cell order in November 2020 and waited 9 weeks for the cells to arrive in the US from China. They arrived in well-packaged boxes with 4 cells in each for 9 boxes total. The upside about the 9 week wait was that it gave me plenty of time to research and order all the other components needed to build my battery system.

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2. Bus bars

With a 32 x 280Ah cell battery system that I wanted at 24V, I had a few options on how to wire it. I needed to have 8 x 3.2V cells in series somehow to generate 24V (really 25.6V). I could build four separate battery packs, each at 280Ah or I could build larger parallel 3.2V x 1120Ah cells, grouping 4 cells in parallel at each level and then attach 8 groups of the 4 cells in series. TI chose to build four separate battery packs. Largely because that I would like to move one or more packs between RVs or to a tent and have redundancy in the system (1 pack can fail and I’m not in the dark- literally!).

Keeping the packs separate means each pack can have a dedicated BMS allowing for four times the max BMS current to be available. This was useful in my large RV which could continuously pull over 200A@24v with the two Victron Multiplus inverters (e.g., we could have the air conditioner running, while microwaving some food and have the second air conditioner start up).

Having four separate packs gives four separate fail points. I could afford to have one pack fail and still continue with plenty of capacity. (This hasn’t happened yet.)

Bus bars are metal connections between the batteries

Bus bars are thin copper or aluminum strips used to join the cells together in series to increase the battery voltage. Most sellers of LFP cells will provide sufficient bus bars for a battery as part of the purchase. In my case, they gave “double bus bars” by allowing stacking two high to double the possible current. It’s possible to buy or DIY even better bus bars than those provided but I’ve found it unnecessary in practice. The EVE cells I bought included two bus bars per battery cell (2mm thick tinned copper bars) and are the perfect length to have batteries snug. I didn’t use the provided screws and preferred to use Loctite 271 on my own studs to eliminate the chance of battery thread damage.

Using two stacked bus bars per cell would easily carry the max 125A my battery pack is fused at. It also helps to clean and coat the bus bars with an antioxidant compound such as Gardner Bender OX-800.

Cell Balancing

Once the cells and bus bars are available, it is time for a one-time cell balance to ensure all the cells are matched. The most common method is called “top balancing” where the cells are matched at full charge. There is a lot of information online about the method but the short version is to connect the cells in parallel and charge them all up to the max voltage they will see, typically 3.6V or 3.65V. This will ensure that the discharging is well balanced across the cells from 100% to 0% and back up to 100% again during charging. You can see my adjustable power supply in the picture below top balancing one of my packs, initially at 3.4V (and showing 7.07A). My other 8-cell packs are ready to be top balanced to the side – you can see the bus bars connecting all the cells in parallel for a huge 3.2V pack.

Once cells are top balanced, the current on the charger screen will drop to 0.0A. This is when the voltage set on the charger is reached across all cells. At that point, the bus bars across the 8 cells can be reconfigured from parallel to series, changing the voltage range from 3.2-3.6V to 25.6-28.8V. The battery could technically be used at this point by connecting to the positive and negative jump points, shown at the top of this wiring diagram. However, it’s typically at this point that the BMS comes into play.

3. BMS (Battery Management System)

The third component in an LFP battery is a Battery Management System (BMS). This is not to be confused with a Battery Monitor, which is used to passively monitor battery levels using a negative shunt (e.g., Victron BMV). And to add to the potential confusion, the BMS actually operates as a negative shunt with a relay to turn it on/off.

In short, the BMS is there to protect the battery cells to make sure you don’t accidentally damage or shorten the life of the battery. In an extreme case, but very difficult with LiFePO4 chemistry, an overcharged battery with no BMS could cause a fire. With a BMS in a drop-in lithium battery or a DIY lithium battery, the battery is very safe.

It’s worth mentioning that a BMS could actually be useful even inside a lead acid battery but a typical BMS costs over $100, so it doesn’t make sense to add one given the price of a lead acid battery. The BMS has numerous protections such as protecting against low/high temperature, low/high voltage (measured at each cell), battery charge/discharge current too high, and short circuit. A great feature of the Overkill and Daly BMSs is that they let you interact with the BMS directly with Bluetooth. This allows tuning of the BMS parameters (rarely needed) and monitoring of the battery and each individual cell for health. This can be very useful in a multiple battery setup.

I used an Overkill Solar BMS and as you can see from its wiring diagram, the BMS is wired inline on the negative side of the battery (3 parallel wires as used to increase current capacity). There are also balance leads to connect to each cell. This allows the BMS to monitor and make slight adjustments to individual cell voltages, allowing for some passive cell balancing. The BMS will prevent power from flowing if any of the cutoff criteria are met.

4. Bits and Pieces

There are a lot of little bits and pieces that you need to bring it all together. An example list for my build is below. All items were sourced on Amazon unless another source is mentioned.

(Might be nice to include pictures of some of these, especially the tools?)

Consumables

Fastenel: M6 25mm socket set screws, ¼” washers, ¼” lock washers, M6 nuts
Loctite 271 used to secure the set screws into the battery cells
Antioxidant compound such as Gardner Bender OX-800 for final “clean” of bus bars
Secure Cable Ties: 60” Extra Heavy Duty Cable Ties to compress cells (2 x horizontal tight tie) and lift pack (2 x vertical loose tie)
2AWG Arctic Ultraflex Battery Cable, Red and Black
Victron fuse holders and 125A MEGA fuses, in case of BMS cutoff failure
Various sized lugs for battery balance wires, BMS negative connection wires, connections to/from fuse, etc.

The following are specific to my install (see below where I talk about the battery box):

HYCLAT 2-4 Gauge Battery Quick Connect (Anderson style) rated at 175A cut into battery pack case and additional quick connect on RV side to switch (next)
Blue Sea Systems 300 Amp m-Series Battery Switches for each battery pack. Installed on RV side of Anderson connectors.

Tools

In addition to typical tools found in any toolkit, there are some specialist electrical tools you’ll want for a project like this. There are sometimes better versions of each tool but these are all good value for money:

And always remember that any time you’re working on the terminals of batteries, there is a chance of sparks. Wearing safety goggles is always a good idea, even when you’re doing something quick with the terminals. Keep the safety glasses next to your battery or in your electrical toolkit.

5. Box

Lastly, once you have everything assembled (or maybe before), you will want some box or containment method for the battery. An enclosed container will protect the battery against accidental damage including water or shorting from a metal tool. An additional important aspect of prismatic (non-cylindrical) LFP cells is that there is a possibility of cell swelling while charging that can reduce the performance and life of the cell. It is not unusual for DIY cells to swell and for them to be still usable for 10+ years. But it’s also quite easy to prevent this provided that the cells are stored vertically and compressed together in any basic way.

Various compression methods are used in the DIY community including tape, velcro straps, zip ties or more fancy methods involving compression plates and rods. In practice, anything that stops the cells from expanding works, even placing them in a custom fit plywood container which may be the cheapest approach.

I chose a relatively simple approach of placing a custom cut piece of plywood (polyurethaned) on each side of the 8 cell pack and zip tying around the pack very tightly in two places. I also used the same large zip ties vertically to provide an ability to lift the cells as a pack. This allowed me to easily lift the cells into and out of the container for maintenance.

Assembled Pack showing plywood “box” (and doing some inverter testing)

There are many options for portable containers but my goal was something close to the size of my 8-cell pack, light, and ideally water resistant. After looking at various containers including coolers, I came across the Pelican Air cases, 40% lighter than their normal cases and available with no foam inside. The Pelican 1557 Air case retails at $182 but was for sale on BHPhoto at $148. Still pricey but worth it for a case that could support 100lbs of weight. Again, if not building portable battery packs, a nice box could be made for $10 of plywood.

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The bonus on a fully enclosed container is that it also provides some temperature and water/humidity protection in an RV environment which can have daily temperature extremes.

I decided that I didn’t want cables hanging out of the box and wanted to close the lid, so I also chose Anderson-like quick-connects that allowed me to cut a hole and flush-mount a connector on the outside of the case. The connectors are rated to 175A and fit the high quality 2awg wire I used for the packs. At the current levels I was using (less than 100A per battery), the 280Ah cells would never get hot enough to require heat venting so an enclosed case was fine.

Final product – 24v battery pack with USB ports, voltage display, and Andersen connector

Finished Product

After all the hard work, more than I anticipated like every big DIY project, I finally had four 8kWh battery packs. They are very heavy at over 100 lbs each, but just portable enough to move between my fifth wheel and truck camper a few times a year with an occasional use tent camping.

I actually added a voltage read-out and USB 3.0 charging ports on the front of one of the packs to make the camping use case easier. The voltage read-out shows the voltage of the cells directly and the voltage after the BMS. This would make it easy to isolate a fuse or BMS cutoff. The USB 3.0 ports are useful not just for device charging but also for running a small USB light or fan.

I may add the external read-out and USB ports to the other packs if I think rotating external use of all the batteries makes sense over time. So far the USB charging ports have proven popular with other tent campers and with 280Ah of capacity at 24V, we would have to be at Coachella before we’d run out of power to charge everyone’s cell phones!