OK, you don’t know a thing about lithium batteries. But when I started this project, I didn’t know anything about them, either. All I knew is that everything I read made them sound complicated. They’re not complicated. In fact, in a couple minutes you’ll be thinking “cheap and easy.”
Buy four little batteries and a gizmo called a BMS. Save a lot of money. Wire the batteries together, and connect a BMS wire to each battery. Done.
Cost Comparison With Four Deep-Cycle 12V Lead-Acid Batteries
- Lithium $450. This includes the battery and recommended BMS management gizmo. Will last for thousands of charge/discharge cycles.
- Consumer-quality big lead-acid deep cycle batteries from Sam’s Club. $600 once you pay for the core charges and tax. Will probably only last for dozens of deep charge/discharge cycles.
- Industrial quality Trojan lead-acid deep cycle batteries. $1,700 once you pay for the core charges and tax. Could get you a couple hundred deep charge/discharge cycles.
How Much Battery Power do you Need?
The first step is to make a guess regarding how much power you need. It’s not critical to be correct. If you guess intolerably low, buy more batteries next month. If you guess high and end up with more battery capacity then you need, brag to everyone that your trailer’s batteries could support the paddock.
That said, be reasonable. Supporting a space heater or an AC unit for half a day is a lot to ask of some batteries. However, if your beer was already cold when you put it into your mini-fridge, the batteries will get your mini-fridge through the weekend as long as half the paddock isn’t using your power to run their air compressors.
After some trial and error, mostly the latter, I chose the lithium equivalent of four big 12V lead-acid batteries. That’s enough power for LED trailer lighting, a small air compressor, coffee, dorm fridge, and an electric blanky, as necessary.
For two track weekends, my fully charged batteries ran all the above and each weekend their state of charge remained above 50 percent, as reported by the really cool phone app.
Let’s put a number on this. I’m talking the better part of 200 amp hours at 12V. With conventional lead acid batteries you’d need around 400 amp hours to get that much power. That’s because lead-acid batteries will have a reduced service life if discharged below 50 percent. The charm of lithium batteries is that not only will they happily discharge all the way down to 10 percent, but they will still be doing it for thousands of more cycles after your 50 percent usable lead-acid batteries have been returned for their meager core value.
Here’s another number. My 25-pound lithium battery pack took the place of more than 300 pounds of lead-acid batteries.
How to buy the Batteries
Unlike lumber, the price of lithium batteries has gone down a lot in the last year. We’re talking like 50 percent. Of course, that means going directly to Chinese sites that look a lot like Ebay “Buy it now,” but it all works pretty well.
My suggestion is to go to AliExpress.com, create an account and look around. Note that when it comes time to buy, you’ll need to give them credit card info. If that causes concern, borrow your wife’s, husband’s, or mother in law’s card. Seriously though, I know it might be uncomfortable to go to some Chinese site and give them your credit card info, but the Chinese are the ones making lithium batteries. The site squared me away when things went a little sideways, and your credit card company will watch your back if things go bad. Besides, as near as I can tell, the Chinese might soon be ruling the world, so it’s past time to introduce yourself.
If the very unlikely occurs and you are out the money you fronted for the batteries, then tell your mother-in-law that she shouldn’t have left her credit card lying around.
AliExpress holds the money until your items are delivered. When my first set of batteries didn’t show up, I notified AliExpress. They looked into it and then refunded my money. It was very much like Ebay.
Other Places to Find LiFePO4 Batteries
Other Chinese sites to consider are Alibaba.com and BangGood.com. I know, I too grin at the names these guys choose for their multibillion dollar eCommerce sites. Alibaba is primarily oriented on larger business-to-business purchases, so its consumer protections might not be as solid as I found AliExpress to be. BangGood has a lot of gadgets, but fewer things in the price range discussed here.
If you want an American site, go to eBay and pay twice as much. Or you can google LiFePO4, find a more conventional U.S.-based reseller, and pay four times as much.
There’s different types of lithium batteries, but LiFePO4 is what you want. If anyone explains to you how you are supposed to pronounce LiFePO4 by actually saying the name of that molecule, suggest to chemical engineer that they maintain their self-esteem on someone else’s time.
To get 12V from a LiFePO4 battery pack, you need four individual batteries. Think of this as just packaging. A 12V lead-acid battery is actually six individual cells, only conveniently packaged so you don’t have to wire them together. LiFePO4 batteries are also available in 12V packages, but they are expensive. The battery design, in the AliExpress picture, will take 8 mm fasteners. Other designs exist, but this one is easier to wire than some others.
The Battery Management System
Here is the only part that will be a little new. Batteries are, of course, all a little different because humans are involved. That means that their charge and discharge specs are a little different. Put four AA batteries end to end and no one cares that the bits are a little bit different, but lithium batteries are more sensitive. There’s a gizmo called a battery management system that works to keep the lithium cells all at the same voltage. It also will protect them from over-charging/discharging. All you have to do is connect a few wires.
My recommendation is to get a BMS from Overkill Solar. There’s a link at the end of the article. Overkill Solar’s BMS are twice as expensive as alternatives that one might experiment with, but its instructions are really good, there’s a whole forum of support discussions, and if you fry the device, it will refund your money. That last idea, since I had no clue what I was doing, was particularly appealing.
I can state with some confidence that the 12V Overkill Solar BMS that is rated at 120 amp hour will really give you that because my coffee maker was pulling almost all of that last weekend. Also, the BMS will communicate with an app on your phone and show you all sorts of interesting battery information. For geeks like me, that is very cool.
Thick, flexible wire. For your inverter to run your coffee pot, you’ll need to provide it with 50-100 amps of 12V. That means thick wires. You will save yourself some headaches if you get 6 gauge welding wire because its thin strands allow it to bend easily.
Assembling the Battery Pack
Four carefully packed individual batteries will show up on your porch. The round metal electrodes on top will have threaded holes. Have some 8 mm bolts handy. Check each battery with your multimeter and confirm which electrodes are negative and positive. It’s a 200 amp-hour battery. Surprises are not allowed.
The batteries will probably come with three or four busbars that can be used to connect the individual batteries. Several busbars are visible in the AliExpress advertisement. Otherwise use some heavy wire and 5/16” ring terminals. That sort of thing is available at your local Home Depot, but some better choices are linked to at the end of the article.
Checking the Batteries’ State of Charge
By balancing batteries that might be at different states of charge, you help the battery pack start out with equally charged cells. It’s like fixing big credit card debts before marriage, vs. finding out about it after the honeymoon.
With a multi-meter check the voltage of the batteries. If there is a voltage difference larger than 0.1 volts, connect the positive poles and negative poles together overnight so they can equalize. See the drawing that follows. If the voltage difference is a lot more, say 0.15V, consider top-balancing. If less, and your battery pack will be spending a couple weeks unbothered before your next event, you can probably skip the top balancing. Your BMS will spend that time fixing any imbalance in your LiFePO4 batteries.
Graphic above shows two batteries connected in parallel so they can equalize. However, if you are in a hurry to pull big power from your battery bank, you’ll be able to charge them up higher and discharge them down lower if you do an initial top balance. For that you’ll need a variable power supply. You can find one for about $50 on Amazon. There is a link at the end of the article.
Assembling the Battery Pack
Just like stacking AA batteries, connect the four LiFePO4 batteries in series, the positive of one connecting to the negative of the other. Setting them up in series likes this makes their voltage add up. You end up with a bit more than 12V, but that’s true for a lead-acid battery also. Electrically it looks like the illustration below.
Go slow and wear safety glasses when touching the terminals. If you screw up and connect bits that you shouldn’t have, 200 amp hours of current trying to discharge all the sudden could be a memorable learning experience. When this sort of thing happens, metal bits weld together and get quite hot. You could end with up a tragic story that explains your pirate-like eyepatch.
LiFePO4 aren’t more dangerous to connect together then a group of lead-acid batteries. But go slow, think about each step, and if you bend a short thick wire in a tight arc, consider what the wire is going to touch when the unconnected half pops out of your hand and tries to straighten.
The Battery Management System
Don’t think of the BMS as daunting or a hassle, think of it as your friend. When I had big lead-acid batteries in my trailer, it never took long before I could no longer get all the power out of them that their marketing department had asserted. The problem with having multiple lead-acid batteries connected together is that you can’t easily tell when one of them is ailing. The problem child battery will be the weak link that dictates performance. Maybe the problem battery won’t charge to a decent voltage, or maybe it will discharge way too quickly, but the result will be that, to your surprise and irritation, your beer gets warm and your electric blanky gets cold.
In contrast, your BMS not only acts to keep your batteries healthy by keeping them safe from over charging/discharging, but via your phone app it will happily tell you the state of health of the individual batteries.
Connecting the BMS
The BMS has two or three big blue wires that go to the battery bank’s negative pole. They are all the same. The only reason there’s several big blue wires, instead of just one, is that the BMS is rated for 120 amps and a single wire of that diameter wasn’t perceived by Overkill Solar to be enough for that amperage. Otherwise it would have had a wire the diameter of your thumb to the BMS.
The BMS also has little wires, one for each battery. The little wires allow the BMS to sense the voltage of each of the four batteries and distribute power to keep them balanced. The schematic above shows how the little black wire goes to the negative of your battery bank and the little red wire goes to the positive. The little white wires, BC1-3, go to the busbars that connect the batteries. When you have the BMS in your hands, which white wire is which is obvious because it’s a flat connector with a red wire, three white wires, and a black wire, all in a row.
If it takes longer than five minutes to put five little ring terminals on the little wires and fasten them to the batteries, put the beer down and use both hands. Some ring terminals are linked to at the end of the article.
Other BMS Bits
The BMS comes with a temperature sensor that needs to be plugged into its little connector, even if you live on the beach in San Diego where it’s 78 degrees year-round. You also get a pair of wires that you can use for an external “discharge” switch. When the trailer is in storage, for example, I click the little switch to prevent the battery pack from sending power to the trailer.
My LiFePO4 battery Bank
There are eight batteries in my 12V bank, so it looks a little more complicated than the depictions in this article. This additional complexity was a result of me having to work my way from bad ideas to ideas that were less bad — my usual engineering process.
I used a piece of wooden shelf to create a base, and rope handles to create a means to safely move the battery bank around. The battery bank lives under the trailer’s work bench, so I used a plastic bin cover and some small bungi cords to create an easily removable cover. The discharge switch at right uses a little generic switch and a plastic hobby box. The BMS is fastened to the rear with duct tape.
In the trailer, my battery pack is stashed in a corner under the workbench that is at the front of the V-nose trailer. Unless I pointed it out to you because I felt it necessary to tell you about it for the third month in a row, you’d never notice it. I fastened a piece of Lexan on top of the battery pack so that anything falling on top of it wouldn’t get all sparky and create trouble. The Lexan is held on top by bungi cords, a charm of which is that they’re not conductive.
Being Obsessive is Interesting, but not Necessary
Like any other obsession, there’s a whole culture of folks out there working to make lithium batteries sound as complicated as possible. There’s people that insist you have to have a variable power supply to first charge all the batteries in parallel—top balancing, and then you should use a calibrated heavy duty discharge device to test them all individually. The obsessive types will also tell you to buy a dedicated LiFePO4 charger, and finally, using the BMS, you have to monitor the batteries frequently because they are delicate flowers.
Should you hear of these kinds of complexities, look around for someone that didn’t do any of that, and it all worked out fine. Obsessiveness is for the obsessed.
In contrast, I wouldn’t do any of that. Sure, I did all that the first time, but you don’t have to be a nut like me. If I had it all to do over again, I would suggest that most reasonably clever car battery chargers should work just fine, only that you should avoid long-duration, call it days, of trickle charging. Also, for typical trailer usage where the batteries only get used once or twice per month, they shouldn’t need any attention. The BMS will work its magic in storage to keep your batteries happy.
Note that whereas lead acid batteries like to be topped up in storage, LiFePO4 batteries do not. So there is no need to make a special effort to top up your batteries before storing your trailer for a couple weeks.