Several years ago, I received a gift of 11Ah prismatic lithium LiFePO4 cells from Gary K., who was clearing out his electric car workshop. These cells have been sitting in my personal version of Warehouse 13, patiently waiting for the right project. Recently, I decided to put them to use. I tested one of the cells to ensure it was still functional, and it worked perfectly. After running multiple charge and discharge cycles, I confirmed that the 11Ah capacity was intact. Here’s how I utilized these cells for my latest project.
Supplies
In the course of researching the previous battery bank project, preparing for this one, and getting interested in doing some solar power projects, I found more than a few YouTube videos that helped me. A search of “off grid solar” yields numerous videos about building batteries and solar-powered power banks, usually for systems designed to run emergency power backup, off-grid cabins and RVs. However, the basic principles apply to building smaller power banks like this one. I list a couple videos, along with sources for the parts I used. Many of the parts are the same as I used on the earlier power bank project, which you can find here.
A very detailed battery build video for a 12-volt 280Ah battery. The basic methods apply to any size battery.
Some more information on top balancing.
LiFePO4 cells in various sizes are available from AliExpress. There are some US companies who buy in bulk from China and stock them for individuals and solar installers who want to build their own battery packs. One of these is 18650BatteryStore. The smallest prismatic cells they carry are 105Ah.
Low power BMS – AliExpress Be sure to select the LiFePO4 option, as opposed to plain lithium when ordering these boards. These low-power BMS boards are not adjustable, so you have to be sure you select one with the correct parameters. You can also find similar boards on Amazon and ebay. Almost all ship directly from China, so shipping time is usually 2 to 3 weeks from any vendor.
16 AWG hookup wire, terminals, automotive fuses – local car parts store
LiFePO4 Battery Charger – 14.6 volt, 4 Amp – Amazon
5.5 x 2.5mm charging socket – Amazon This matches one of the charging cables that come with the LiFePO4 charger.
For battery testing I use the Atorch DL24 tester from AliExpress YouTube is the best source for learning how to use it. One of many videos available is here.
SPDT switch – AliExpress This is the main power switch. It has three positions: ON-OFF-ON. The top ON position connects the battery to the plugs. The bottom ON position connects the battery to the charging socket.
Anderson Power Pole style connectors – Amazon Most of these on Amazon are clones. If you want the real deal, order from here.
Automotive USB Plug – Amazon These come with integrated volt meter display that I use as a power-on indicator, and it gives me some idea of remaining battery capacity.
Lights – the USB plug is used to keep phones and tablets charged. For lights, I build my own from recycled flashlights, and automotive and RV lights like this one from Amazon
Files for the 3D-printed parts are available upon request.
Step 1: Assembly
Apparently the LiFePO4 prismatic (identified as rectangular, instead cylindrical, and usually wrapped in blue plastic) cells will expand and contract as they charge and discharge. The general recommendation seems to be to package them in a way to counteract this a bit. In fact, the instructions that came with some new cells I bought recently had specifics regarding how much pressure should be applied. Usually, wrapping the cells in fiber-reinforced strapping tape is sufficient. A thin separator is used between each cell to ensure the metal cases don’t touch as a result of wearing of the plastic covers. I use plastic sheets from old report covers, or notepad backing cardboard cut to size. Scrap pieces of .25 inch plywood or hardboard go on the ends. The whole thing is held firmly with quick clamps and then wrapped with strapping tape. Be sure to pay attention to the position and polarity of the terminals. Usually the cell polarity should alternate so that the final wiring in series is made as easy as possible.
Step 2: Initial Charging and Testing
According to the video tutorials, building a LiFePO4 battery of any size usually consists of basic steps: 1) acquire the cells, 2) package them, 3) charge the cells, 4) balance the cells (if needed), and 5) add a battery management system to avoid damaging the cells by overcharging or over-discharging them.
The cells I got from Gary have date of manufacture, and cell capacity testing documentation from 2007. The individual cell voltages today vary by 0.1V, and therefore I’ll be running them through a full charge, discharge, charge and balance cycle before I connect them to the BMS for final use. This may be more than needed but I want to be sure I get the maximum capacity possible. As an aside, I bought eight new 300Ah cells for another project and they all measured 3.295V with my Fluke volt meter. I won’t be needing to balance those.
Initial Charge and Discharge
After packaging the cells, I connect them in series and use my 4-amp charger to charge them. Then I use the Atorch battery capacity tester to discharge at what I consider a typical load for my intended uses, about 2 amps. This should take 5 hours.
Charge and Balance
For the balancing phase, I first charge them in series with the 14.6 volt charger. This will stop whenever the total hits 14.6 volts, however, not all of the cells will be at the same voltage, or state of charge (SOC). To achieve that, the cells get reconnected in parallel, and then connected to a constant-voltage power supply set to 3.65 volts. Any cell that is below 3.65 volts will draw current until it’s at that voltage. When the power supply is set to constant voltage (CV) it shows the current it is drawing. For me this started around 4 amps and then declined to around 100 milliamps, which is a good-enough stopping point. (The picture shows 3.6V – I later got another charger that allows settings to 1/100th of a volt.)
Step 3: Battery Management (BMS) Installation
2 More Images
BMS Installation
Once the cells are balanced, it’s time to install the battery management system (BMS). Follow the instructions for the model you bought. My BMS boards arrived in the mail from China, without any instructions. I went back to the product page on AliExpress and the product description includes a hookup diagram. You can see it in the picture above.
The next picture shows the actual board with connections installed. The main power leads are connected to the P+ and P- pads on the BMS board. The other connectors have appropriately-sized lugs and connect the cells in series. Individual wires go to each cell and are used to keep the cells in balance.
Once all the wires are connected, I use some cardboard with strategic cutouts to insulate the BMS board and then strap it to the top of the battery. Once everything is connected, I run another capacity test until the BMS shuts down. This gives me the total usable capacity.
Step 4: Power Bank Assembly
Once the battery is complete, and 3d parts have been printed, it’s time to put it all together. It includes:
- On/Off switch
- 7.5Amp Fuse
- USB ports with voltage display
- 12V plugs for lights, and other devices
- A case to totally enclose the battery. Some 3mm screws hold the top and bottom together.
- Mounting point for a lamp so it can be used as a lantern or flashlight
- Charging plug
- Handle
Step 5: Final Words
Here is a comparison of the two projects. The new power bank is slightly bigger, allowing for a larger handle, which is more comfortable for my large hands.
I’ve now built three versions of the power bank, here is how they compare:
UPS 7Ah AGM Weight: 5lbs 15oz Capacity: 4Ah $42
Eco-worthy LiFePO4 Weight: 2lbs 9oz Capacity: 10Ah $40
Custom LiFePO4 Weight: 4lbs 6oz Capacity: 11Ah $Free!
