50Ah LiFePO4 battery

Late in the autumn of 2021 my buddy Dann VE6TD asked me if I wanted to do a LiFePO4 battery box build project along with him. I thought to myself “Sure! I’ll use that empty Goal Zero Yeti 400 enclosure I’ve had for some time” and make it look like a commercial project when completed, at least from the outside. Naturally this is part of any Emcomm kit. It would be less expensive than buying a BioEnno or Dakota Lithium and would have more features, but at the expense of 80+ hours of my time to figure it out and build it. As like all my projects this is a guideline to help you build your own. The build notes as I proceeded were documented on Twitter and spoken about on Ham Radio Workbench Podcast episode 154.

Build list of parts:

  • 25Ah 3.2V LiFePO4 prismatic cells “Topband”
  • 40A 4s1p BMS with Bluetooth
  • 10A Buck/Boost converter
  • DC Solid state relay
  • 10A and 30A self-resetting 12V breakers
  • 4 wire style battery monitor and shunt
  • USB convenience outlet
  • PowerPole connectors, fans, power switch

The enclosure itself is incredibly well-made with a steel subframe and molded high-impact plastic wraparound enclosure. That wraparound would later serve to hide components that would not fit elsewhere, but frustrate me along the way to perform the fitment. I guess nothing worth doing well is easy . . .

The most difficult decision to make was the choice of batteries. The design point was to fit in as much capacity as possible in the available space. It took many hours of research on sites like Aliexpress to find a cell that would fit into the given space. The sweet spot was 25Ah and I landed up with two sets of 4, which means the pack is a 2p4s style meaning 2 cells in parallel and then 4 sets of cells in a series string to get to the desired voltage. Given the chemistry of this battery choice the pack’s voltage is 14.6V (3.65V x 4) which falls within the acceptable range of most radios. The other value that is important are the charge and discharge rates, stated in the unit ‘C’ which loosely means the cell has a 25Ah capacity (1C). This set of cells can discharge at 3C (75A, more than enough) and charge at 1C (pretty quick).

The next choice to be made was how to charge the pack. The enclosure came with two PowerPole connectors and two fans and a large heat sink to work with. I wanted the ability to charge the pack without purchase of an external adapter, and in a perfect world to be able to charge it from any 12V source. Given the battery’s voltage is higher than the nominal 12V available from an automotive ‘cigarette’ or convenience outlet, the idea to use a buck/boost module was hatched.

Pre-testing of the buck/boost module before buttoning up your project is heavily advised. The unit I purchased said it would do 10A with external cooling, but the sad reality is only 3A and the output is quite noisy as seen on my ‘scope. This is one area of the project where lots of modules are available however verify with testing before trusting it in real life. I skipped the hard testing of the buck/boost and only saw it’s true ability once I’d discharged the pack for load testing. Sigh.

Once I verified the functionality of all parts I began thinking about mounting them in the chassis. A buddy with a 3D printer and good modeling skills built a few parts for me to help keep the cells all together with spacing between them to allow for any expansion that may occur during charge or discharge.

Not all cells are created equally even from the same manufacturing run, and this is important to note when you are running cells in parallel then series as I am. I went through a process to top up each cell individually and observe their natural fall-off over a range of hours/days and then double-checked my work before selecting cells in pairs. I tracked this in a simple spreadsheet as seen below. Then I assembled the pack with spacers and began interconnecting the cells.

The most nerve-wracking part of working with batteries is, for me, interconnecting the cells themselves. This is where mistakes are easy to make and mixing up your pluses and minuses can result in an unexpected spontaneous deconstruction which is to be avoided at all costs. To mitigate this risk I used a red and black Sharpie marker on the top of the bolt to give me a very clear visual indication. Given the custom-made spacers I had built, I built up bus bars for inter-connectivity as commercial choices weren’t easily located. I wrapped a piece of heat shrink around the mid-points on the bars as you can see below.

LiFePO4 back all connected up

Along the way I created four or five drawings showing the schematic of connections and it was only one week after the project was complete that I finished the drawing with an as-built version as seen below. I would amend each drawing as I discovered what worked and what didn’t, and am pleased to say I did not trip out either of the circuit breakers during the entire process – #winning!

Once all components were tested at least a few times on the bench with about 3 miles of jumper cables, and the near-final version of the schematic was done, I switched focus to the front panel. Careful measurement of selected components was needed so that the front and rear diameter/size was known before sending it off for 3D modeling and printing. I had a blank to work with that gave me an easy way to visualize.

After the final panel came back from printing it needed some re-work as I had provided incorrect values. I corrected the holes with a Dremel tool.

Blue would not do, it threw off the appearance of the enclosure, so I researched how to finish this panel printed out of PLA and learned a little sanding and some acrylic spray paint would work. When I started out I wanted a smooth satin black finish that would match the appearance of the enclosure however PLA doesn’t always print up like this given the resolution of the printer at hand. Working with what little sandpaper I had on hand and with the deadline approaching, I switched my desire to a ‘woodgrain’ finish πŸ™‚ In the end it was 6 coats of satin black spray paint with sanding in between most coats and one final coat of lacquer. I think it looks awesome.

The operation is simple. Any 12V source plugs in on the left and runs the fans to cool the buck/boost and lights up the round voltmeter (which I later swapped out for a combined volts/amps) tells me what voltage is coming in. I can also charge via the right port at 14.6V from a dedicated charging source. In either case the BMS determines when to allow charging and not – part of it’s job.

To run a radio or other 12V load, turn on the switch on the left and it lights up to tell you that voltage can flow (see schematic). The battery monitor on the right gives current and energy consumption via the 4 wire monitoring and shunt. A dual USB is provided for convenience of charging a cellphone or other 5V appliances.

I hope this inspires you or gives you ideas for your own pack. Be safe, mind your reds and blacks and have fun!