Building a 3 cell LiFePo4 portable battery

Part #19 of the "Roger Writes" series - January 2026

Background

I've been looking for a high capacity 12V power bank for a couple of things:

  1. To power a compressor cool box (averages 600Wh per day).
  2. To power some amateur radio kit (25A max, about 500Wh per day)

Original plan

I've used the 315Ah LiFePo4 cells in a couple of projects, they are notionally 3.5V per cell, so I used 4-cells (for 12V) and 16-cells (for 48V). The smaller capacity cells aren't much cheaper (the 100Ah are 80% of the price of the 315Ah!). So I looked at using DC:DC up converters with 2x 315Ah cells (to keep the price and weight down).

Modified plan

The 315Ah cells are usually around £50 per cell. But someone on FB marketplace was selling 3 for £60 (total). Why so cheap? Well, it's best if you use banks of matched cells, and he had dropped / damaged the 4th one, and the importer isn't bringing in those cells any more. So you can't make up a set of 4!

At that price I couldn't resist!!

I was originally going to use 2-cells (of the 3) as per my original plan, but found that the high power DC:DC boost converters struggled at 5.3V (the lowest usable voltage from 2 cells). So decided to go with 3-cells (so 7.8V min, 10.95V max).

Spec

3x 315Ah LiPoFe4 cells, giving around 3200Wh of storage. With 4x 40A DC:DC boost convertors, switched via relays, giving 22A...31A out at 14V depending on battery voltage, and over 12A at 24V (for the PD/USB sockets).

These were wired to:

  1. 2x 65W PD (running from 24V so they can output 3.25A at 20V), and 2x 2.4A USB.
  2. 2x 14V cigarette lights sockets, upto 15A each
  3. High power output going to a XT60 and a speakon connector
  4. Another High power output going to a XT60 and a speakon connector
I have used the Speakon connectors for 12V DC for year, as they are touchproof, and well rated. So have a reasonable number of devices with them wired to them.

Charging

Normally a BMS (Battery management system) would be used. This is wired to the separate cells, and limits the voltage differences, so that the cells charge/discharge evenly. But to reduce cost, I'm just using a radio controlled balanced charger. This evens up the cells during charging (which is the most important side of things). It will only charge at 6A (so 52 hours from empty), but as this will be a weekend thing, that's not a problem.

Discharge

These cells are rated at C (315A) continuous and 2C (630A) peak. With all outlets at maximum, it's only about 40% of C, and in normal use it will be much less.

Testing

I've yet to fully test this yet, but it should run the compressor cool box and the radios for a weekend...

Images

The three cells in the basic case. With insulators between cells, and the case providing side pressure

The high and low power fuses (2x100A for the 4x DC:DC boost converters, and a 10A for the charging and relay switching)

The batteries and DC:DC boost converters in position (with insulation sheets between the DC:DC convertors)

The front panel layout

The front panel and relay mess before it was all bolted down.


The Roger Writes series

I research / dabble with lots of things, and figured that if I write my notes here, I can quickly reference them, also, sometimes, they are useful to others!
Here is what I have so far:




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This page was lasted updated on Tuesday, 13-Jan-2026 18:08:07 GMT

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