DIY LFP battery pack

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Over the years I have been collecting 18650 LiIon batteries out of old laptop batteries. They usually have between 4 and 6 cells and it’s tough work trying to collect them. Also, you end with a whole mix of cells of different types.

More recently I have been able to get hold of faulty e-bike batteries for free. These typically contain 50 or so 18650 cells. Most of the time there is nothing wrong with the batteries themselves and it’s the BMS (battery management system) or some other circuitry that has gone faulty.

Some of these packs contain Lithium Ion Phosphate (LFP) pouch cells. Due to their voltage characteristics they are more suited to 12v applications than LiIon chemistry batteries.

As a radio ham all my gear operates off 12v so this excited me and I wanted to tackle a 12v DIY LFP battery pack.

Using four lithium iron phosphate cells in series, gives us roughly ~12.8-14.2 volts pack when full. This is the closest thing we’re going to find to a traditional lead-acid or AGM battery.

A 4S lithium ion pack has too high voltage (~16.8v) when full. In contrast there are some radios which require more voltage than the low side of a 3s lithium-ion pack can provide at the end of its voltage curve. If we still want to use a 4S lithium ion pack, we need to integrate a DC DC regulator, to manage voltage output. Or, as I alluded to in the second paragraph, we can also use lithium iron phosphate cells, which have 14.2-14.4v fully charged. This is perfectly fine for most radios, but read the voltage requirements for your radio.

This is how I get the packs.

According the label on the battery, the one I chose to modify started out as a 12Ah 38.4v e bike battery. As you can see here it is 24 cells in a 2p12s configuration.

My plan was to modify it into a 6p4s configuration giving me a 12.8v battery at 36Ah. As it turns out the pack did not need to be completely split up. All that I needed to do was to reconfigure the wiring from what you see in the photo above to this. I then connected my hobby charger to each of the individual 6s packs to do a capacity test. As you can see from the sharpie markings on each of the packs capacities were very similar and only a few percent lower than the factory rated 36Ah.

I then installed a 4S 30A BMS and balance leads and taped the whole lot up again. This BMS does have a balance charging capability but just for belts a braces I decided to install my own directly connected balance leads as well. This would allow me to use my hobby charger to do the balancing if and when I decided that I wanted to do that.

With the physical dimensions of the pack not having changed it was then easy to reinstall the pack back into the original housing after making a hole hear and there for the balance lead and power cable. Charging can easily be done by applying the correct voltage (14.6v) to the power leads. The BMS will take care of protecting the 4 cells in the battery against over charging and will ensure that each of the cells are balanced.

It was a fun project and my total cost was the $3.60 that I paid for the BMS. I never plan to run the battery anywhere near the BMS rated capacity of 40A but it’s good to know that I have this capacity should I need it.

Keep in mind that as long as you stick with 4 of these LFP’s in series the voltage will be correct for 12v applications. It’s up to you to decide how many of these in parallel you want to get the desired capacity. For a QRP rig you might just be fine with just a 4s pack giving you 12v at 6Ah.

Honey Badger Knives – Ball bearing flippers

A word of caution. While LFP chemistry is safer than LiIon chemistry these cells have high current density and if shorted can be a little dangerous. Be careful when working on the battery. Remove metal objects from the work bench, use insulated tools and remove your watch if it has a metal strap.

Of course if you prefer you could always buy an LFP battery from folks like Bioenno Power who have been very supportive of the amateur radio community.

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