LiFePO4 - Charging Adventures

I recently took ownership of some larger 20Ah LiFePO4 cells and the charging adventures that followed are worth sharing.

The first LiFePO4 cells I purchased that were actually delivered were 26700 sized with a claimed capacity of 4000mAh. Charging them for testing was easy enough with a variation of my CC/CV circuit. They never delivered 4000mAh.

Ditto the 14Ah cells I brought for my ride on mower. More like 12.5Ah in capacity they took more effort on my part but I have an old HP power supply on my bench that I can manually set up for CC/CV charging. 

When I received some 20Ah cylindrical cells things started to go awry. I didn't like leaving my HP 20A power supply running unattended with a charging current greater than 2.5A because of the gauge wiring I was using to connect to the battery. However, I couldn't readily charge these cells at less than this. At lower currents the energy just seemed to disappear. 

 

So I built heaver gauge charging cables and charged them while I was present. Surprisingly, the rated 20Ah capacity was achieved when discharged at 1.5A.

During all of this I looked for an alternative. I first tried some supposed LM2596 buck convertors. Advertised as 3A and 40V input I thought these would work really well with some 200W 30V solar panels I have. The first time I connected one to a 30V supply, with no load, for testing the module burnt out forcing a retreat outside till the fumes dissipated.

From a 12V supply they could deliver 2.5A to 3A but the heat they dissipated meant they could only be used intermittently. And some care is needed adjusting the trim-pot which is difficult to set within 20mV of 3.65V.

I had some server power supplies. One model, a HP unit, appeared suitable but the 3.3V rail could only be coaxed to 3.45V. I didn't persist with modifying the unit because I tried the 5V rail expecting it to shut down. Instead, it happily dumped +20A into the cell. Which gave me a different idea.

Take the 5V rail and then regulate it to 3.65V with some current control. Assume a 10A charging rate. 10A x (5-3.65)V means dissipating 13.5W of heat. That sounds achievable. Except the dropout voltage for a LM317 alone is 1.7V. So a purpose built regulator or a lot of time searching/testing would be needed. The design time for this approach seems high. It would be easy to do this from a 12V rail but the additional 70W of heat generated needs a big heat sink.

Then I found some LM1084 regulators. Similar to the LM317 but the drop out voltage is much lower (around 1V) and each one is good for 5A. I plucked a CPU heatsink from the junk box, mounted the regulator and tested the concept with heavy gauge wiring.

Partial success. I can now charge at a little over 4 amps continuously with no concerns over heat dissipation. My bench power supply has been released from battery charging duty.

At present I'm getting on with other projects. But my planned next steps are:

• adjust the 5V rail to perhaps 5.3V to get more current.
• Parallel a second or third LM1084 to deliver much higher currents
• Investigate further a high current switch mode supply. A comprehensive post setting out development of such a supply which almost met my requirements was found here. Well worth a read!