5A-LiFePO4 cell charger-V2_1
, on Flickr
I decided to try a 5A version of the single cell charger using LM338 linear regulators. I used an old ATX computer power supply that i modified to work as a bench top supply. it can source up to 20A on the 12V line so it is ideal. it was rescued from an old dead desktop computer that would otherwise been thrown away.5A LiFePO4 Charger
, on Flickr
I built the circuit on a chunk of perf board. I thought i could get away with a chunk of aluminum as a heatsink, but at 5A it got way to hot. So i bolted the bar to the CPU Cooler and fan i also removed from the same junk computer.5A LiFePO4 Charger-2
, on Flickr
this photo shows the unit charging a 10Ah LiFePO4 cell. it is almost at end of charge.
the circuit is built using one LM338 as a 5A current limited source. the second LM338 is set up as a 3.65V regulator. i originally used 14AWG wire from the charger to the cell. this proved inadequate. at 5A there was almost 0.2V drop in the cable itself. I upgraded the wire to 8AWG to limit the drop.
theoretically the circuit should supply 5A until the output reaches 3.65V and then the voltage should hold steady at 3.65V with the current tapering off until end of charge.
It does not work this way in practice. the Current limiter limits the current to 5A. the voltage regulator acts as a simple pass element until the voltage rises high enough for it to start regulating. that happens at about 3.45V. At 3.45V the Voltage regulator starts regulating and using power itself. as it starts regulating it uses more and more power internally reducing the amount of power available for charging. this results in a less than ideal but still safe charging curve of 5A CC until the cell hits 3.45V and than the Voltage slowly rises as the current reduces until the cell hits 3.65V @ 100mA charge current.
Full Charge takes about 5Hours. the time could be cut by an hour or so if the charger maintained the 5A to 3.65V and then only tapered the current. this is impossible with this ultra simple circuit.
I still need to do a whole bunch of cells. And every once in a while i need to charge a single cell. so I'm thinking of building a larger version that will be able to do 10A using a LM317 and a couple of MJE2955 transistors as a constant current source. That will work for the CC part. I'm thinking i do not want put the regulators in series, but in parallel. the CC source will work up to 3.65V, switch off and then a 3.65V CV source will switch on. I'm still working on a schematic. so far my attempts look far too complicated. so i'll keep using this circuit until that happens.