Thursday, March 28, 2013

48V LiFePO4 Battery Repair, AKA: Something goes right for once this month

Amidst countless failures this month, most of which to do with my VW Golf, and will be saved for another post, I was successful at recommissioning my 48V bike battery.  And I'm pretty psyched about that, since a new one would have been well over $500 shipped.  Let's have a look at the story.

I disassembled my battery management system and soldered a wire tap onto the B1 terminal.  The theory was that if I could work on getting just the one cell up to voltage close to where the others sat, the battery pack would start operating "normally" again (instead of cutting out the moment I tried to use it, even right off charger).  I brought to "work" where there is a nice voltage AND current regulated, bench-top power supply.  The solution plan was simple: measure the current open circuit voltage of the rest of the cells, average it, set the power supply V to that, hook it between B- and B1 (the bad cell), and wait.

I soldered a wire to the second-from-right-most terminal, which is B1.
The cell started charging at about 1.5A, immediately, and slowly leveled off .  The pack voltage is seen on the left multimeter.
The cell started at 2.8V, with the power supply set to 3.6V (the nominal open circuit voltage on the other cells).  The current draw started around 1.5 amps, then slowly leveled off to 1 amp, and after about 5 hours it was down to .5 amps.  The open circuit voltage ended up around 3.2V before I ran out of time.

I then left it on charge via the bulk charger w/ BMS over night, with the hopes that the BMS would finally do the rest of the balancing.  I honestly have my doubts that it does any real balancing, however, and I really should consider a better BMS.

Anyway, long story short, I took the bike out and it ran beautifully!  The voltage barely sagged - even when riding uphill.  Under full load, it was down to around 45 volts.  My sense is that there is still a "runt cell", and I am going to invest in a nice power supply so I can maintain battery packs without having to use the one at work.  

Suffice to say that this mission was a success, and chalk up one win in a long list of losses, for the month of March.  Hopefully next month will be a little kinder.

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