Yes another cell has bit the dust. The story goes like this: drove 10 miles started getting an alarm from the MINIBMS. This means a cell, maybe more, was at 2.7 volts or less. If a LiFePO4 cell goes to less than 2.5 volts, it's probably a goner. One thing is for certain, you'll definitely lose capacity and if you keep stressing the battery it could reverse itself or even blow up! But you can't just whip out your voltmeter and say "Aha! I have found the cell!" It's hard to test a cell unless it's under load. Also, this low cell will also cause serious "sag" (lack of gusto) in the pack. Your pack is only as strong as it's weakest link.
Charged up the battery bank, then did a top balancing. In the end, three batteries did not shunt - no red light on the circuit boards. I whipped out my individual charger and charged each one of them up. This can take some time for each battery - 4 -6 hours because the charger I have is only 2 amps.
Out for another drive, 20 miles (mostly 35-45 mph), and no alarms. Repeat charging and top balance process. All batteries were shunting when I looked.
This time I went for the stress test - drove the first 10 miles of my commute which has a few steep climbs at 55 mph. Alarm happened at 6 miles. Bummer!! Came home, recharged the pack, top balanced and one cell did NOT shunt. This was one of the batteries I had individually charged earlier.
Rather than try to revive it, even if that's possible; I wired around it. Once I did, I noticed that it had leaked electrolite previously, which I couldn't see due to the BMS board covering the vent hole. I then drove the first 10 miles of commute again and NO ALARMS!! (Yeah!...the crowd goes crazy.) I was pulling approx 150 - 200 amps going uphill. This is good because the next 15 miles of my commute is pretty much flat or downhill.
Check out Jack Rickards website www.evtv.me he is very knowledgeable about LiFePo4 batteries and believes that bottom balancing the cells and not over charging or under charging is the key to long battery life... he does this without a BMS and is of the school of thought that because BMS give off heat they are the cause of most LiFePo4 battery fires.
ReplyDeletealso check out my EV1E at www.electricar.us/blog
Hi Mark, Your website inspired me to do my own. The MINIBMS will not do a bottom balance. You should also look at the Thundersky yahoo forum, Jack takes a real beating regarding his stance on BMS and bottom balancing. When I don't get in the way of the MiniBMS, it works very well.
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