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Old 06-26-2012, 11:55 PM   #8 (permalink)
Electric MG Midget
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I think you want to bottom balance because it sounds like your BMS only protects you from overcharge. The guys who run without a BMS bottom balance and use a SOC gauge and pack voltage to know when to stop driving. They seem very successful at doing this, and the packs don't drift according to their data. They stop their charge when the average voltage of the pack gets to 3.6 volts. The batteries can handle a higher voltage, so the lower capacity cells end up at 3.9 volts or so when they shut off the charge. By bottom balancing you'll be better protected on the low voltage side. Using the BMS to shut off the charge when the first cell hits 3.65 volts means that you haven't completely charged your pack, but the amount your missing is probably less than 5%. If you've tested your SOC gauge, and know the voltages of your cells are safe when the gauge says to stop that will work as well. My BMS protects on both ends, and I just plug it in at night, and drive the next day. I've had no issues with it. I haven't checked my voltages for the last 6 months. I hope this helps with your decision. It sounds like you know enough about all of this that you'll be safe either way, but I think safer with a bottom balance.
Frank '77 Electric MG Midget
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