I'm cycling my packs before I put them on my new bike. Most every 6S 5AH Turnigy 20C is giving me 4.7AH from 4.15V-3.5V. One cell ended up at 4.1AH and has been replaced, but the rest are fine. Probably 50+ cycles on it, lots of 1.5C bulk charging, minimal balancing, quite substantial monitoring. The cells average 3-4 mOhm internal resistance, which is roughly 1mOhm higher than fresh cells.
What's really interesting, is that my bother built his pack the same time I did, probably has <10 cycles on it, but has less capacity. He never monitors it, never balanced it, and stores it at whatever SOC it was at when he was done with it. I just started cycling his packs, and so far 4 of them were evenly discharged down to 4.5Ah. Very few cycles, no bad cells, all with 2-3mOhm internal resistance, but lower capacity than my 'used' cells. One of his packs even had a REALLY weak cell that showed 17mOhm resistance! I'm guessing the remaining packs will show more issues.
The difference? I bulk charge my pack before use, putting only enough energy in so that I come back home around 3.85V/cell. Stored mostly around 3.8-3.9V/cell and generally shallow discharges. My brother had his bike sit in the shed over winter, at 100% SOC, when ambients were below freezing.
http://www.youtube.com/watch?v=OoiSIlL6bOs&feature=youtu.be
I thought it was interesting that the one 'bad' cell in this pack was binned as lower capacity from the factory. Are most common failures due to these lower capacity cells?
What's really interesting, is that my bother built his pack the same time I did, probably has <10 cycles on it, but has less capacity. He never monitors it, never balanced it, and stores it at whatever SOC it was at when he was done with it. I just started cycling his packs, and so far 4 of them were evenly discharged down to 4.5Ah. Very few cycles, no bad cells, all with 2-3mOhm internal resistance, but lower capacity than my 'used' cells. One of his packs even had a REALLY weak cell that showed 17mOhm resistance! I'm guessing the remaining packs will show more issues.
The difference? I bulk charge my pack before use, putting only enough energy in so that I come back home around 3.85V/cell. Stored mostly around 3.8-3.9V/cell and generally shallow discharges. My brother had his bike sit in the shed over winter, at 100% SOC, when ambients were below freezing.
http://www.youtube.com/watch?v=OoiSIlL6bOs&feature=youtu.be
I thought it was interesting that the one 'bad' cell in this pack was binned as lower capacity from the factory. Are most common failures due to these lower capacity cells?