I've been thinking about this idea for a bit. Typically high capacity cells have lower discharge limits, so there's a trade off between high current output and capacity. e.g. 25R high discharge lower capacity; 35E higher capacity, lower discharge; 30Q in between.
My thought is around how to get both high capacity and discharge without adding more parallel groups to a pack using high capacity cells, in the form of helper packs.
My current battery setup is 28Ah or 35E cells in a 52V pack (40A discharge, 60A peak), in series with 3x8Ah 6S lipos, making 20S. My general observation is that if I use the pack gently, the 28Ah pack will discharge at even rate as the lipos, with the larger pack having a higher resting voltage per cell after a long ride (expected given the higher capacity). If I thrash the bike with lots of bursts of acceleration, the lipo packs often have a higher resting voltage per cell after the ride. This makes sense, since a higher rate of discharge affects capacity. Also, under a high load, the 6S lipos will sag about 0.2V (out of 24V), while the 18650 pack will make up almost all of the sag.
My thought is to add some low capacity lipo packs (2x6S + 1x2S) in parallel with the 18650 pack to increase the discharge rate and reduce voltage sag. They would function like a large capacitor bank, and keep the 18650 pack from getting anywhere close to it's discharge rating, and hopefully get better us of the stored capacity by keeping the cells happier. It would probably take a few additional monitoring inputs to determine how effective they might be, but I'll probably just use general observations to see if it actually helps. I'll be able to monitor sag and the resting voltages afterwards, so that may be enough. If I can add another 50A of peak discharge with a few little packs, I have a feeling it will make a noticeable difference when using the packs hard.
Anyone running a setup with helper packs like this?
My thought is around how to get both high capacity and discharge without adding more parallel groups to a pack using high capacity cells, in the form of helper packs.
My current battery setup is 28Ah or 35E cells in a 52V pack (40A discharge, 60A peak), in series with 3x8Ah 6S lipos, making 20S. My general observation is that if I use the pack gently, the 28Ah pack will discharge at even rate as the lipos, with the larger pack having a higher resting voltage per cell after a long ride (expected given the higher capacity). If I thrash the bike with lots of bursts of acceleration, the lipo packs often have a higher resting voltage per cell after the ride. This makes sense, since a higher rate of discharge affects capacity. Also, under a high load, the 6S lipos will sag about 0.2V (out of 24V), while the 18650 pack will make up almost all of the sag.
My thought is to add some low capacity lipo packs (2x6S + 1x2S) in parallel with the 18650 pack to increase the discharge rate and reduce voltage sag. They would function like a large capacitor bank, and keep the 18650 pack from getting anywhere close to it's discharge rating, and hopefully get better us of the stored capacity by keeping the cells happier. It would probably take a few additional monitoring inputs to determine how effective they might be, but I'll probably just use general observations to see if it actually helps. I'll be able to monitor sag and the resting voltages afterwards, so that may be enough. If I can add another 50A of peak discharge with a few little packs, I have a feeling it will make a noticeable difference when using the packs hard.
Anyone running a setup with helper packs like this?