I'm in the same boat... Currently I unplug my 2 12s packs and put them in parallel to charge, and in series to run on the bike. I'm planning to build a proper battery box though, so it's no longer going to be convenient to unhook and parallel the batteries.
I anxiously await a nice simple BMS that will do this.
trappermike: The difference between those 2 types of BMS is how the balancing occurs.
Resistive bleed ones (which this BMS is) work by simply bypassing current once the battery is at the correct voltage. When the first cells get to their max voltage it just burns off the extra energy as heat, until all of the other cells catch up. It balances only during charging.
Advantages are that it's relatively simple and doesn't rely as much on electrolytic caps (which don't last forever)
Disadvantages are that in a heavily imbalanced pack, a lot of energy could be wasted, and a lot of extra heat pumped out. Balancing doesn't continue as the pack discharges, so a weaker cell will still drain until it hits the low voltage cutoff, and shut down the pack, even if there's still power in the other cells.
Capacitive ones work by switching capacitors from cell to cell, charging the capacitors from the highest voltage cells, and supplying that energy to the lowest voltage cells. They usually run continuously.
Advantages are usually better efficiency, less heat, and full time balancing, which could give better range on a pack with a weak cell, since that cell will be continuously topped up by the stronger ones during discharge and idle.
Disadvantages are lots of relatively unreliable caps (assuming electrolytic) and just a more complicated operation (could be more prone to errors in a poor quality design),
It's a bit of a tradeoff really. A cap BMS is nice, because it should stay balanced through the entire discharge period, and should be more efficient during charging. The down side is that under some circumstances it can actually make things worse. If you have a cell that has a higher internal resistance than the others (possibly a bit weak, but still usable), the BMS will balance it properly during idle, but can cause it to imbalance under load/charge.
For example, a pack with one slightly high impedance cell:
During charge, the weak cell will receive less current than the other cells, due to its higher resistance. the voltage across the cell will climb a lot faster, not because the cell is charging faster, but because it's accepting charge slower.
Unfortunately, a capacitive BMS will see the higher voltage on that cell, and start shuttling it away to catch up the lower cells. The end result is that the cell which is already accepting the least current is having current shuttled away from it, causing it to charge even slower.
In theory, all of the cells will reach their full charge voltage around the same time, and after a short time in CV mode, the charger will shut off. (while the weak cell is still less charged than the others)
A resistive BMS, OTOH, would charge them all normally. The high impedance cell will actually end up with a higher voltage across it, (and therefore a lower voltage on the other cells). It will charge, but due to its impedance, will reach it's final voltage at a lower than ideal SOC(state of charge). At this point, the BMS just burns off any overvoltage applied to the cell, keeping it at the final per-cell charge voltage (kind of forcing it into the CV portion of the charge early) it will be held there until all of the other cells are at full voltage, then the charger will go into CV mode and terminate.
It's still not perfect, but at least in this situation it's just kept the weak cell topping up the whole time, which will bring it a lot closer to charged.
It's all pretty variable though. Slower charging would probably allow a more even charge. More time floating at the full pack voltage would allow the weak cells to catch up. (at the expense of holding your healthy cells at a high voltage longer than necessary)
I think if you're running relatively low currents for both charge and discharge, it probably doesn't matter which design you use. Capacitive will be better with uneven capacity cells.
If you're running higher currents, it probably still doesn't matter in most situations, but if there is a problem, it won't make things worse.