My electric car is running and almost ready for routine use. It's a 1971 Karmann Ghia, a 9" Netgain Impulse9 motor, a Kelly KDH14800D controller, and a 144v 100ah Thundersky battery pack, with a BMS from Alliance Renewable Energy (a.k.a. Lightning Motorcycles).
The issue at the moment is getting the battery pack fully charged and balanced. What we're observing in the process has really educated me on the value of BMS's. There are some in the conversion community saying you can go with no BMS, that the charger will take care of it okay and not to worry. Eeeep is what I have to say to that, especially with what I'm seeing right now. I'm more convinced than ever that a BMS is necessary with lithium batteries.
Lead-acid batteries for all their flaws are somewhat forgiving of abuse. You can run with an unbalanced lithium pack, overcharge, overdischarge, and while that might damage the batteries a bit you'll still be able to use them just with a shortened life-span. With lithium batteries on the other hand they can quickly turn into a useless brick, or if you're using a flammable battery chemistry they can catch fire and destroy your vehicle. My Karmann Ghia is so beautiful (and cost quite a bit) that I don't want it to be destroyed, and the battery pack cost enough to put together that it's best to protect it for long life.
The charger is an Elcon PFC3000. It runs at either 240 volts or 120 volts, and will put up to 3 kw charge rate into the battery pack. I was trading off between cost and charging speed.
It is a somewhat smart charger - when it thinks the battery pack is nearly full, it lowers the charge rate to match. I can measure this using a clamp-on AC power meter and during full charge it will draw 11 amps or more at 240 volts, but in the more modest charge period when it thinks the pack is full it will draw 1 amp or so at 240 volts.
The BMS has an on-off signal it can use to tell the charger to go on, or off.
The BMS algorithm is to keep the charger on until any cells reach a maximum voltage (set at 3.9 volts; this is set a bit below the maximum cell voltage to lengthen the lifetime of the pack). When a cell is at max, the charger is told to turn off. Shunt resistors bleed off voltage from the highest cells. The charger is told to turn back on when cells fall below another voltage level. It means that as the pack nears full charge, the charger will be cycling on and off until all the cells are fully charged and have almost identical voltages.
This is probably a standard algorithm for BMS's ... it's certainly what it looks like the pingping packs I have are doing, because the pingping charger keeps cycling on and off when it's at the end of the charge cycle.
The problem we're having at the moment is a) the pack voltage is high enough the charger is only putting out a mild charge current, b) the cells in the pack are way out of balance.
Many of the cells in my pack at this moment are in the region between 3.7-3.9 volts (during charge) but a few are near 3.5 volts (during charge). The problem is that you can't really deduce the state of charge in a cell that's at 3.5 volts. When the cell goes above 3.6 or 3.7 volts it's real easy to deduce the cell is almost fully charged. But the voltage characteristics on lithium cells are such that a 3.5 volts cell is indistinguishable from almost charged or almost discharged.
Basically - if I were to drive the car right now there would be a danger of damaging the weaker cells because those cells might actually be almost discharged right now, and taking the car for a spin could send the cells below the low-voltage threshold that causes damage.
The charger sees a nearly full pack however, and is putting out a modest charge current. If there were no BMS a vehicle owner trusting the charger to do the right thing would have no clue how out-of-balance their pack is, drive away with an unbalanced pack and end up with a dead cell or two because of cells going below the low voltage threshold. Also the unbalanced pack could cause the high cells to be over-charged, damaging those cells because of going over the high voltage threshold.