An absolutely great explanation on how it works. Thanks, I really learned a lot from it. Your comments about what I will call pulsed motor response is the same comment that was stated by Jim Olson from Cyclone USA the first time he tried a lifepo4 pack with a 500w Cyclone kit. Ether the pulsed protection or the speed limiting protection sounds like something I would consider. Now that you have done such a great job explaining the discharge protection can you please explain the charging protection? From what I have read (and this is something I don't like) is that the BMS is just bleeding off the voltage on high cells. I have also read that with some BMS they cut off the charge once the first cell reaches a full charge.
You mention Ã¢â‚¬Å“Gary Goodrum's LVC boardsÃ¢â‚¬Â. Is this only a LVC or a BMS with LVC with ether pulses or a reduction is current draw. I like the idea of using individual cell chargers but would like the addition of a low voltage control.
First of all, Andy has it completely right, with regards to how the LVC works. The only minor correction is that the voltage detector hysteresis is .15V, so a 2.1V detector will reset at 2.25V.
I do have LVC-only boards, which I have been selling, but these are being updated, along with the new v4.0 version of the BMS unit itself, which we are just about finished testing.
A BMS really only has two required functions, cell level low voltage protection, and charge management. If you use individual cell chargers, all you need is the low voltage protection part. I used this exact setup for quite sometime with my a123 and PSI-based packs. I have 16 of the VoltPhreaks 2A individual chargers that I used to "top-off" each cell, or block of paralleled cells, and used a 48V/15A Zivan NG1 bulk charger in parallel with its voltage set a bit below the CC/CV set point of the VP chargers. This works quite well, but the problem is the setup is bulky, and all the connections can be a pain. I fixed the latter issue by using a single 18-pin AMP 4.2mm PE Series set of connectors, the female version wired into the packs, and the male wired to all the chargers. Still, having to drag this whole setup around with my folding bike go to be too much, which is really what got me to hook up with Richard, and start down the BMS path.
From the beginning, the goal has always been to use a single bulk charger and have the BMS work like a bunch of individual cell chargers. This is different than the way RC-type balancers work, which is to simply bleed off current from all the cells that are higher than the lowest cell. Anyway, the behavior of the individual cell chargers that the BMS tries to mimic is the ability to let each cell receive a full charge, at its own pace. The reason this is important is that even cells that are carefully matched when the pack was built will eventually drift apart, and end up with slightly different capacities, internal resistances, thermal characteristics and states of charge. With individual cell chargers, each with their own CC/CV charge profiles, the cells will simply take however long they need to get full. With a bulk charger, charging all the cells in series, this is a problem because when the first cell gets full, it won't let any more current in. Since all the current has to go through all the cells, however, that means the cells that aren't full yet stop receiving current, which causes them to get even farther out-of-balance. SLA cells are different. They have the unique ability to "absorb" a bit more current, even though they are full. The trickle mode of an SLA charger is there to keep the full cells topped off, while allowing some current to flow to let the lower cells catch up. All Lithium-based chemistries don't have this "self-absorption" capability, so when the first cell is full, that's it, no more current flows to the rest of the cells. By adding a shunt circuit to each cell, a certain amount of current can bypass the full cell and flow through the shunt, so that it is available for the next cell in series.
In addition to providing this shunting capability for each cell, the BMS needs to do one more thing in order to mimic the individual cell charger behavior, and that is basically to have a separate CV mode for each cell. This ensures that no one cell (or block of paralleled cells...) goes over the CV set point. Without this feature, the first cell to hit the cutoff will suddenly have its voltage rise and keep rising until the bulk charger's CV mode kicks in. LiFePO4-type cells are more tolerant of this sort of over-voltage condition, but repeated instances of this will eventually affect cell life. LiCo-based LiPo cells, on the other hand, can't tolerate significant over-voltage conditions at all, and can quickly go into thermal runaway, which causes the cell to explode in a fireball and burn at over 2000F until all the Lithium is gone.
Not all BMS designs work the same, when it comes to handling what happens as the cells hit the CC/CV crossover point. What our BMS does is detect when each cell's voltage reaches this set point, and then it trips the same opto-controlled output that is used by the LVC circuit, which is obviously inactive during the charge process. The charge control logic then uses this signal control the duty cycle of a PWM circuit that reduces the current by however much it needs to in order to keep the cell's voltage at the limit. Without the shunt circuits, what would happen is that the first cell to hit the set point would control the current available for all the cells, so that in effect, only the first cell will have a proper CV mode, and will get completely full. In combination with the shunt circuits, however, the net effect is that you have individual CV modes for all the cells, so just like the individual cell charger setup, each cell can get completely full, at its own pace.