Ianhill wrote:So the BMS you use is 20s max and its possible to overlap the BMS so they both maintain some of the same cells for a portion of the pack ?.
I get you can series the bms but is this how you would achieve 28s with 2 20s bms overlapping them so both bms would see 20 cells on each BMS but the second BMS is overlapping its first 12 cells for 28s total and no magic pixies getting out.
No...two different things. These BMS have unpopulated solder pads for an additional 4 cells per BMS. IE: that would make these into 24S each or 98 volts.
Series running two or more BMS is a bad idea. I was partly wrong. It sounded great in my head, but then I drew this thing out and realized I had made a HUGE mistake. It is possible to series string multiple BMS, but also a really bad idea. The reason why is Load- on the BMS and Batt+ are not the same thing...in fact they are exactly 180 degrees opposite of each other. In order to string two BMS together in series, that means the two 16S packs are also in series. The positive end of the first pack connects to the negative end of the second pack. To series wire two BMS means Batt- on the negative most BMS is the final Batt- for the whole 32S battery pack. It's Load- needs to connect to Batt- on the second 16S pack where the second BMS also connects its BATT-. So far so good...sort of. Lets not forget that the Batt+ on the negative most 16S pack needs to connect to Batt- on the second 16S pack to make a 32S pack. Well that means that Load- on the negative most BMS is also connected to it's own Batt+. This would create a dead short across the negative most pack via it's own BMS. So much for protecting the battery pack! I assume the BMS would shut off, but still...completely unworkable.
To anybody reading my nonsense about series connecting two BMS...it's a really bad idea...don't do this!!! LOL. My mistake was I was thinking of the BMS as both the positive and negative connection to the battery pack it protects and it's not. A BMS is only the negative connection of a battery pack and then the balance cable is all the intermediate cell connections. There's just no way to connect together two battery packs and 2 BMS without a really high power diode between the two 16S packs to prevent current flow from Batt- to Batt+ of the negative most 16S pack. You would need a diode that can handle 2X the current that the pack is capable of delivering just to be safe. It's doable, but at a minimum you are going to see a .7 volt drop across the giant silicon diode that is going to need a heat sink. Essentially...never mind, not really doable or practical...just buy a 32S BMS.
A BMS serves three roles.
1. Overload protection
2. Cell under voltage shut-off
3. balance charger
!. This is easy, current sensors attached to high voltage mosfets that turn off the mosfets when X amps is reached. I could build this in a couple of hours.
2. This is pretty easy too, but I would need an op-amp for each and every channel of the pack that compares cell voltage to a threshold voltage and then sends a shut-off signal to the mosfets when the threshold is crossed. One channel would be a couple hours of work, 20 channels would be 20X the effort.
3. You can buy balance charger boards. It used to be that RC chargers didn't balance charge, you needed a separate balance board connected to the balance connector on the pack to balance charge. Now days, this is built into RC chargers. So...with sufficient numbers of balance boards (number 3) and a signal off those boards for under voltage and a separate current sensor/mosfet switch (number 1), you could build a BMS any size you wanted as long as your mosfets could handle the total voltage.