Tfisher309 said:
I thought the purpose of a BMS was to protect the battery by controlling it's output?
It is there to protect the cells, but it cannot control the output the way you are likely thinking. It can only turn off the entire power output, completely, or leave it on, fully. It's just switch, activated or deactivated by the readings it takes from the cells (and any current monitoring it may do--not all of them actually react to overcurrent).
So if a cell rises above HVC (full charge) it turns the charge port off. If a cell drops below LVC (empty) it turns the discharge port off. If it does current monitoring, and pack current exceeds it's limit for however long it is allowed to do so (depends on the BMS), it turns off the discharge port.
The controller is what actually modulates current within the system, and limits it gracefully and variably, rather than just shutting down completely on overcurrent.
For you to pull 370a your BMS must have a high maximum rating, like 10c.
To help you find and understand specifications:
A BMS is not rated in "C"; that would be for the cells, since a C rating is a number used as a multiplier for the Capacity (Ah) to give a current (A, Amps). A 10C capable cell that has a capacity of 5Ah could output 50A (10 x 5 = 50).
A BMS may have multiple ratings, but two typical ones would be a Max A rating, usually meaning Peak for a few seconds, and a Continuous A rating, that it can handle all day long. Some of them, especially ones meant for higher currents, may require cooling airflow thru / over them in order to operate at these ratings, and may be severely limited in capability without that, even though they may not tell you this on the seller's pages. .
Is it important to match the max output of a BMS with the max capacity of a controller? If they can't be matched which should be higher?
The BMS, first and foremost, must be able to protect the cells against anything outside their limitations. Next it must be able survive the worst-case current draw from the controller without stress or damage.
So the first step is to determine the maximum battery current your system will ever draw, continuously, and peak (and for how long it will peak, since if it's very long, that's going to end up being your continuous rating). Let's say that's 400A, just to give some big numbers in later examples below.
Once you know that, then you use cells that, in the configuration (number of parallel cells in each group) can handle that current, easily, without stress, and with some extra capability (say, 20-50% to toss out some numbers, depending on the performance and lifespan you expect from the pack) so that as they age the pack still handles the full load of the system. (and if you ever expect to upgrade the motor or controller to something more powerful, make sure you account for that when choosing the pack capabilities.) (if you use cells at their max ratings, they're stressed and will have more heating and voltage sag, and shorter overall lifespan--use cells that can handle say, twice what you need of them, and they'll have much less sag, heating, and longer lifespan, under that 400A load).
For the 400A example, then you need cells that can handle 400A, at least, preferably more. Let's say you decide to use 800A rated cell groups, and use individual cells that can handle 30A each--you need 800 / 30 = 27p or 27 parallel cells in each group. If you use cells that can handle 100A each, you only need 8p, or 8 cells in parallel in each group.
(note we haven't mentioned capacity, or Ah yet, that's essentially unimportant to the current-delivery ability, though it is related to it in that cells that can supply higher current usually have less capacity). (we alos haven't mentioned pack voltage or number of series cells (groups), as that also doesn't matter for the purposes of determining current (A). )
Then you use a BMS that can easily handle that worst case current as well, so that it is not stressed and the FETs on it don't heat up too much. If you use one that has a current detection / shutoff ability, then choose a limit that is above what the controller would ever draw (so it doesn't trip the BMS and turn off your ride), but below what the cells should ever be allowed to see.
The battery is the heart of your system, so if it cannot supply, for any reason, everything your motor and controller will ever ask of it, with as little voltage sag as possible, and as little cell and BMS heating as possible, the system will not perform as well as it could, and might not perform the way you need it to.