I've answered the last part first, because as you point out it's what the thread is for:
This thread however is very specifically about BMS's - do you have any thoughts regarding to the specific questions, i.e.:
2) Can you recommend a BMS based on the above? Am struggling to find one that looks like it's from a reliable source.
Unfortunatley I don't where to get one. I don't trust any of the cheap stuff, I've seen too many threads about packs damaged by them. Some of the "good ones" have terrible support, or none, so when something goes wrong you can toss it and go buy another (expensive) one. There's thread "Bluetooth BMS" that covers at least two of the currently popular ones.
But you can use just a *monitoring* system, and have it's alarm output shut off a contactor external to the battery, that disconnects the battery from the controller. Any BMS could be used this way, too. Just don't run the main battery wires thru the BMS, rather thru the contactor, and use the BMS FET control signals to control the contactor instead, and then you can use a 10A BMS to run a zillion amps if you like...the only catch is you have to also change the shunt in the BMS to match the new limit you want it to run at.
1) Which is the relevant A figure for choosing the BMS? Do I need to go with the continuous discharge of the pack (110A), the maximum continuous draw of the motor (62.5A), or something else?
I did answer this one, although I went on a bit so maybe you missed it.
I'll try again, first with a "simple" answer about the pack current limit (the motor limit is discussed later, as it's wrong/irrelevant):
You have to pick a BMS that doesnt' allow more than what the pack itself can handle. I would rate that at 70-80% of what a cell spec sheet says.
As previously noted, that is not really what the number of cells x the max cell rating is. (even though most battery sellers will rate their packs that way, and quite a few pack builders here on ES seem to, I'm not sure they understand the issues, and I wouldn't do it. Probably everyone has a different idea on what limit to actually use).
It is what the cells could probably handle inside the pack, which you could guess at based on the cell's spec sheet data, if they give a discharge rate vs temperature chart. Assume that cells in the center will reach higher temperatures than the ones on the outside, but assume all of them will be a lot warmer (or hotter!) than the lab-tested single cell running at ambient room temperature with airflow around it.
So...if I were doing this I'd derate the cells to at most 70-80% of their max specs, when built into a pack.
If they are rated for 10A I'd tune that down to 7A, meaning a 10p pack is good for 70A continuous.
Peak currents, for a few seconds at a time, can be allowed to be higher, back up to the max the cells are actually rated for, so I'd give it a peak output of 100A. This would probably have a lot more voltage sag than the 70A.
If you want to use the cells as hard as possible, you certainly can, but their lifespan is going to be shorter than if you don't push them right to the edge (and probably over it).
There certainly are people that do this, sometiems because they keep buidling new bikes with ever greater power/etc., so they don't actually use any one build long enough to wear it out.
I use my stuff for commuting and everyday riding, shopping, etc., so it has to be reliable, and last as long as possible. So I don't push stuff to the limits if I can help it (one-off experiments notwithstanding; I've broken plenty of stuff in those
).
I also don't use a BMS; I manage things manually and periodically check balance, etc., but because I'm not pushing things to their limits, things stay in balance for years just fine. I also have a battery pack that (IIRC) is rated, at the cell level, to deliver about 10 times what I continously draw, and about four times what I draw at peak right now, about twice what I could draw in some experiments I did last year.
I have a couple of packs with BMS built in. One of them has wierd problems *because* of the BMS, both in charge and discharge (mostly in charge and balance). The other seems to work fine, but (like all of them) the BMS drains the pack if it's not kept topped off regularly, and would kill the cells if just left sitting long enough.
skinardlasvegas1 said:
So if we assume that this controller is programmable for current limits, there shouldn't be a problem?
Sure; as long as it's current limit is less than what the battery can handle, it should be fine.
(sorry for restating things and forgetting what was already said; when people make multiple threads for their project, it's hard for me to keep track of what has been said and what hasn't...when it's all in one thread it's easier for me to reference).
my motor supposedly draws 62.5A continuous if I let it,
The motor doesn't draw just 62.5A. It draws whatever it needs to do the work asked of it, given a controller that allows it to and can supply that (as long as the battery doesn't sag in voltage so much trying to supply the controller that it reduces the power the controller has available too much).
If you put a high enough load on the motor, it'll draw quite a lot of current. Usually the windings are somewhere in the milliohm range. So let's just say it's 100mohm per phase, making 200mohm for a current path. Put 48v across that, and you get 240A. Two hundred forty amps. That's stall current, at zero speed, so as the motor begins spinning, it'll go down as BEMF inside the motor cancels out some input voltage. The faster it spins, the less current can be drawn, and the lower the torque the motor has. This is stuff you can see, at least indirectly, on http://ebikes.ca/simulator if you want to.
If you kept the motor from spinning, it would draw 240A until it caught on fire (which might not take very long
). Not a normal condition--but an example to show you that it doesn't just draw 62.5A. And that "3000w" is just a number that has nothing to do with what the motor actually does when in use. Most of the 240A would be wasted as heat, at stall...there's some "saturation" point at which the motor can't *use* more current; dunno what point that is for this motor.
So...it's not just 62.5A. Even if it were, that current doesn't translate to what the battery has to supply.
The battery has to supply whatever teh controller pulls, whcih depends on the total *power* the motor is pulling, which again depends on the load on the motor. So if the controller is limited to 62.5A, then sure, the battery would only see that much current.
It's just not the motor that's limiting it.
This means that from a ratings perspective, I've allowed for nearly 100% headroom.
Unless I'm completely misunderstanding where you're getting the headroom, I don't see any at all...see the above for why.
