Quick BMS question
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lnanek
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The controller decides the amount of current to draw. The battery has no choice in the matter. If the controller attempts to draw more current than the BMS allows, the BMS will disable discharge. In my case that means I have to get off the bike, turn the battery and BMS off with a key, then back on again. My first battery had a BMS with a max discharge of 20A, so if I set my controller to draw 30A max, this happened every time I tried to go full throttle up a hill, for example. Setting the controller to draw a max of 20A fixed it to never trip the BMS.
If the BMS trips at 30A, but the battery has poor cells or wiring and has enormous voltage sag at 15A, the controller may have a minimum pack voltage and turn itself off. BMS similarly also has low voltage protections, not just high current protections. So if the BMS max current is set too high (Bluetooth capable BMS generally allow setting the limit) for the battery and doesn't trip, the low voltage protections may kick in when voltage sag from drawing too much current occurs.
Also many BMS have temperature probes, some for both the BMS FETs and the battery cells. So a BMS with a temperature probe by the cells will detect overheating from drawing more current than the cells of the battery are rated for and disable discharge in that case as well.
If the BMS trips at 30A, but the battery has poor cells or wiring and has enormous voltage sag at 15A, the controller may have a minimum pack voltage and turn itself off. BMS similarly also has low voltage protections, not just high current protections. So if the BMS max current is set too high (Bluetooth capable BMS generally allow setting the limit) for the battery and doesn't trip, the low voltage protections may kick in when voltage sag from drawing too much current occurs.
Also many BMS have temperature probes, some for both the BMS FETs and the battery cells. So a BMS with a temperature probe by the cells will detect overheating from drawing more current than the cells of the battery are rated for and disable discharge in that case as well.
docw009
10 MW
BMS limits current? Perhaps some of the more elaborate models with bluetooth do, but mine don't appear to do so. I thought the listed current was the safety rating on the MOSFET's, I've only identified current shunts on a couple of BMS. Most don't show anything visible.
Emprirically, I've used a 20A BMS because larger one won;t fit in my battery case. I've seen them handle 24-26A peaks.
Emprirically, I've used a 20A BMS because larger one won;t fit in my battery case. I've seen them handle 24-26A peaks.
lnanek
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You could check the spec sheet. Even cheap 20A Daly BMS with hard coded settings lists a "over discharge current detect" separate from a "continue discharge current", though:
It can't reduce the current like the controller, though. It can only detect the current, and disable discharge via the MOSFETs if the current exceeds the amount. Like a smart fuse basically. Product description says:
Although beware of this brand since there's whole threads about them failing to trip correctly. MOSFETs typically fail shorted, unfortunately.
It can't reduce the current like the controller, though. It can only detect the current, and disable discharge via the MOSFETs if the current exceeds the amount. Like a smart fuse basically. Product description says:
Although beware of this brand since there's whole threads about them failing to trip correctly. MOSFETs typically fail shorted, unfortunately.
Not the way that's meant by that normally.
The only limiting a BMS can ever do is completely shutting off it's output, disconnecting all power from the system.
The controller is what actually limits current in the normal meaning of the phrase.
To limit current that way, a BMS would have to have a complete DC-DC with current limiting features within it, as capable power-wise as the controller, so it would be as large as the whole controller...so no reason to have this in the BMS since it's already in the controller. (plus, since doing this kind of limiting lowers the voltage output to do so, the controller would have to then deal with a wildly varying input voltage all the time, whenever the BMS was in limiting mode).
Bgt2u
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Yes, l am confused on what the maximum discharge rate is on the cells that l am using. They are Tesla 2170 cells. I know that they are 3.7v @4800mah.
The battery doesn't require amps. It has the capability to supply a certain amount of them, and this shoudln't be exceeded, or it will stress the cells or damage them.
The BMS is there to protect the battery from this kind of overload (and from overvoltage and undervoltage, which also stresses or damages the cells).
The controller has to supply the motor with the power the motor demands to overcome the conditions it's pushing against, and all that power comes from the battery.
So the battery, both cells and BMS, must be able to supply it. That means you must size the battery and the BMS to be fully capable of handling the worst-case demands of the controller (which in turn must be able to supply the worst case demands of the motor under your specific riding conditions and usage).
The controller has a current limit to protect it's own hardware and the battery. Cheap ones are a fixed limit on the battery side. more advanced ones may be user-alterable limit, and some have both battery and motor current limits; the only one that matters for this discussion is the battery current limit.
Usually you size the battery and BMS to handle everything the motor/controller will ever need, and so the controller current limit is set to what the system will require to do the job you want it to do.
If you haven't worked out what all of that is and just get or already have a battery your'e just going to use anyway, then you set the controller current limit to below what the least-capable part of the battery is (cells or BMS or interconnects, etc), to prevent damage to the battery.
No, it won't. You want a BMS that is capable of more amps than you will ever draw thru it, so that you are not stressing the FETs.
If the BMS *also* has a programmable overcurrent setting, you can set that to below the ability of the least-capable part of the battery, but higher than the controller's battery current limit, so that the controller first has a chance to keep things under control, and if it can't *then* the BMS just cuts all power based on that limit to protect the cells/etc.
BTW, all of this is integral with the other info in the other thread you have, as it is all part of one system. Keeping it all together would be better than discussing it separately.
Some of the info in the other thread already helps answer things in this one.
Some of the info in the other thread already helps answer things in this one.
Bgt2u
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That is what l kind of thought. I figured controller would set the limits on what the battery puts out. I am using Tesla 2170 (21700) cells in my battery pack. The pack will be 72v, (20s and 22p).Since l do not have any specs on those cells, (other than being 3.7v @4800 mah), I was just going to go with a 300a BMS to be safe. Is that okay?......That is all that l really need to find out.
Bgt2u
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Yes, l was thinking that also. I started this thread, because l did not think people were getting what l was trying to ask, so l started with a clean, but very similar slate.
FWIW, when I find people aren't getting the question I'm asking, I'm usually asking the wrong question, or in the wrong way, or I don't understand enough of how things work to ask the right question.
I have a feeling that the last one is the case for this question--the stuff you are asking about is all one system that starts at the *other end* of the electrical load than where you are starting.
You should be starting at the job end--what the system has to do for you, which you have to define. Then what motor can do that job for you. Then what controller can feed that motor. Then what cells can feed that controller. Then what BMS can protect that battery but still feed that controller.
I have a feeling that the last one is the case for this question--the stuff you are asking about is all one system that starts at the *other end* of the electrical load than where you are starting.
You should be starting at the job end--what the system has to do for you, which you have to define. Then what motor can do that job for you. Then what controller can feed that motor. Then what cells can feed that controller. Then what BMS can protect that battery but still feed that controller.
