Relation of BMS, controller and motor

jai134

100 W
Joined
Sep 18, 2021
Messages
105
Which one decides. My battery pack is good for 240A cont and 540A peak, my controller 200A cont and 500A peak and my BMS 250A continous and 375 peak. I read a post that states "battery:phase ratio at 1:2.5 is good". Is it 250A (my BMS continous amps) that sets the limit?
 
jai134 said:
Which one decides.
"decides" what, specifically?


My battery pack is good for 240A cont and 540A peak, my controller 200A cont and 500A peak and my BMS 250A continous and 375 peak. I read a post that states "battery:phase ratio at 1:2.5 is good". Is it 250A (my BMS continous amps) that sets the limit?
Which limit?

The lowest current limit in a system is the highest current you can use.

So the lowest peak limit is the highest peak current you can allow your system to use, meaning you must set the controller's peak current limit to no higher than that (if it isn't already that). In this case, 375A is the lowest peak, so you must not exceed that, or you may damage your BMS.

The lowest continous limit is the highest continuous current you can allow your system to use, meaning you must set the controller's continous current limit to no higher than that. In this case, 240A is the lowest continuous, so you must not exceed that, or the cells may be damaged.

If your controller does not have separate limits, you should set whatever limit it does have to no higher than the lowest of those limits, because you can't then control the peak current it might pull.

Regarding battery:phase ratios, that depends on your specific controller's design. If it does not have separate limits you can set, but only one, and no ratio setting, then you must look at it's manual (if any) to find out what ratio it uses, so you can determine battery current limits should be (or are), as battery current is the only thing that matters for your specific question.

So...if your controller has a battery current limit, you would directly set that to the current(s) noted above.

If it has only a phase current limit, then you must find out what ratio it uses between that and battery current, from the manual or the manufacturer, so you can guesstimate what to set the phase current limit to so that the battery currents do not exceed your known limitations.

If it has both phase and battery limits, then you set the battery limit to those noted above, and the phase limits to whatever your motor can handle (if you're not sure, better safely conservative).
 
Thank You so much! I have to read it slowly a few times to understand.
Regarding settings, it is in percent (and that does it even more confusing for me) so the BMS's 375 Amps peak current is 75% of controllers peak Amps.
I think that my controller, it's a Kelly KLS 72501 8080i, has lowest continous current, 200A. Then "Battery limit" is around 40%?
As I understood the post I referred to in my first post, "Battery limit" is continous current and "current percent" is peak even though it doesn't say so in the manual.
 
If your controller only has a 200A battery (not phase!) current limit, then that means it would never draw more than 200A.

(If it doesn't have a battery current limit you can directly set, then you'd need to use whatever math the controller manufacturer provides to you directly in order to determine what that limit is. Just because you find someone else with the same *brand* of controller has a specific ratio and way their controller is setup, does not mean that yours will be the same, unless yours is exactly the same model *and* uses exactly the same firmware version, and possibly is setup exactly the same way theirs is in all other settings as well, because any of those things could change the way it behaves compared to theirs).


If it is drawing more than 200A, then either that is not it's battery current limit, or it is not obeying it's current limit until some set time has passed, allowing for unknown levels of peak current that you cannot control. For that, you'd have to experiment with the limits available to you in the controller setting, turning them down by whatever ratio is necessary to bring them below the worst-case system limitations.

So if you are seeing say, 400A battery currents but 200A is the most your system can tolerate, then 400A / 200A is 2:1, or 200%. To ensure you don't see more than 200A, you'd need to change the battery current limit in the controller by half, and retest.

If this still doesn't reduce it enough, you would need to either experiment further with even lower limits, or determine the math formula needed to use the new result plus the old result to give you the *real* ratio the system is using (which might even be a curve rather than a flat ratio, making this task very difficult as you can't know what curve it is if it's not in a manual or available from the controller manufacturer). I'm not good enough at math to do that sort of thing. :oops:


Note also that in order for you to say that the BMS's 375A peak current is 75% of the controller's peak current, it means you *must* already know the controller's peak current and have done the math (0.75 x controller peak amps = 375A). If you do *not* know the controller's peak current, then you cannot know that the BMS's peak current is any specific percentage of it, because you don't have the numbers available to calculate it. If you do know the number, either because you set it or because it's in the manual, what is the specific number you already know for the controller's peak amps?

If you don't know that number, then the 75% is not a valid ratio; the ratio is unknown and would need to be determined experimentally and by math from the results. If BMS peak current is a known limit number, call it BMSpkA, then the math is ControllerPkA / BMSpkA = actual ratio between them. Then you can use that to plug the actual limit numbers you want to use to get the ratio you need, if using a ratio is the only way to set a limit in the controller.


Also note that while peak amps are important to limit, the continuous amps are often more important in that they create continuous heating, which builds up in the components (cells, BMS, controller) and if this heat reaches damaging levels can cause catastrophic failures.

The same is true of peak amps, but if those are limited to a few seconds, every few minutes or less often, the heat they generate has time to dissipate, as long as it doesn't reach damaging levels during the few seconds, and doesn't exceed the limits of any part of the system (cells, cell interconnects, BMS, controller, and wiring / connectors between all those things).




jai134 said:
Thank You so much! I have to read it slowly a few times to understand.
Regarding settings, it is in percent (and that does it even more confusing for me) so the BMS's 375 Amps peak current is 75% of controllers peak Amps.
I think that my controller, it's a Kelly KLS 72501 8080i, has lowest continous current, 200A. Then "Battery limit" is around 40%?
As I understood the post I referred to in my first post, "Battery limit" is continous current and "current percent" is peak even though it doesn't say so in the manual.
 
You are really very kind! Thank You again :) According to the manual the controller can handle 200A continuous and 500A for 60 seconds.
So, if the my battery pack deliver 240A continously and the BMS can handle up to 250A continuously it is no problems there. But my controller can only handle 200A cont so I have to reduce "Battery limit" to 80%?
My BMS has peak current at 375A but both motor and controller can handle 500A. Here I need to set "Current percent" to <75% to be safe?
Problem is that the manual says something like this:
"Phase Current Percent. The max motor current is (The value * Peak Current of the Controller) Factory default 100%.
Battery limit Current: Limit the max value of battery Current"
The manual from the shop where I bought the motor and controller says that, if I have a current of 200A in Phase Current setting and sets battery limit to 50% I will have 100A as a result of 200 * 0,5.
I am starting to beleive that is wrong but that's why I don't understand how settings correlate to each other.
In this case, if I set phase current to 75% and battery limit to 80%, I end up with 300A which is too high....
(I'm sorry for stupid questions and bad english) I really appreciate your kind help!
 
jai134 said:
You are really very kind! Thank You again :) According to the manual the controller can handle 200A continuous and 500A for 60 seconds.
<snip>
My BMS has peak current at 375A but both motor and controller can handle 500A. Here I need to set "Current percent" to <75% to be safe?
Then you just need to set the peak limit to ensure it doesn't ever draw that 500A, that it only ever draws whatever the lowest peak current any of the system parts can handle.

So if the lowest peak current anything in the system can handle is 375A, but the controller's peak current limit is 500A, and it uses a percentage of that 500A to create the limit, then you need to change it so it's 375A / 500A = 0.75, or 75% of that 500A, for the battery current peak limit setting, if it has that specific setting available.



So, if the my battery pack deliver 240A continously and the BMS can handle up to 250A continuously it is no problems there. But my controller can only handle 200A cont so I have to reduce "Battery limit" to 80%?

No, because the controller is what determines the limit. The battery doesn't push current, the controller pulls it. So there is no need to change the continous battery current limit, if it does indeed have that specific setting, to protect the other parts of the system against excessive battery current, *if* the controller's limit is already lower than what the rest of the system is capable of.




Problem is that the manual says something like this:
"Phase Current Percent. The max motor current is (The value * Peak Current of the Controller) Factory default 100%.
Battery limit Current: Limit the max value of battery Current"
The manual from the shop where I bought the motor and controller says that, if I have a current of 200A in Phase Current setting and sets battery limit to 50% I will have 100A as a result of 200 * 0,5.
I am starting to beleive that is wrong but that's why I don't understand how settings correlate to each other.
In this case, if I set phase current to 75% and battery limit to 80%, I end up with 300A which is too high....

If there are no battery current limit specific settings available, and everything in it is done by setting ratios of phase currents, then you'd have to follow whatever math the controller *manufacturer* provides for that specific controller model / firmware version / etc., in order to set it up correctly, and then experimentally verify with your ammeter or wattmeter, etc., that it is actually only drawing the current you want it to under worst-case heaviest-load conditions.

If it doesn't exceed the limits you need it to obey, you're all set. If it does, you'd have to change the settings and retest until it does obey the limits.

If it has battery-current-limit-specific settings fields, *and* phase current limit fields, then you set them per the controller manufacturer directions to give you the limits you need for each.

If all battery current setting limits are done as percentages of the phase current *limit* settings, then you'd first set the phase current limits to whatever your motor can handle, and *then* calculate the percentage ratio needed for those limits vs battery current limits you need, to get the numbers to enter into the battery current percentage limit fields.
 
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