Lithium batteries in series - BMS question

Terrible Juan

100 µW
Joined
Jun 22, 2022
Messages
7
Hi all

I have two identical lithium ion batteries (36v / 14ah) with 30 amp BMS for each one. If I connect them in series, am I limited to 30 amps or 60amps? This is for a temporary project but I want to rest some concepts.

My main concern is the motor I am connecting is rated for 34 amps. I want to make sure I limit the current properly through my Kelly controller. Thanks!
 
30A. Current in series remains the same, only voltages add. (in parallel it is the opposite).

However, if the discharge FETs (and charge FETs, if those are also seriesed) are not rated higher than the full total series voltage of both packs (and any voltage spikes that happen from the motor system during this event), then as soon as either BMS shuts off the FETs (for instance, to protect against a low cell being overdischarged), the full total voltage will be across just that single BMS's FETs, and they'll fail from overvoltage.

A common failure mode is shorted (stuck on), so rather than a power loss event, the system may appear to continue to operate normally, but since the BMS was trying to shut down to protect the cells against a potentially destructive situation, but can't, that situation may now happen.

For instance, if a cell had run low (empty), it will now continue to be discharged (instead of being protected) and may reverse voltage from the current flow thru it, which is a fairly bad thing to do to a cell.


As long as the FETs are rated for a high enough voltage, then nothing bad should happen...but if they're not.... :(


Additionally, your controller will need to do the current limiting to the *least* current handling ability par of the system. So not just to what the motor can handle, but also to what the battery can handle.

If the battery can only handle 30A maximum then the controller must be setup to not draw more than 30A in it's battery current limit settings (preferably less, to not push the battery to it's limits, which will give the battery a longer lifespan).

If the motor can only handle 34A phase current, then the controller must be setup to not allow the motor to draw more than 34A phase current. Note that the motor can still be damaged by overheating if it is drawing high phase currents at low RPM for a long enough time.
 
amberwolf said:
Additionally, your controller will need to do the current limiting to the *least* current handling ability par of the system. So not just to what the motor can handle, but also to what the battery can handle.

If the battery can only handle 30A maximum then the controller must be setup to not draw more than 30A in it's battery current limit settings (preferably less, to not push the battery to it's limits, which will give the battery a longer lifespan).

If the motor can only handle 34A phase current, then the controller must be setup to not allow the motor to draw more than 34A phase current. Note that the motor can still be damaged by overheating if it is drawing high phase currents at low RPM for a long enough time.

Thanks. I appreciate all this info. I have been scanning this forum for the past couple weeks and I see your name and avatar all over the topics I've been interested in. Very glad to have your expertise on this.

So to finalize, my Kelly controller is KBS-X 24-72V rated for 55amp continuous with 130amp 10 second boost.

Here is what I'm working with:
30amp BMS limited batteries
Motor rated for 34 amp (41 amp max)
Controller rated at 55 amp continuous

Kelly says to adjust the current limit by % according to what the actual controller is capable of. So with my battery being the limiting factor here, should I the max current output for:

30 amps BMS limit / 55 amps controller output = 54% ?
or
30 amps BMS limit / 34 amps motor rating = 88% ?

Minus some % less for safety as you mentioned.
 
With two packs in series, if either one of them trips, the full voltage of both packs will appear across the FET switch. This will be twice what it was designed for. Depending on the voltage rating of the parts, it may fail. One way around this is to place hefty diodes across each pack output (reverse biased). Normally, the diodes don't do anything, but if one BMS trips, it keeps the voltage across it from exceeding 1 packs worth.
 
Terrible Juan said:
Controller rated at 55 amp continuous

Is that phase (motor) amps, or battery amps? They are different (because of the way these motor systems work) and it's often important to know which one that is. (most generic controllers rate only battery amps as they ahve no way to even measure phase amps, but some Kellys do phase amps, and most (possibly all) FOC-type controllers (regardless of brand) do phase amps, and some higher end controllers do both.


Kelly says to adjust the current limit by % according to what the actual controller is capable of.
It wouldn't make sense to set the controller's current limit to what the controller is capable of--then it isn't doing anything but protecting itself.

What you need it to do is to protect the motor and the battery.

Kelly controller setup can be a little complicated, because not all of their stuff is the same, and their software uses wierd terminology and wierd settings (like using specific numbers you have to actually type into a field to enable or disable a function, rather than just having a button to do that, or a 1 or a 0, etc...and you have to just "know" what those numbers are supposed to be, etc). Fany at Kelly may be able to help you when you have questions about the setup software, but beware that there are likely to be language barriers making it more difficult than it has to be. :(

I don't have enough direct experience with Kelly's controllers and software to directly give you the right specific settings, but:

Some Kelly controllers are rated (and may limit based on) the phase amps, meaning the motor current. Others are rated (and may limit based on) the battery amps. Some of them may have two separate limits, one for phase and one for battery.

The ones with only one limit probably use a ratio (possibly programmable) of that limit to "limit" the other one.

You need to set protection according to whichever limit your specific controller model uses. If it has both kinds of limits, you need to set them both appropriately. If it only has one, and uses a ratio of that one to limit the other one, you'd need to set the one limit you have control over so that the one you don't is still within the limit you want. If that doesnt' sound complicated then you're doing better than I am. :lol:
 
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