BMS Cutoff Problem

Batteries, Chargers, and Battery Management Systems.
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Davideus   1 mW

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BMS Cutoff Problem

Post by Davideus » May 05 2020 11:38am

Hello,

I do have a oversized esc (72v 20Ah battery, 41A motor & 100A esc) and frequently experience bms cutoffs. If we exclude individual cell disbalance, could this be small (weak) battery pack problem? Maybe esc pulls too much juice from battery and bms switches the power off for security reasons?

Individual 2500 mah 18650 cell max continuous discharge is 3C. So it's 2.5A x 3 x 8 = 60A
So also maybe battery don't have juice to power the motor when it spikes and bms pulls the trigger?

Thanks for any help <3
Last edited by Davideus on May 05 2020 11:42am, edited 1 time in total.

john61ct   100 GW

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Re: BMS Cutoff Problem

Post by john61ct » May 05 2020 11:41am

Yes, LVC is a critical BMS function.

Good ones let you adjust it, since V drop under load depends on the current.

Imbalance can be a cause, just as much as the pack being too old, small or weak.

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Davideus   1 mW

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Re: BMS Cutoff Problem

Post by Davideus » May 05 2020 11:49am

john61ct wrote:
May 05 2020 11:41am
Yes, LVC is a critical BMS function.

Good ones let you adjust it, since V drop under load depends on the current.

Imbalance can be a cause, just as much as the pack being too old, small or weak.
So what size of pack i should use for my config?
One other way is if it helps, I can adjust in ESC config max amps to 60A. Should this solve my problem?

john61ct   100 GW

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Re: BMS Cutoff Problem

Post by john61ct » May 05 2020 12:03pm

Davideus wrote:
john61ct wrote:
May 05 2020 11:41am
Yes, LVC is a critical BMS function.

Good ones let you adjust it, since V drop under load depends on the current.

Imbalance can be a cause, just as much as the pack being too old, small or weak.
So what size of pack i should use for my config?
One other way is if it helps, I can adjust in ESC config max amps to 60A. Should this solve my problem?
Many factors in that first question

Getting a pack made from cells with a higher actual C-rate discharge capacity is one approach.

Adding to the Ah capacity is another.

Your last idea, actually reducing amps drawn, compromising on performance (fun) is certainly easiest.

But the pack will continue to wear out in fact accelerate in loss of power capacity and Ah stored at the same time

so best to start planning / researching / saving for a newer / better / stronger / larger pack sooner rather than later

Quality costs a **lot** more than the usual crap out there.

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Davideus   1 mW

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Re: BMS Cutoff Problem

Post by Davideus » May 05 2020 4:39pm

john61ct wrote:
May 05 2020 12:03pm
Adding to the Ah capacity is another.
So, if I take this approach, adding another 20s8p in parallel config, consisting with same cells (Same pack I have now), total of 40Ah should be enough?
Connecting in parallel config 2 separate packs with 2 separate BMS is Ok?

I have this ESC:

Image

john61ct   100 GW

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Re: BMS Cutoff Problem

Post by john61ct » May 06 2020 1:49pm

Well if it turns out the problem is caused by the cells used in your pack are just crappy, that would not be a great approach.

Going parallel, the two packs just need to be roughly the same voltage-SoC curve, if one is much stronger (power C-rate capacity) or even much higher Ah capacity, that is no problem.

So I would get a much better pack made, from known-good high-discharge cells as if you were going to replace your current pack.

And then you could use one or the other or both depending on the day's need.

BMS I'm not sure, likely depends on the type, how it's wired etc hopefully someone else can help with that.

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