Overkill (JBD) Cell Overvoltage cut-off ....delayed too long??

jbatx

I make stuff with things
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Jun 8, 2021
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Austin, TX
Hey folks,
I have a strange issue with an Overkill (JBD) BMS delaying cell over voltage charge protection longer then it should ( I think ). I cannot explain it ...100%

The set up...

1. I'm working with a 16s 48v overkill bms
2. 100ah cylindrical cell modules from batteryhookup
3. Using the arduino lib to monitor and post data

FYI This is a two battery system. Both are 16s 48 with the same brand cells. Both have independent identical BMSs. Battery 2 is the one with the issue.

The issue...

One of the cells appears to be damaged. It will spike up as high as 3.86 while it's buddies are around 3.5* and it will discharge faster than them too. Over night, the voltage diff will normalize to about 5-10mv as the bms balances. I can see this graphed over time. The question is, why is the BMS letting it get that high? I have single cell cut off set to 3.65. The BMS does cut charging but it seems to be abnormally delayed in doing that.

The only thing that I can make logical is that the rapid polling by the arduino lib is causing a processing delay in the BMS. If that's true and you combine it with a cell that is rapidly increasing in voltage starting 3.6*, then I guess it makes some sense.

The BMS manual describes a delay between reaching the triggering voltage and the actual cut-off. However, I don't see mention of that as a configurable option.

I suppose, I could lower the over voltage cut-off.

Does this makes sense at all?

I have a replacement cell and I'll install it. HOWEVER, I don't want to ignore the issue of the BMS's delayed response.

What should I do?

I have attached a screenshot of the battery dashboard. you can see where battery_2 voltage diff jumps up rapidly above 400mv then starts dropping after the bms blocks charging. This is the current state as of just a about 60 seconds ago
 

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Last edited:

LVLAaron

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There is an adjustable setting that tells the BMS how long it should wait before turning things off in under/overvoltage. Do you know what your setting is?
 

jbatx

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I didn't see that in the android app. Is that in the desktop app? ...I'll look again once I'm on site again on. Thursday
 

LVLAaron

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Jul 16, 2021
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It should be in the desktop app. I know it is in the iOS app.

I only used the android app for a few minutes and realized it kinda sucked.
 

jbatx

I make stuff with things
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Jun 8, 2021
Messages
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Location
Austin, TX
[UPDATE]
I set the cell over voltage to 3.55 on batter_2 before I left the site yesterday. This had the effect of capping cell 1's spike to about 3.67. ...Far better than 3.8+. ...temporary stop gap
 

Ampster

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Kenwood, California
One of the cells appears to be damaged. It will spike up as high as 3.86 while it's buddies are around 3.5* and it will discharge faster than them too.
That cell clearly has less capacity than the others and will limit the useful capacity of your pack.
 

curiouscarbon

Science Penguin
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Jun 29, 2020
Messages
2,301
recently built a 4S pack of 25Ah topband cells from batteryhookup. one cell syncs up fine at top of charge but drops 30-60mV early in discharge and then drops below 3.0V before all the others. luckily ordered extra and now going to capacity test them and try to swap in a cell that’s closer to the rest in capacity.
 

jbatx

I make stuff with things
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Jun 8, 2021
Messages
182
Location
Austin, TX
recently built a 4S pack of 25Ah topband cells from batteryhookup. one cell syncs up fine at top of charge but drops 30-60mV early in discharge and then drops below 3.0V before all the others. luckily ordered extra and now going to capacity test them and try to swap in a cell that’s closer to the rest in capacity.
Did you top balance?
 

jbatx

I make stuff with things
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Jun 8, 2021
Messages
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Location
Austin, TX
The bms disapative balancing has apparently helped out. The data graph I have shows that the goofy cells are less goofy and are not drifting so far. ....I don't know why that makes any sense since it does seem like an cell capacity issue as @Ampster pointed out 🙏.

I am going to raise the hvc tomorrow morning by .02 see how it goes.
 
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