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Tesla Module Overdischarge
#1
Hello all,

I have a Tesla model S module (a single 24v, 5.2 Kwh unit). Recently I was on thanksgiving break and my RV PV system was compromised and upon returning home (to the RV) the module is reading 9.8V.

I am not very familiar with this chemistry and am wondering if anyone more knowledgeable than I can help me understand if the module can recover from such a large over-discharge. 

If the pack is shot is there any salvage value? 

If I am asking in the wrong place or anyone knows of somewhere with users that have knowledge specific to this battery please let me know. 

Cheers,
Jack
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#2
The tesla module is a 6S configuration, so there's six groups of cells in series. You will need to check each group to see if they're all equal in voltage. Typical ranges for these cells are 2.8V to 4.2V. At 9.8V that means you're looking at around 1.6V per series. Is draining them down that low bad? Probably, but we've recovered cells that are discharged under 1V without issues. They're not LFP chemistry, which is really bad when it's discharged completely, leading to swelling etc. as gases are built up. So your cells may stand a chance.

The problem is, how far did it discharge? How was there not a low voltage disconnect or something to prevent it to go into such a deep discharge? In any case, test each series to make sure they are all equal and balanced. Otherwise you will need to balance them carefully and not bring it up to full charge immediately. The worst thing you can do now is to bring it up to full charge and not balanced, leading one to go over 4.2V which would be dangerous. Then continuously check for voltages for the next few cycles to make sure all the cells are healthy.
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#3
(12-04-2019, 03:09 AM)not2bme Wrote: The tesla module is a 6S configuration, so there's six groups of cells in series. You will need to check each group to see if they're all equal in voltage. Typical ranges for these cells are 2.8V to 4.2V. At 9.8V that means you're looking at around 1.6V per series. Is draining them down that low bad? Probably, but we've recovered cells that are discharged under 1V without issues. They're not LFP chemistry, which is really bad when it's discharged completely, leading to swelling etc. as gases are built up. So your cells may stand a chance.

The problem is, how far did it discharge? How was there not a low voltage disconnect or something to prevent it to go into such a deep discharge? In any case, test each series to make sure they are all equal and balanced. Otherwise you will need to balance them carefully and not bring it up to full charge immediately. The worst thing you can do now is to bring it up to full charge and not balanced, leading one to go over 4.2V which would be dangerous. Then continuously check for voltages for the next few cycles to make sure all the cells are healthy.

Thank you for the reply,
I will use a tenergy cell meter tomorrow to check the voltage of each group of cells, and balance them accordingly.
As far as recovering the cell, I assume a low amperage is best for bringing them back up towards a more normal SOC? I should be able to reprogram my inverter to charge quite slowly probably at around 3 amps. 
Do you think this is the right approach?

Thanks again
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#4
One thing that would bother me, is Tesla cells lack the PTC and CID devices that normal Li-Ion cells have. So if a cell does go bad, it will pressurize until it explodes.
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#5
(12-04-2019, 04:01 AM)Geek Wrote: One thing that would bother me, is Tesla cells lack the PTC and CID devices that normal Li-Ion cells have. So if a cell does go bad, it will pressurize until it explodes.
Hi Geek, is there a way to know when a cell has gone bad before it explodes? Would a battery meter such as the tenergy which can check the capacity and internal resistance be enough to know if a cell has gone bad?

thanks
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#6
(12-04-2019, 04:26 AM)jagarwal Wrote:
(12-04-2019, 04:01 AM)Geek Wrote: One thing that would bother me, is Tesla cells lack the PTC and CID devices that normal Li-Ion cells have. So if a cell does go bad, it will pressurize until it explodes.
Hi Geek, is there a way to know when a cell has gone bad before it explodes? Would a battery meter such as the tenergy which can check the capacity and internal resistance be enough to know if a cell has gone bad?

thanks

Personally, I would remove a couple of cells, and test them first. Charge them slowly, capacity test them, then cycle them a few times. Check for self discharge. Having no experience with these batteries, I don't know if this is possible without dismantling the whole thing. In which case you won't end up with a carton of fireworks should something go wrong.

Even if these cells did cycle without any problems. I would still be very wary of using it near anything I didn't want to catch fire.
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#7
Just out of curiosity, did you have a BMS on this system? What was the failure that caused the pack to discharge so low?
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#8
(12-04-2019, 04:26 AM)jagarwal Wrote:
(12-04-2019, 04:01 AM)Geek Wrote: One thing that would bother me, is Tesla cells lack the PTC and CID devices that normal Li-Ion cells have. So if a cell does go bad, it will pressurize until it explodes.
Hi Geek, is there a way to know when a cell has gone bad before it explodes? Would a battery meter such as the tenergy which can check the capacity and internal resistance be enough to know if a cell has gone bad?

thanks
If you charge at low current to slowly bring the cell voltages back up, any problem cell will hold down the voltage rise.
I'd charge slowly while monitoring for pack balance, & if the balance goes out much, start other checks before continuing.
If the pack comes up to normal cell voltages that are fairly even (very likely it will), then charge it gently again, eg 25% usual charge rate towards "full".
Watch the balance closely when approaching full - balance is likely to go wild (not from bad cells, just balance out here) , so slow right down to charge current = balancer bypass current or manually charge lower cells to catch up.
Running off solar, DIY & electronics fan :-)
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