# tesla S battery repair



## brian_ (Feb 7, 2017)

Purplebus said:


> 3.) Does anybody know how the 18650 cells are split into the 6S. The divisions of the stamped SS plates are almost random. They don't add up to the 74P groups?


They do add up to the parallel groups; that's 74P in most Model S battery modules, but I think it's fewer in earlier lower-capacity models, and it's apparently 86P in a P100D module. In general each plate connects to the negative ends of all cells in one group and to the positive end of all cells in the next group. The plates at the ends of the modules (electrically) will obviously have half as many cell ends, connecting to just one group. That means one side should have three 148-cell plates, like this: 








... and the other side should have two 148-cell plates plus two 74-cell end plates, like this:








(note the module terminals on the left end of this side)
If you look closely at these images, you can see one cell end visible through the holes (I don't know if it is positive or negative) looks a little darker than the other cell.

The cell groups are basically in two rows, with the series connection running along one row (i.e. down the length of the module), across the module, then back along the other row.


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## Purplebus (Sep 14, 2019)

Thanks Brian

By overlaying the C2- and C2+ areas on the top and bottom I was able to isolate Cell 2. All of my 13 blown fuses are in this region. I think I have a solution to save my $1400 battery. I will create an auxiliary pack of 13 new 18650 batteries in parallel and solder the pos/neg to the respective stainless plate in the region of the damaged cells. ( I will just leave the damaged batteries in place with their blown fuses) I will charge this new 13 pack to be equal to the other cells. 

I am leaning toward the ZEVA BMS. If I purchase and install this I think it will balance out all my batteries and provide protection in the future.

Thoughts?


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## brian_ (Feb 7, 2017)

I'm glad that made sense  - I wasn't sure if my description was clear enough.



Purplebus said:


> By overlaying the C2- and C2+ areas on the top and bottom I was able to isolate Cell 2. All of my 13 blown fuses are in this region.


That's a 74-cell _group_, not a cell (each 18650 is a cell). 
That failure pattern is not surprising. Once any cells in a parallel group fail, either internally or by blowing their fuse, the other cells must carry more current per cell, because the total current is the same in every group (since they are connected in series). That overloads the cells that haven't failed yet, causing more cells (or just fuses) to fail. This is the nightmare of paralleling electrical components.

If EV batteries were actually designed to be repaired (they're not - even module-level replacement doesn't seem to be common under manufacturer's warranties and dealer servicing), the would have modules with a small number of groups in series, ideally only one... but no one wants the cost of assembling 96 modules into an EV battery. Tesla is actually going the other way, splitting the Model 3 battery into only four modules, so they are 23S and 25S (two of each).


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## electro wrks (Mar 5, 2012)

Purplebus said:


> I over charged my battery back in my DIY VW bus EV conversion. I am running (5) tesla S batteries in series. I think the damage is only to one module. I have removed it. There are 10 (18650) batteries with blown fuse wire and testing very low volts. 5 of the 6 groups were reading 4.3x V. I bleed the power down to 3.9x volts on these. The remaining group reads .504V. Can this be fixed?
> 
> 1.) If I am able to isolate and charge most of the 18650 cells with blown wires so they are very close to the adjacent 18650, can I just re establish the fuse wire?
> 
> ...



You are very lucky not to have had a fire. With the hard earned knowledge learned by the DIY community, it borders on suicidal and criminal not to have a good BMS when using these modules. You should get one and set it up right away. Usually when these modules are overcharged, without a good BMS, a thermal runaway results. Was there any indication of this in your mishap? Photos of the bad module(s)?

I don't believe the conductor plates are stainless steel. AIR, they are nickel plated aluminum or made of an aluminum/nickel alloy.


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## brian_ (Feb 7, 2017)

electro wrks said:


> I don't believe the conductor plates are stainless steel. AIR, they are nickel plated or made of an aluminum/nickel alloy.


I didn't think they were steel, either, but I didn't know. I did find an InsideEVs article which says that they are aluminum; it also has a still from a video which shows a module with the plate removed, showing the cell groups more clearly. Note that this article is primarily about the Model 3 battery, and that this Model S/X information is only some background at the beginning.


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## electro wrks (Mar 5, 2012)

The conductors could be electroless plated: https://www.cybershieldinc.com/electroless-nickel-plating-aluminum/


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## GreenTechFusion (May 18, 2019)

I some experience with the Tesla modules and customers that have overcharged or damaged their modules. Of course they did not have BMS however that is not what caused battery failure. It is the overcharging. If you don’t have a proper charger or solar charge controller you can manually specify the exact constant current and constant voltage settings to not go past the 24v max. Most regular inverters/chargers and some solar charge controllers are default around 28v typical charge settings.
So, as for trying to repair? Not a good idea to try and add blown fuse wire to fix blown ones because what I found is most of the cells also have their CIDs tripped. 
So do not try and reset those, you will def have fire later.
I would just tear apart bad module to get the good 18650s and sell them or make other project with them.
Cheers

John at GreenTechFusion.com


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## electro wrks (Mar 5, 2012)

GreenTechFusion said:


> Of course they did not have BMS however that is not what caused battery failure. It is the overcharging.



The point is that a good BMS would sense the overcharge condition, temperature and/or voltage-wise, and stop the overcharging from damaging and/or overheating the modules.

It seems to me this is a no brainer with the cost of a good BMS costing ~ what it costs for just one module.
Maybe you're referring to one module or paralleled modules being used in a 24V solar/wind storage system. This type of system would still benefit from a good BMS that monitors all six cell groups in the modules.


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## Purplebus (Sep 14, 2019)

yes I am kicking myself for not doing the BMS earlier.

I have decided not to repair the module or put it back in service in the traction battery pack. 

I am going to repair it by tying in the satellite P74 batteries using one of the DIY power wall kits to take the place of the damaged group and then use it to power my 24v inverter. They make some pretty cheap chargers and BMS for 6S configurations. I will post this to the community when this is complete.

I figure this will cost about $300 for the charger, 74ea 18650, and the bms and I will have a 5.2 Kwhr power wall.


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## Hollie Maea (Dec 9, 2009)

Over charging should not by itself blow the cell fuse, which makes me think that the over charging caused internal shorts in the cells, which then caused the fuses to blow.

As others have said, it's a miracle you didn't burn down your car.


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## remy_martian (Feb 4, 2019)

Get a replacement module and sell your defective module to cell scavengers on eBay for $600-$700 or so. They'll be thrilled with a pack that has over 400 good cells to scavenge.

For the $500-$700 out of pocket, I would not take any of the risks you're proposing, or waste my time on doing the sketchy repairs.


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