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Lithium battery refurbishment (e-bike)


ProDave

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Today I was helping a friend with her e-bike.  Short story, 3 years ago "the battery died"  Only recently did she get a new battery pack and charger.  My first job was to connect that and working (connections slightly different)

 

I brought the old battery home so have a look at. Initially with a view to replacing the cells.  However on investigation it is actually the charger that has failed. The battery was no doubt run until it was flat, then left 3 years.  It appeared to be completely dead.

 

The battery pack is made up of 1865 lithium cells.  Banks of 7 cells in parallel and then 12 of these banks in series.

 

Initially trying to charge it as one stack resulted in some stacks starting to accept a charge while others not. It quickly dawned on me that was not going to work well.

 

So I did a quick trial on one parallel stack.  I put it on charge for a few minutes on a current limited power supply and it came up to a terminal voltage of about 3.6 volts.  I don't know if that is normal, a quick search suggested it should be more like 4.2V.  I don't know if it would come up further if I just left it on charge.

 

Basically I am asking is this a lost cause?  Or is it actually possible to get totally flat lithium cells to accept a charge again and have any useful capacity?

 

If so I take it I am on the right lines to put a known charge into each stack individually before attempting to charge them as one string?

 

P.S Some things about this e-bike battery system astonished and frankly shocked me.  The DC input connector from the charger to the battery pack is a tv aerial plug.  and if that is not bad enough, the DC output plug is a standard IEC (kettle) plug.  If you were daft enough, there is absolutely nothing stopping you plugging an IEC mains lead with a 13A plug on it, straight into the battery pack :o

 

 

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If the cells are totally flat rate don't fast charge them as that's a known fire risk in model circles. Many chargers will refuse anyway. You can try a slow trickle charge to see if the voltage comes up but best do it somewhere safe. I don't rate your chances of restoring the pack very highly. 

Edited by Temp
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Usually a cell below 2.5v won't recover. Sometimes you can charge a zero volt cell by temporarily jump starting the charger with a good cell (the charger won't begin until it sees is expected voltage from the cell). But this is usually done at individual cell level, so tricky to do in an assembled pack.

 

I would check all the cells in the pack or at least every parallel string and see what the voltage is as a starting point. 

 

Does it have a BMS? (Battery management system)

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If you can get each parallel pack to accept charge it might recover somewhat. 4.2v is the max charge level per cell. If the cell jumps from zero to 3.6v that's fairly typical I think , just leave it on and see if it reaches 4.1 at least (4.1v per cell is 90%full), might take a few hours depending on your charger.

 

If the battery does not have a BMS (quite likely) then even if you can charge all the cells up to 4.2 then the pack will probably become unbalanced again when it is cycled as the previously flattened cells will have an increased internal resistance. 

 

 

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Just to clarify all cells were reading 0V so well and truly flat as can be,.

 

The one parallel pack I have tried so far I was charging from a constant current / voltage bench power suoply. After sitting all night that pack is still holding 3.2V.  I will try later ans see if I can get that to say 4V and then repeat on each pack in turn.

 

No BMS. That quite surprised me as I always heard that was vital with lithium cells.  I guess cheap Chinesium strikes again?  I guess the only safeguards are the charger will have a maximum voltage at which it cuts off.

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0v often just means "turned off" bt the cells own internal BMS rather than dead/flat. It needs to see volts to waken up, per @Miek above.  If the whole group is @ 3.2v , what charge rate are you at? How much charge has actually gone in?

 

 

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I charged at about 2A for half an hour so not much. assuming that 2A is split equally between the 7 parallel cells that 285mA per cell so should be a safe rate to charge at.

 

This first attempt was just establishing a principle that they could revived from dead.

 

So each cell has an internal management system? I did not know that.

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it's not really management, just overcurrent protection and low-voltage switchoff.

 

/my first Dewalt lithium pack was bought dead and cheap in the hope that it wasn't really dead but had a dry joint or similar. Infact the cells has just turned off and giving each 3v for a few seconds woke'em up and it then charged fine...

 

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10 hours ago, ProDave said:

 

 

So each cell has an internal management system? I did not know that.

Not all cells have this. If you can identify the cells then search for the datasheet you should be able to find out. 

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