Salvaging cells from battery

[Justin.Clements]

Hi,

I have a dead 48v battery 14s9p using samsung 22p cells. I discussed it a bit on a post here when I was deciding on a replacement.

http://www.pedelecs.co.uk/forum/threads/battery-advice-wanted-a123-pouch-cells-or-18650-cells.30397

I've since replaced it with a LiFePo4 battery with prismatic pouch cells.

After my last post I accidentally pulled on the charge leads and when I came to charge the battery I found the negative charge lead had come loose from the BMS. It charged when I pushed the lead onto the bms and I didn't have time to sort out soldering it on. The next day I did the same and managed to get it to charge for a couple of hours then it stopped so I jiggled the lead and there was a very small puff of smoke.

I ordered a new bms from China which has taken 6 weeks to arrive and when I went to fit this yesterday I discovered the battery voltage had dropped to 41.2v. I started checking the balance wires and the first 9 strings are at 3.9v, the next 2 at 0v 1 at 0.11v and the last one before the positive connection is at 3.49v

I'd be interested to hear peoples ideas on what happened, but I'm also wondering where to go from here.

I was looking at some plastic solderless battery holders/connectors on ebay and thought I could either build a 14s 7p battery or try to revive the 0v batteries and rebuild.

How hard is it to remove the spot welds and dimantle the battery, and whats the best way of doing that?

All advice very welcome!

Thanks,

Justin

 

[Deleted member 4366]

It's not that clear in your photo, but it looks like two of the cells at the front are leaking.

 

[Justin.Clements]

It's not that clear in your photo, but it looks like two of the cells at the front are leaking.

There is some rust colouration. Would that definately indicate leaking cells or could it just be rust? The bottom block of cells show rust but 3.49v.

 

[spanos]

D8veh has repaired more than most by some way so I defer to him

But the 3.49v reading is from the series, right (aka block or cell string)

You could have 2 duff leaky cells and a still get that reading due to good cells in the series . You’ll need to isolate those cells and read again to be sure

 

[spanos]

I have dismantled far more packs then Ive repaired. I use plastic tools to pry the welds off

I use tools.....well, I use kiddie plastic cutlery set and a spudger ! (from repairing iPhone screens).

The welds don’t take much to leaver off. I try to dismantle in cell blocks . The bugger in doing that is cutting up the plastic mounts/holder/spacer things. I have gone as far as cling filming some metal stuff pliers to twist and break parts. And very very careful use of a craft knife

It’s just nice to have ready welded blocks to reassemble something useable. Only the duff block taken down to individual cells.

I generally don’t cut any wires at the bms but at the cell side. Basically don’t have flappy wires that might touch when working

I usually make safe by removing main outputs and the bms. Then the balance wires . Then dismantle block by block

 

[Deleted member 4366]

There is some rust colouration. Would that definately indicate leaking cells or could it just be rust? The bottom block of cells show rust but 3.49v.

Brown stains on the top of the cells is normally a sign of leaking. If it were rust, you'd see it on the bottoms too, but you never do, except when the bottom is adjacent to a badly leaking top.

 

[Justin.Clements]

D8veh has repaired more than most by some way so I defer to him

But the 3.49v reading is from the series, right (aka block or cell string)

You could have 2 duff leaky cells and a still get that reading due to good cells in the series . You’ll need to isolate those cells and read again to be sure

Yes the 3.49v reading is from the bottom block of 9 cells on the left hand side in the picture. The block above them gives 2.41v and the next 3 blocks above them are at zero (more or less) there are 2 more blocks above them at 3.9v each and all the 7 blocks on the rhs are at 3.9v.

Thanks for your tips on dismantling the welds, I might try plastic tyre levers. In terms of shorting the cells with the wires, the balance wires all lead to a plug which I've disconnected from the bms so they are isolated at that end and I will disconnect them from the cells first before prising off the welded tabs. The positive main wire which is on the side I'm looking at taking apart leads to the XT90 connector so again should also be isolated. I'll disconnect the negative charge wire from the bms and isolate it in a terminal strip before starting. Does that sound all sound ok? I don't want to short anything and burn the house down!

 

[Justin.Clements]

Brown stains on the top of the cells is normally a sign of leaking. If it were rust, you'd see it on the bottoms too, but you never do, except when the bottom is adjacent to a badly leaking top.

Thanks, I guess I will have a clearer idea of any leaks once I have removed the cells. I will try and use your tip from another thread of connecting a good cell and a low voltage cell in parrallel for a few seconds to try and revive them enough to charge.

 

[Justin.Clements]

I usually make safe by removing main outputs and the bms. Then the balance wires . Then dismantle block by block

I was thinking I would try and remove the lower 6 blocks on the lhs in one go, by prising off the outside tab connecting the top two blocks on this side. Would that be a bad idea?

 

[Justin.Clements]

While this battery is a work in progress I have another one that I think just needs a new bms.
It charges if I connect the positive charge cable and negative discharge cable but won't charge using the negative charge cable. This is after changing the bms to a salvaged one from a previous battery. Prior to that the battery was just cutting out when any load put on motor. The charge cable does show a voltage slightly lower than the discharge cables.
It's a lifepo4 16s
I've been looking at 2 bms boards on ebay that look quite different but have no idea if one might be better than the other?

https://www.ebay.co.uk/itm/16S-30A-48V-LifePO4-Lithium-Cell-Battery-Protection-BMS-PCM-Board-with-Balance/182253838480?epid=1590340348&hash=item2a6f2ce090:g:geQAAOSwU-pXvCl~

https://www.ebay.co.uk/itm/48V-16S-cells-30A-LiFePo4-Lithium-iron-Battery-BMS-Protection-Board-w-Balancing/322968149370?epid=1488911089&hash=item4b3266dd7a:g:eD4AAOSwUlxaNJ1i

 

[Deleted member 4366]

Both about the same as far as I can see.

The problem with charging like that is that if there's any out of balance cells, there's nothing to switch off charging when one goes high. Once a LiFePO4 cell is full and you continue to charge it, its voltage climbs astronomically fast, so be careful.

 

[Justin.Clements]

Both about the same as far as I can see.

Thanks Dave, I'll get one ordered straight away.

I was worried about charging in this way. I wasn't sure if it would bypass the bms as some bms boards seem to have shared charge and discharge connection points. However this one doesn't. So far I only did it as a test after riding about 1m to discharge the battery a bit. But thanks for the info about run away charging for individual cells.

The charge leads do show a voltage so they must be connected to the cells through the bms. When I changed this bms to the one I'd salvaged from a replica battery I used a hair dryer on some heat shrink tubing and the balance wires got quite hot. The glue in the connector metlted a bit although all the wires were still inside. I'm now wondering whether some glue is intefering with a connection and one or more of the balance wires is no longer in contact with the board. I assume that would prevent charging. It seems unlikely as the connector was connected to the board at the time but I might open it up and have a look. I think it's more likely that this bms is shot and should be replaced anyway as the battery it came from had puffed.

 

[Deleted member 4366]

When the charge mosfets are shut, some charge still leaks through, so you see a voltage. If you tried to draw any current from it, the voltage would collapse. Your meter doesn't draw any current.

You should measure the cell voltages directly on the BMS at the bleed resistors or the back of the multi-pin connector, or somewhere like that.

LiFePO4 won't catch fire if you over-charge a bit, but I don't know whether the cells get damaged if you go too high.

 

[Nealh]

If you are going to play and reconfigure the Sam 22P battery chuck any cells that are low under 2.8v and consider the 3.49v may be ok once charged but won't really know until a load is put on them.

 

[Justin.Clements]

When the charge mosfets are shut, some charge still leaks through, so you see a voltage. If you tried to draw any current from it, the voltage would collapse. Your meter doesn't draw any current.

Thanks Dave. So if a charge mosfet were to fail that would prevent charging? New bms on order now, but just trying to gain some knowledge.

Also the replacement bms I bought for the other battery (14s9p 18650 cells) has a shared charge and discharge connection point. So I assume it shares mosfets too? Would I be better getting a different one if I'm able to repair this battery?

https://www.ebay.co.uk/itm/48V-14S-45A-Li-ion-Lipolymer-Battery-BMS-PCB-Board-W-Balance-For-Ebike-Escooter/322750118046?ssPageName=STRK:MEBIDX:IT&_trksid=p2057872.m2749.l2649

 

[Justin.Clements]

If you are going to play and reconfigure the Sam 22P battery chuck any cells that are low under 2.8v and consider the 3.49v may be ok once charged but won't really know until a load is put on them.

Thanks Neal. I'll probably look at working on this battery after the lifepo4 one now. I've got one working battery and my partner only uses her bike in the Summer. I'm sure I'll be back asking questions then.

 

[Nealh]

Reconfigured you may still get a reasonable 13.2 or 15.6 ah battery, the 22P cell is 10a rated so some what better then the 22 or 26F's.

It's not often you see 22P's used, though yours must have been a special at 19.8ah & 52v/14s.

 

[Deleted member 4366]

Thanks Dave. So if a charge mosfet were to fail that would prevent charging? New bms on order now, but just trying to gain some knowledge.

Also the replacement bms I bought for the other battery (14s9p 18650 cells) has a shared charge and discharge connection point. So I assume it shares mosfets too? Would I be better getting a different one if I'm able to repair this battery?

https://www.ebay.co.uk/itm/48V-14S-45A-Li-ion-Lipolymer-Battery-BMS-PCB-Board-W-Balance-For-Ebike-Escooter/322750118046?ssPageName=STRK:MEBIDX:IT&_trksid=p2057872.m2749.l2649

I doubt that a charge mosfet has failed. I've never seen it. Most likely, something is wrong with the circuit that switches it. You can easily test that by bridging the left leg of a discharge one (should be around 12v) to the corresponding leg on the discharge one to see if it switches on. The mosfets switch on when you apply 12v to that leg, like a relay. The other two legs are the in and out.

It doesn't really matter that the charge and discharge mosfets are shared. The logic circuits switch them off when any cell is too high, on when all cell voltages are in the happy working zone and off when any cell gets to low. In theory, you could get discharge stuck switched off if a cell went too high, but the bleed resistors should drain them back into the working zone, which would switch the discharge back on again. If the bleed resistor got stuck, you'd need a new BMS anyway. I'm not sure how they allow charging after it's switched off when too low. There must be some way the charger switches it back on, but I've never studied one to see how.

 

[Justin.Clements]

It's not often you see 22P's used, though yours must have been a special at 19.8ah & 52v/14s.

It was made by em3ev a couple of years ago. I chose the 22p's to keep the price down a bit as the higher capacity cells cost more per ah.

 

[Justin.Clements]

Hi Dave,

You can easily test that by bridging the left leg of a discharge one (should be around 12v) to the corresponding leg on the discharge one to see if it switches on. The mosfets switch on when you apply 12v to that leg, like a relay. The other two legs are the in and out.

I wasn't quite sure what you meant but thought I would open it up again and have a look and decided to test the individual cell voltages to check they were balanced. I found that 14 cells were between 3.55 and 3.58 one cell was at 3.33 and one cell was at 0. This is probably a daft question but would a dead cause the bms to cut off charging?