18v tool battery repair

[hotponyshoes]

I have an 18v 2ah power tool battery that was working fine.

I dropped it from a decent height and it was completely dead. No indicator light or anything.

I opened it up and found a break in a metal joining tab.
The cell at the damaged end was reading 0v the other's all about 3.7v or so.

I ordered one single new cell, fixed that and repaired the break in the metal strip.
Put it back together and put it into an led torch to "test" it.
Worked fine so I left it in the torch and used it until it went flat.

Then tried to recharge it. Charged fine for a couple of minutes then the charger shut down with a red error light.

Took the battery apart again.
The new cell I fitted was reading about 3.9v and the others all between 3.2v and 3.8v.
I did record the exact voltages if it matters?

I thought maybe the charger was detecting a cell imbalance?
So I tried running it flat in the torch again and recharging a few times. Seemed to help as each time the charger would run a bit longer but I couldn't get it to charge up past one indicator light so I gave up.

That was a couple of weeks ago.
Just opened it up to have another look at it.
I've now got voltage readings of 3.7v on the new cell. 3.7v on one of the original cells, 2.4v on another and 2 cells with 0v.

All cells are wired in series, so I'm not sure why 2 more seem to have died?

Any way to fix or test this?
Any reason why I have messed this up?

Should I have replaced all 5 cells together?
Or should I have disconnected each one and balanced charged them some how?

A genuine new battery is about £30 and this is the first time I've ever needed to try and repair one so I don't want to be spending a fortune on special chargers or anything

 

[Pigeon_Droppings2]

I have a DeWalt 18v that I dropped too...was planning a similar repair but never found the time.

The reading up I did suggested the cells needed charging and discharging in a controlled way to measure the capacity individually. They sell test rigs for doing it automatically but I think there were folk on line doing it manually too.

Reckon all the cells needed individually testing...at least that's what I was going to do!

I even bought the machine to do it too but who knows where that's gone now!

 

[hotponyshoes]

That's what my reading up seems to suggest.

First problem I had is the cells seemed to be spot welded. I couldn't melt the 'solder' on the bad cell so I had to break it off then clean up the tab.

I don't think it would be worth the cost in parts or time to rebuild the whole pack..

 

[Pigeon_Droppings2]

I don't think it would be worth the cost in parts or time to rebuild the whole pack..

Thats where I ended up I think!...unless it's just out of interest that you do it. I had the same issue with spot welding too.

When I looked was 3+ years ago....there were folk stripping and selling the individual cells (tested). Not all cells are created equal I think so the replacements need to be matched.

The plan I had was to strip the cells, test the capacity of each and switch out any busted cells with the same make/capacity/spec...probably not worth fixing when the new ones are £30 unless you're determined.

 

[Turbo]

To start with you should not run a li ion battery flat. If a li ion cell goes below 2.5v it could brick the cell, 3.3 v is the recommended minimum.

By run flat, if you meant run it until the battery pcb cut power that's normally OK but it should protect the cells from going below 3.3v.

Older batteries didn't monitor the voltages on all cells and they sometimes would run the unmonitored cells flat. Newer batteries monitor all cells and are safer.

Check if each cell has a pair of wires going up to the bms pcb.

What brand are your batteries?

 

[Turbo]

To recharge the flat cells get an old phone charger preferably putting out 3.3v and maybe 300mA, that means you cannot overcharge anything.

Strip the jack plug off and fit small Croc clips and clearly mark each + and -.

Connect them up to one of the flat cells and charge it for a while, checking the voltage every 5mins.

Charge until the cell holds a steady voltage of at least 2.5v. If that doesn't happen the cell is likely toast. Keep a constant eye on things and monitor the temp of the cell.

If the cell is protected it disconnects it if it goes below 2.5v, stopping the cell going dead flat. Applying that small charge should reset the cut out.

Charge each low cell until all hold at least 2.5v, 3.3v better. Once they are at that voltage the standard charger should take over and finish the charge.

I have an opus smart charger that will finish the charge itself.