Battery pack repair

[cwah]

I'll balance charge first. Then discharge it to see if there is any weak cell (i think it probably has at least one).

Then replace the faulty cell with the other pack.
Then balance charge again and leave it for 24h fully charged to see if there are any self discharge issue.

Then cycle again and give it to her lol.


But I'm thinking to give her 1 pack with her current cell and the other one with zippy compact. Should work good then?

 

[Synthman]

That looks exactly like the slightly used battery I bought last year. The BMS supplied doesn't seem to have a balancing feature, and it wasn't quite charging to 42v and taking a very long time to do so. What I did was remove the BMS and swap it one I had from an older battery.

 

[wurly]

Would it be a good idea to measure the internal resistance of each cell? it wouldn't take long and you could at least see if they are comparable to each other. You could do the same with the other pack and hopefully make one pack from two.

 

[cwah]

Yes I'll check it once it's balance charged. My charger can do that.

After more than 12 hours balance charging... it's still far from being finished....

Damn that so long

 

[cwah]

Anyone know where I can buy these epoxy glass sheet for cheap? I've seen on ebay and they all look expensive

 

[cwah]

After 15 hours the balance charge isn't still finished. And I have another pack to repair.

I'm thinking to directly discharge the high voltage cell with a 3 ohm resistor.

So during balance charge, if I can see 1 cell charging much faster, I should be able to directly detect the faulty cell.

What do you think of this circuit? It would be a 4.15V cell with 3 ohm, so I = 4.15/3 = 1.3A. The red and black clamp directly go to the high voltage cell.

Would that work?

 

[oigoi]

It would work if the wattage rating of the resistor is high enough 4.15V X 1.3A = 5.395W
Do you know how many watts the resistor is rated at?

 

[cwah]

Its rated at 25W so it should handle that easily. I just wasn't sure the wiring is correct I don't want to create a short lol

 

[Deleted member 4366]

That should work , Cwah.

Some chargers struggle to balance when one or more cells are on maximum, so it's a good idea to discharge the high one.

I just got a battery with the same cells. One cell is down to 0.7v. All the rest are over 4v, so it' doesn't look good for that cell. My charger can charge the one on its own, so we'll see what happens, but I think I'll be looking for another cell.

 

[cwah]

I've tried it and the metal plug became red after few seconds...

I've looked at it again and realised my resistor is 0.3 ohm and not 3 ohm....... So I've put 60W on my cell for few second.

It went down to 4.1V... Let's re-do it

 

[cwah]

My little discharge trail

So 0.3ohm * 6 = 1.8 ohm.

I discharge at 2.3A or 9W. If the cell is 8AH, it means 0.25C.

 

[Scimitar]

What is a gerbil? I currently have a Proxxon to cut but it doesn't cut big items or very straight

Neither does a gerbil.
If you squeeze them tight to straighten up the cut, it just seems to make it worse, somehow.

 

[cwah]

Ok so what do you recommend to make clean cut? i haven't found the tool yet

 

[oigoi]

Anyone know where I can buy these epoxy glass sheet for cheap? I've seen on ebay and they all look expensive

You could use polycarbonate sheet instead

Clear Polycarbonate Solid Polycarbonate Sheet A3 (297x420mm) inc, FREE DILIVERY | eBay

 

[cwah]

Oigoi, polycarbonate sheet should not be used as first layer, because it's plastic it just melt under high temperature. Epoxy glass are used because of it's electric insulation, ability to support high temperature and being flame retardant.

 

[cwah]

I finally finished balance charging.... pffiiioouuu!

I doing a discharge test now:

The 2 red cells seem to be slightly lower capacity... but still good? Do you think it's a BMS issue? I'm thinking to maybe just change the BMS to the one sold by Bestekpower.

 

[Deleted member 4366]

0.04v difference is nothing. It just looks a lot because of the scale of your graph. You need to run them down to about 3.3v to see if there's a difference.

 

[oigoi]

Oigoi, polycarbonate sheet should not be used as first layer, because it's plastic it just melt under high temperature. Epoxy glass are used because of it's electric insulation, ability to support high temperature and being flame retardant.

By the time it gets to a high enough temperature to melt the polycarbonate your wiring will all be fried anyway!

 

[cwah]

Ok let's wait and pray then....

Oigoi the main case is aluminium. There is no point for me to use polycarbonate as protective layer.

 

[Geebee]

A Dremel/Proxxon can do a nice straight cut but it does requires practice and the correct speed for the cutting tool, mind with the panel on the unit in the carboard it wont have to be perfect.