Battery pack repair

[cwah]

I can get 8 AH out of the pack:

I think I should get a new BMS..... Shall I get it from Bestechpower or EvAssemble?

Geebee, I always thought it wasn't convenient to cut it with a rotating tool. Like cutting wood just burn the wood and cutting metal just wear so quickly the bits. It's good when it's very tiny task but when it's bigger it's not very funny.

 

[cwah]

it passed the charge no problem:

it seems good? Going to wait 24h fully charged to see if there are self discharge issue.

 

[Deleted member 4366]

I balanced and charged my one. It's identical to yours. Even the colours of the wires are the same. Mine has one bad cell. If left disconnected after charging, it self-discharges slowly, so that's probably what's happening with yours rather than a problem with the BMS. Check it every day for a few days to see what happens. 3.85v at the drop-off seems very high compared with other cells.

 

[cwah]

I think it's at 3.75V that every cells fall in capacity. It has 8AH capacity which is very correct.

I'm going to leave it completely charged until sunday and see what happens

 

[cwah]

After 24 hours sitting fully charged, battery 1 cells dropped between 4.12V and 4.13V:

Battery 2 seems to be weaker, with 3 weak cells:

 

[Deleted member 4366]

After 24 hours sitting fully charged, battery 1 cells dropped between 4.12V and 4.13V:

That's nothing, and it's quite normal. My one drops from 4.05v to 3.85v overnight.

 

[cwah]

So shall I conclude it's a bms issue? I have the same result with the pack number 2

 

[Deleted member 4366]

I can't see anything wrong at all, other than they all see a bit low on capacity. Maybe they self discharge over a long time, and they need to be left on the charger for a long time to re-balance. If you switch off the charger immediately it goes green, they won't have a chance to balance. You can see how long it tool when you did it. I suspect that the batteries had been sitting around for a long time and were out of balance before she got them. She should try one now to see if it's better.

 

[cwah]

But how do you know it's not a bms issue? Bms should start bleeding as soon as 1 cell reach 4.2V right?

 

[cwah]

She also told me initially she was able to do 25km with 1 charge. And now she can only do 5km.

So it was balanced initially right?

 

[Deleted member 4366]

I think that the bleeding starts before 4.2v because I was charging a battery, and when it was nearing fully charged, the bleeding had already started. I could feel the resisors had become quite hot. I''m not sure about the exact sequence and how the bleeding works. Maybe it starts at about 4.15v and the charger switches off at 41.7v and the BMS cut's off if any cell goes over 4.2v. There's lots going on all at once. If you have a high cell, bleeding starts at 4.15v, but the charger is pumping it up faster than it can go down. The BMS will cut off when it reaches 4.2v and the charger will go green. That cell will then bleed down to 4.15, so you've only balanced one cell by 0.05v, so you'd have to keep re-charging to allow the other cells to catch up all the way, or leave your charger on for a long time.

 

[cwah]

Ok I'm going to charge it full and see if resistors get hots so I know if it's bleeding or not:

I would prefer to send her a working battery rather than going back and forth to fix it.

 

[cwah]

I tried a bulk charge with the bms plugged in...

Charge stopped when 1 cell reached 4.2V. Cells are ranged between 4.13 and 4.2V. But I didn't feel any heat on the resistors.......

I think it's not auto-bleeding?

 

[Deleted member 4366]

You know that the bleeding resistors are on top of the cells and not on the BMS PCB? I can't see them on any of the photos. Is your BMS PCB a double-decker?

 

[cwah]

I can't see any resistor on top of the cells:

Here is the other side of the bms:

I think the resistors are the ones written 390?

 

[Deleted member 4366]

That's them, so you can only feel the heat with the PCB detatched, which is OK for charging. The Ezee battery has exactly the same BMS, but there's nothing on the back of the PCB. Instead, they moved all that stuff to the cells.

 

[cwah]

hmm... I do not feel any heat.... maybe because bleeding too slowly?

 

[Deleted member 4366]

Put the charger on.

 

[cwah]

I did that it remains cold

 

[cwah]

Ok, here is the values from celllog:

BMS stopped charging since few days.

I plugged the charger in for 24h, and didn't see any difference. Then disconnected the charger for 18h to see if there is any auto-bleeding... nothing.

Charging is stopped and bleeding is not happening. Resistors are cold. Cell value remains the same.

I'm going to replace the BMS.

What's the best brand? Bestechpower? Or bmsbattery?