Blown cell !

Dedicated to the conversion sold by the now defunct Electric Car Company

Moderator: mattcarr

Grumpy-b
Posts: 991
Joined: Sat Dec 01, 2007 12:06 pm

Re: Blown cell !

Postby Grumpy-b » Fri Mar 08, 2013 9:32 am

Not sad, just interested to know whats going on. Very poor cells rise very rapidly ahead of the rest of the pack, and with them doing it early in the charge the current is much higher and the bleed resistor makes little difference, they just head on upward.

Keep on watching, its beter than a lot of TV.

Grumpy-b

genawin
Posts: 66
Joined: Wed Apr 11, 2007 5:39 am
Location: north somerset

Re: Blown cell !

Postby genawin » Fri Mar 08, 2013 10:06 am

I agree Keep watching we all need to know if there's a problem or all is well , i have measured up to 4.4v but i have doubts about my BMS and think the cut backs are the Zivan hitting 132v and nothing to do with the BMS.
Grumpy has said that there is no balancing wilst charging which would explain why voltages vary so much and then they come quickly back into line after charging when the balancing takes place, which surprised me - i used to leave my car plugged in for hours on cut back thinking that it was balancing as most systems do.
Has anyone seen over 100% on thier BDI , i've seen 101 with a flashing spanner until it came back to 100 ?

Grumpy-b
Posts: 991
Joined: Sat Dec 01, 2007 12:06 pm

Re: Blown cell !

Postby Grumpy-b » Fri Mar 08, 2013 10:30 am

Im probably not strictly correct about no balancing during charging, as it does operate but for most of the charge there is such a high current going into the pack, that the small bleed makes no difference, it will start to have some effect when the current is very low at the end of the charge. Hence at a high current the lower capacity cells will go very high quickly. Dont like the sound of a 4.4v

Grumpy-b

genawin
Posts: 66
Joined: Wed Apr 11, 2007 5:39 am
Location: north somerset

Re: Blown cell !

Postby genawin » Fri Mar 08, 2013 10:43 am

"Dont like the sound of a 4.4v" , I agree but no heat was involved and i estimate at less than 2amps - my inductive wouldn't even pick it up.

mattcarr
Posts: 389
Joined: Sat May 12, 2007 2:27 pm
Location: Hampshire

Re: Blown cell !

Postby mattcarr » Sun Mar 10, 2013 4:00 pm

I think the charge current is important. Remeber the pack is in series and all cells are getting the same current. In the bulk charge phase, you can cram in very high currents and the voltage will stay fairly stable. Towards the end of the charge cycle some cell voltages begin to climb higher than others and this is when the bms will begin to bleed individual high cells and when the whole pack voltage begins to climb it then will throttle back the charge current. You have to think in terms of percentage for the bleeding. At 18 amps charge current the 1 amp bleed is only 5% but when the charge current is down to 8 amps them that is more than 10%. Remeber these cells should all be very closely matched. It takes less than 10 minutes for the charge to complete on my car from when the pack voltage hits 86 volts and climbs to 102 volts. I do think the pack in my car is fairly well matched.

genawin
Posts: 66
Joined: Wed Apr 11, 2007 5:39 am
Location: north somerset

Re: Blown cell !

Postby genawin » Sat Mar 16, 2013 6:08 am

P3160001.JPG
P3160001.JPG (23.57 KiB) Viewed 19406 times
Here's the naughty cell
P3060002.JPG
P3060002.JPG (31.11 KiB) Viewed 19406 times
As you can see it welded it's case to this one , i put the pack back together and tested the 2 neighbouring cells for a week wilst obtaining 3 good used cells from Grumpy, they were good.
P3130001.JPG
P3130001.JPG (36.17 KiB) Viewed 19406 times
When the new cells arrived i pulled the pack apart again cleaned all the "extra" casing off them and sealed them up with silicon.So all back to normal and i've got a couple of spare cells.
One thing that seemed odd was both times after i removed and reconnected the BMS wires from the battery tops the charger would repeatedly refuse to play and i needed to disconnect and reconnect the 12v over several cycles which inturn reboots the BMS and then eventually settels down , i have been doing some single cell balancing on the replaced cell to speed up the balance, i toyed with the idea of a bottom balance but i feel that the way the BMS functions this would be un necessary.
I would like to thank grumpy for his help with this problem he is a great asset to our little forum.

P.s. I feel i should point out that some of my methods of doing things mean i take chances and i wouldn't recommend them to others.

Grumpy-b
Posts: 991
Joined: Sat Dec 01, 2007 12:06 pm

Re: Blown cell !

Postby Grumpy-b » Sat Mar 16, 2013 9:35 am

Glad you got there.
Doing a bottom balance is a real pain and takes a lot of effort to achieve, but by doing so you can ensue thatthe cells have taken in a similar amount of current and so are similar in stored energy. All the BMS will do is to even out the voltages, not the amount of energy taken into them all. The best way to let the BMS balance out the cell voltages is to leave the car unplugged for about two days. This will allow the BMS to take the higher voltage cells down to the lowest denominator without interference from the charge cycle.
Good luck, I hope the car goes well now.

Grumpy-b

mattcarr
Posts: 389
Joined: Sat May 12, 2007 2:27 pm
Location: Hampshire

Re: Blown cell !

Postby mattcarr » Sun Mar 17, 2013 10:32 am

Glad that you got your car operational again Mart. That cell looks pretty awful - I can imagine that it took a fair bit of heat for it to end up like that. I suspect that it must have been some sort of internal failure to have caused that.

Now you have got 2 Ev'ie's at least you wont have to fight the wife to get to drive one !!!!.

Matt


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