Matt Gruber
1 MW
That's when cell voltage goes below zero, and reads negative, during deep discharge.
Have u seen this?
Have u seen this?
Have you seen cell reversal?
- Thread starter Matt Gruber
- Start date
Ypedal
100 TW
Yep.. got one right next to me here..
Nimh Nexcell brand D cell. Spewed white crystals and went reverse.
Nimh Nexcell brand D cell. Spewed white crystals and went reverse.
Matt Gruber
1 MW
how many volts reverse?
what did it do to pack amps and volts?
what did it do to pack amps and volts?
Ypedal
100 TW
What i suspect happened, not 100 % sure.. but.
The pack was left sitting for a month or 2 while i was using some Nicad packs for testing.. Nimh self-discharge something awful.... it's insane..
So.. after sitting for so long, i put them on the bike to drain them a bit before re-charging, never recharge an already fully charged nimh pack.. it will OVER charge..
So.. when i went for a ride, i noticed really slugish performance, and the DrainBrain confirmed my suspision.. more voltage sag under load than usual.. and lower operating voltage by 1 or 2 volts...
I put them back on the chargers.. ( 2 x 36v 8ah packs in series at 72v ) .. and after an overnigh sit.. there was a 1v difference between the packs..
A few e-mail to justin and i decided to dig into it.. opened it up and it was easily aparent i had a bad cell...ie: white crystals.
---
Sidenote for NIMH users : .. if you leave your bike sitting for a while.. take it easy for the first few rides.. drain it very slowly and then re-charge.. after 3 or 4 cycles you are good to go with full capacity, but packs become un-ballanced when sitting
The pack was left sitting for a month or 2 while i was using some Nicad packs for testing.. Nimh self-discharge something awful.... it's insane..
So.. after sitting for so long, i put them on the bike to drain them a bit before re-charging, never recharge an already fully charged nimh pack.. it will OVER charge..
So.. when i went for a ride, i noticed really slugish performance, and the DrainBrain confirmed my suspision.. more voltage sag under load than usual.. and lower operating voltage by 1 or 2 volts...
I put them back on the chargers.. ( 2 x 36v 8ah packs in series at 72v ) .. and after an overnigh sit.. there was a 1v difference between the packs..
A few e-mail to justin and i decided to dig into it.. opened it up and it was easily aparent i had a bad cell...ie: white crystals.
---
Sidenote for NIMH users : .. if you leave your bike sitting for a while.. take it easy for the first few rides.. drain it very slowly and then re-charge.. after 3 or 4 cycles you are good to go with full capacity, but packs become un-ballanced when sitting
safe
1 GW
- Joined
- Dec 22, 2006
- Messages
- 5,681
This is a strong argument for using "tubes".If you were using "tubes" to contain the cells you could have done a "regular checkup" of the cells and probably found this one as being a problem early on and removed it. It certainly makes maintentance a lot easier if you can physically remove your packs and look at the cells individually.
I've had it happen with NiCd's before. The reversed cell developed a high resistance.
I'm sure it's happened with some of my SLA's when they were run completely flat, but they seem to survive that.
I'm sure it's happened with some of my SLA's when they were run completely flat, but they seem to survive that.
xyster
10 MW
Yep. Seen it. Felt it. Melted it. The PTC's cut off the power so it never read negative, just dead. Top row, 2nd from left, ruptured li-ion 18650.
<img src="http://endless-sphere.com/forums/files/img_1872_432.jpg">
<img src="http://endless-sphere.com/forums/files/img_1872_432.jpg">
Matt Gruber
1 MW
DEAD can be OPEN or SHORTED.
I've tested 3 nicad tool packs and every bad cell shorted on deep discharge. at least 10 cells.
I've never seen reversed voltage, but i am looking...
I've tested 3 nicad tool packs and every bad cell shorted on deep discharge. at least 10 cells.
I've never seen reversed voltage, but i am looking...
xyster
10 MW
The ruptured cell probably shorted. The heat was intense enough to melt the can as it expanded, and cause smoke to be emitted.
Ypedal
100 TW
I can test the cell again when i get home.. but if i recall correctly...
1- The cell had vented..
2- under no-load it showed 0.xx mv
3- under load ( the whole pack.. under 50w ) showed -0.xx mv while the rest of the pack went on about it's business.
1- The cell had vented..
2- under no-load it showed 0.xx mv
3- under load ( the whole pack.. under 50w ) showed -0.xx mv while the rest of the pack went on about it's business.
Hello
Yes I suspect one of my NIMH packs has one cell that is farting in the wind on me, I am seeing sag under load that is a lot worse than anything like it was before, I am going to do a little more testing and then if it still does it they are coming off and will get stripped and prodded.
I can run my machine on my lipo packs for now which isnt a problem nice backup to have eh but I actually only use the Lipos for recreational use my NIMH packs are my day to day batteries so they have to work.
I will strip the pack, load it up and then check with a meter across all the cells to see If I can spot the bad boy. They say you are not supposed to swap out bad cells and you should replace the whole packs if this happens
wont be doing that at the price we pay for NIMH!! ha ha it will get the bad cell swapped and be happy with it!! he he.
Other than that I have been very happy with my NIMH batteries and I think for the price they are still great and very high performing batteries.
Thanks for these posts its kicked me up the backside to make me sort mine out now, got to get my bike A1 for the spring.
Cheers
Knoxie
Yes I suspect one of my NIMH packs has one cell that is farting in the wind on me, I am seeing sag under load that is a lot worse than anything like it was before, I am going to do a little more testing and then if it still does it they are coming off and will get stripped and prodded.
I can run my machine on my lipo packs for now which isnt a problem
nice backup to have eh but I actually only use the Lipos for recreational use my NIMH packs are my day to day batteries so they have to work.I will strip the pack, load it up and then check with a meter across all the cells to see If I can spot the bad boy. They say you are not supposed to swap out bad cells and you should replace the whole packs if this happens
wont be doing that at the price we pay for NIMH!! ha ha it will get the bad cell swapped and be happy with it!! he he.Other than that I have been very happy with my NIMH batteries and I think for the price they are still great and very high performing batteries.
Thanks for these posts its kicked me up the backside to make me sort mine out now, got to get my bike A1 for the spring.
Cheers
Knoxie
Matt Gruber
1 MW
i find with nicads, the cells never reverse (at least for me)
they short, and don't have to be removed. i've run tools for years with a shorted cell. it has only a tiny effect of 1.2 less volts.
to maintain voltage, just add a new cell EXTERNAL to the pack. wire across the bad cell(short it for sure so as to not confuse the charger). this avoids soldering on the good cells, which i've seen, can cause early failure if not done just right.
For tools where an external cell is in the way, I tear off the tab from the bad cell so i dont have to solder a wire on the old but still good cells. Then solder the tab on the new cell.
they short, and don't have to be removed. i've run tools for years with a shorted cell. it has only a tiny effect of 1.2 less volts.
to maintain voltage, just add a new cell EXTERNAL to the pack. wire across the bad cell(short it for sure so as to not confuse the charger). this avoids soldering on the good cells, which i've seen, can cause early failure if not done just right.
For tools where an external cell is in the way, I tear off the tab from the bad cell so i dont have to solder a wire on the old but still good cells. Then solder the tab on the new cell.
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