Reverse Charging question, my fault?

[sstmax]

Hi there, I've got one of those cheap r/c helecoptors for my son.

The controller takes 6xAA cells and charges the Helecoptor's Li-poly cell from the controller.

I took 6 brand new GP 2700's from their packet, ran a break-in on them on my Maha 9000 charger and put them in the controller. After about a month of light use, I took the cells out to charge them (at 1400mAh) in the hope of preventing any reverse charging nasties given we're talking about a 6 cell pack.

First 4 cells only needed ~5-600ma, 5th cell needed almost 1000ma and the 6th cell needed about 1500ma. All cells terminated ok.

When I pulled the 6th from the charger I noticed that the cell had vented from the -ve terminal. I noticed that it had also vented on the terminal of the controller itself, so this happened prior to charging.

Now, some questions:

How would 1 cell discharge so much more than the others?

Was this just a case of a bad cell or could I have done anything else to prevent this? I thought I was being good as it was charging the cells often rather than letting the controller run the cells right down!

Should I bin the vented cell or try to recover its capacity?

Thanks.

 

[SilverFox]

Hello Sstmax,

Welcome to CPF.

Are you sure your cell vented at the - end? The only vents in AA cells are at the + end. It is possible to vent at the + end and have the electrolyte run down iside the heat shrink and come out at the - end, but you should see evidence of this at the + end also.

Did you happen to record the Break-In capacities you got?

If you did, then you could do a charge/discharge cycle and compare the results.

It sounds like you did everything correctly. You may just have a problem cell. However, by comparing your initial Break In data, you may be able to see it there. If you have the time to do another Break In cycle, you may also see it there as well. Just make sure that if your cell is actually venting that you don't drip on the charger. It makes a bit of a mess.

If the cell has vented, replace it.

Tom

 

[ltiu]

Hi SSTMAX,

I am curious:

I wonder what the runtimes are of the chopper per charge? How many minutes of on/off fly time per charge?

How many charges (to the chopper from the remote) per batch of fresh 6x AA?

 

[sstmax]

Hello Sstmax,
Are you sure your cell vented at the - end? The only vents in AA cells are at the + end. It is possible to vent at the + end and have the electrolyte run down iside the heat shrink and come out at the - end, but you should see evidence of this at the + end also.

Hmm, you've got me thinking. The evidence of venting is only visible at the -ve electrode and is also on the -ve spring contact inside the controller.

The helecoptor was a christmas present and I had originally put 6 alkalines in it due to a lack of nimh cells. I only replaced them with nimh cells when the alkalines died (only 2 cells were actually dead).

I guess it's possible that the alkaline's vented in the controller and the -ve terminal on the GP nimh cell became contaminated from the spring inside the controller?

Do/can Alkaline cells vent from the -ve terminal?

I'll clean the -ve contact on the cell and do a refresh/analyse cycle on it to see if it has lost capacity. I'll report back on this.

Did you happen to record the Break-In capacities you got?

If you did, then you could do a charge/discharge cycle and compare the results.

I didn't record them but am pretty sure that the 6 cells were closely matched.

Hi SSTMAX,
I wonder what the runtimes are of the chopper per charge? How many minutes of on/off fly time per charge?

How many charges (to the chopper from the remote) per batch of fresh 6x AA?

Unfortunately, the helecoptor is too new to know and now it's being run on nimh cells, I'll probably (hopefully) not find out exactly how many charges it'll give the chopper from the remote!

Details on the helecoptor are here:

http://www.firebox.com/product/1420?src_t=sbk&src_id=picoz

internal battery here (only 50mAh):

http://www.rc-spareparts.com/picooz...8.html?zenid=58e79b4712eeb8b34e39adbbd32291ef

Thanks for your help guys!

 

[VidPro]

one could be SELF discharging faster than the others too.
you can get a lot of "out of balance" from self discharge differences just as much as capacity differences when its been a whole month.
(with non LSD cells)

just charge more often, would solve that issue.

as for the negative venting, your assumptions are probably most accurate, an alkaline can lose containment on either end, then the slop from that, hangs around to confuse you 🙂
like silverfox said, a ni-mhy loosing containment on the bottom would be much less likly, along the lines of a miracle occurance 🙂

 

[sstmax]

Did another break-in cycle on this cell and it came out at 2448 which I guess is just slightly low. How much of a dent in the capacity would one expect if the cell had vented? Doing a refresh/analyze on this and another GP cell from the same pack to compare their results and will post these up shortly.

 

[SilverFox]

Hello Sstmax,

If your cells are the GP270AAHC, here is the data sheet for them.

2448 mAh looks pretty good...

Tom

 

[sstmax]

Ah, thanks for that SilverFox.

Looks like the cell is largely unharmed in that case.

one could be SELF discharging faster than the others too.
you can get a lot of "out of balance" from self discharge differences just as much as capacity differences when its been a whole month.

I hadn't thought of that. This cell needed ~2x the charge of the other cells. Perhaps it has a faster discharge rate than the other cells in the pack or maybe the leakage from the alkalines somehow contributed to a faster discharge??