saving reverse charge NiMh's

[Packhorse]

I have a 34 x F (14ah) NiMh pack. Max voltage is 50volt.

Its 5 or 6 years old and has been working well but I did a discharge test at 2/10C and measured the voltage across all cells. I found 4 that were reverse charges at -0.3volt.

They are Nexcell cells and use a Nexcell 42v 3a charger.

EDIT: What if I simply removed the fault cells? I wonder how the charger will react.
Sure, I will loose capacity but will I be better or worse off?



So, what can I do?

Can the faulty cells be saved?

I guess I could replace the faulty cells but I dont know how long the other cells will last.

I dont want to rebuild the whole pack. I'd rather rebuild it with Li Ion but it will be costly.

Thoughts??

 

[SilverFox]

Hello Packhorse,

0.3 volts on the negative side isn't that far off from 0. There may be some life left in them.

The first step is to do a 16 hour charge at a 0.1C charge rate. With a battery pack in poor condition you may want to extend that to 24 hours. This charge will make an attempt to balance all the cells in the pack and give you the best chance of recovery.

Follow this with your 0.2C discharge and then check the cells again. With that many cells in series it is difficult to pick a shut off voltage that eliminates the possibility of cells going into reverse polarity and balancing that with decent run time. There is no problem removing cells from the pack as long as what it is powering still will work with the reduced voltage.

Tom

 

[m4a1usr]

I have a 34 x F (14ah) NiMh pack. Max voltage is 50volt.

Its 5 or 6 years old and has been working well but I did a discharge test at 2/10C and measured the voltage across all cells. I found 4 that were reverse charges at -0.3volt.

They are Nexcell cells and use a Nexcell 42v 3a charger.

EDIT: What if I simply removed the fault cells? I wonder how the charger will react.
Sure, I will loose capacity but will I be better or worse off?



So, what can I do?

Can the faulty cells be saved?

I guess I could replace the faulty cells but I dont know how long the other cells will last.

I dont want to rebuild the whole pack. I'd rather rebuild it with Li Ion but it will be costly.

Thoughts??

I've been building Nimh packs for more than a decade and I can tell you from experience that any cell that falls below resting voltage is going to end up being junk at some point and should be removed from any multi cell pack ASAP. Having lost way too many packs due to cell reversal it's been quite the lesson. And not just to the pocket book. Resting voltage for any Nimh cell is right around .9 volts. They are after all a 1.25 cell and are literally drained for all intent and purpose for use when at 1 volt. One of the problems I see with you setup is you do not have the ability to do a slow charge. It's well known that multi cell Nimh packs need a maintenance charge every 60 to 90 days when not being used. And it's a half amp (500mah) charge. Not a 3 amp charge.

If your continuously charging your pack at 3 amps I'm not surprised at what you are experiencing. Do google search and your going to find lots of information on the various RC web pages regarding Nimh cells. The German racing teams have had the best data to pay attention to. From Fast Electric Boats to F5 pylon racing aircraft. Some extremely savy guy's who learned and past along their lessons learned. Ironically right now as I type this I am doing maintenance charging on my remaining Nimh packs. Do your self a favor and get a better charger if your staying with Nimh packs. Myself I have been using an AstroFlight 112D now for 10 or so years and you can adjust the charging rate to what you need at the moment. Not what you stuck with. Plus you can discharge your pack up to 8 amp hour

 

[Packhorse]

The charger I have has several different charge modes but of course I can not access any other than putting it on charge and letting it do its thing.

1st is charge at 3 amps
Then cool off for 2 hours
then 500ma for 2 hours
then trickle charge at 150 ma for 5 hours
then it goes into storage mode of 40ma for 6 days
then it wakes up and goes back to 150ma for 5 hours.

http://www.dive-xtras.com/pages/support/chargers/chargermanuals/cudachargermanual.pdf

So it is a "smart charger"

I have left it on for 20 hours so should have made it well into the sleep mode.
I will try a slow charge over the next few days on the individual faulty cells and see how that goes.