Useable Mah For NiMh

[stefanj]

OK My BC900 just finished trying to save some Tenergy 2600 Mah AA NiMh batteries. I refreshed them at 1000Mah charge/ 500 discharge. Took 12 hours for the first cell to finish (the other 3 were still running when I left for work). Sarting voltage (all off the charger screen) was 1.12v. First run Mah was 514mah. Finished Voltage was 1.41 and 1737 Mah. Is this battery saved? Would it benefit from another refresh? I really like this charger as I was able to figure out how to set it up pretty quickly. I took periodic Cell temps with my infrared therometer and none of the cells got warmer than 102° F.

 

[Turbo Guy]

First run Mah was 514mah. Finished Voltage was 1.41 and 1737 Mah. Is this battery saved? Would it benefit from another refresh?

Everyone has difference opinions as to how to judge a battery for fitness for duty. For most of my AA / AAA applications capacity fresh off of chartge means little to me. I have this habit of charging cells ,placing them in a bag with the charged date on it and then pulling a cell or two each week and running discharges at close to the same rate the cells will be used at.

The BC-900 has a Refresh mode that will automaticly cycle the cells until they stop improving. If you want a faster cpacity test the Test mode does a fair job. One problem with BC-900 is that it always tests warm cells which if fine for charge and go.

 

[Hoggy]

First run Mah was 514mah. Finished Voltage was 1.41 and 1737 Mah.

Are you sure you did a "refresh-discharge"? 12 hours would suggest you did, but the above comment says you might have done a "discharge" - unless you just didn't record the in-between numbers(which is also very likely).

Anyhoo - to try and bring well aged cells up to par, I'll first try to do a break-in on the C9000, then a 'refresh' on the bc-900.
If you don't have a C9000, what I would do is a 'refresh-discharge' on the bc-900 at 100dis/200ch(AA&AAA). Doing a slow discharge like that (and especially at a pulsed 500ma with the bc-900(vs. the C9000 pulsed 1000ma)), should break up much crystalline formations within the cell. At least that is what I've gathered from other, more knowledgeable CPF members.

As far as when to throw out a battery - that's your call.
I think the normal suggestion here of 80% of rated capacity is too rigid for many folks that can use the lesser batteries in less demanding applications. BUT, that is totally up to you and your situation.

 

[LEDAdd1ct]

I think I saw the 80% figure for lith-ion cells. I am not sure if the same recommendation applies to Nickel-Mh. But of course, one's own budget probably dictates when a Ni-Mh cell really is "gone."

 

[Hoggy]

I think I saw the 80% figure for lith-ion cells. I am not sure if the same recommendation applies to Nickel-Mh. But of course, one's own budget probably dictates when a Ni-Mh cell really is "gone."

Well, among others, Silverfox often talks about the 80% figure for discarding NiMH, in particular. But he has a definite need for top-quality cells and seems to have too many to want to deal with the 'lesser' ones.. Others may/may not be in such a situation though..

 

[Turbo Guy]

Nicad,NiMh.LiIon,LiPoly,LiFePO4 it really makes no difference the standard recomendation to remove from service wheh cells drop to below 80% of their original capacity.

As the cells age they develop crystals in them which inturn leads to the seoerator(insulator) breaking down and micro shorts developing. These lead to increased self discharge rate. Also as the cells age (are cycled) the plates (simple term) materials are consumed. This leads to reduced capacity and increased IR (internal resistence). Increased IR leads to increased temperatures during charging and discharging (at high rates) and also leads to lower voltage under load.

Each much weigh the replacement cost versus safety and performance.