NiMH break-in

[Prune]

I read in several places that while NiMH have no memory effect, brand new batteries usually need a break-in to get to a full charging state -- a couple of cycles of discharging (to about 1V) and slow charging (i.e. not using a fast charger even if it is a smart charger); I've even seen it written on the package of one brand of NiMH batteries I bought. (However, I find that I cannot discharge them fast even if I short + to -, because these batteries have a very large output resistance.) Can anyone comment on this break-in thing? Is it really necessary? Can it be shortened?

[Prune]

Hmm, according to this, it's true. Reading this whole battery book, what a mess. And apparently fuel cells will not be able to replace batteries in many applications.

Maybe when the ITER spinoffs begin operation in thirty years we'll have enough energy to spare to put the inefficient antimatter production in large scale, and then we'll finally have a perfect portable power source (I think currently the maximum achieved efficiency is much less than 1% of energy converted to antimatter in accelerator experiments).

[setmenu]

What capacity is the cell you mention?

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[Prune]

My AA cells are 2.3 Ah, AAA are 0.8 Ah, and 9V format are 0.2 Ah. I don't see why it matters.

[joe carrow]

I think that shorting positive to ground is discharging them "fast", at least, as fast as they will go on their own.

I had a NiMH charger that would do fast discharge, and charging at different rates. It ran a fan during discharge, but I don't know if it was applying any sort of negative potential or just shorting it through some sort of shunt.

As far as whether or not it's necessary- that's a matter of performance. Are the batteries performing as well as you would like? If not, what's wrong with them? There's still some improvement to be had with the best lithium batteries; though there is some overlap between weight/energy and volume/energy ratios for good NiMH and bad Li-Ion batteries.

Joe

[setmenu]

I f I understood correctly, shorting batteries to discharge them is not a good idea!

This site is has plenty of helpful 'care and feeding' information about all manner of cell chemistries:


http://www.batteryuniversity.com/index.htm




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[jc2]

I have worked as a battery engineer for the last 5 years for a cell phone comapny and have done quite a bit of testing on NiMH and Lithium Ion batteries. For NiMH you generally see a rise in capacity over the first 5-10 full cycles of use, but the rise is on the order of 3-5%. So basically you don't gain much. Second, I certainly wouldn't short any battery to do a fast discharge, the cell impedance of a NiMH battery is on the order of 50milliohms or so, so by shorting it you will be getting many Amps, and those cells are generally not designed for currents higher than 1C rate or 2.3A in the case of the AA size. Too much current, especially a constant current will cause heat buildup and gas to form inside the cell and damages the cell.

The things you want to do to prolong your NiMH batteries is to not let them overcharge, the common method of charge termination of -dV/dt overcharges the cells by 5-10% depending on the detection threshold, anything more than that just heats up the cell and deteriorates the performance. Don't store NiMH in a discharged state, the self discharge will continue to drain the battery until they are useless.

[joe carrow]

Jc2, thanks for saying something that's actually informative

And shame on me for not pointing out the hazard of shorting a battery- I really do know better

(from now on, I'll try not to post before coffee)

[Prune]

Will someone post a discharge circuit? I cannot spend $125 US on a Cadex conditioning charger.

[setmenu]

Originally posted by Prune
Will someone post a discharge circuit? I cannot spend $125 US on a Cadex conditioning charger.

The simplest thing I can think of is to just select a suitable load
resistor , and discharge the cell/pack at the rate recommended by the cell manufacturer.
Usually the data sheets seem to consider a cell voltage of 1-1.1 v
fully discharged.

Depending on how many cells your pack contains you may be able
to find more economic conditioning charger sold for radio control
model applications.
That said, if you intend to use nimh cells for a number of applications purchasing a good one will pay off eventually by
keeping your cells in top condition.


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