What causes a high self-discharge rate?

[chrome-addict]

I notice that with every 4-pack of AA NiMh's (non-hybrid) I buy, the cells self-discharge at different rates ...it seems there are always 1 or 2 that completely self-discharge (down to or below 1.0v) after 1-2 weeks while some of the others are only down 2-3%. I've noticed the same with cells that have very similar discharge capacities. I guess my question is what causes some cells to have a higher self-discharge rate, and can anything be done to bring them up to the level of the other cells?

Also, should cells be grouped by discharge capacity or self-discharge rate, when used in devices requiring multiple cells?

Any input would be greatly appreciated ...thanks.

 

[VidPro]

some things i know can
reverse charge , like when used in series, and one cell goes down totally, and gets screwed
Dropping it hard on the cement, from hand height.
todays Highest capacity of cells after much use.

Also, should cells be grouped by discharge capacity or self-discharge rate, when used in devices requiring multiple cells?

probably both , as you want to avoid the reverse charge thing that happens when one in a SERIES set is depleated. the better "balanced" in remaining capacity, even after self discharge, the better it would be balanced.

my high-caps have a LOT bigger issue with being self discharged, than loosing total capacity, so i now have them sorted for how long they survive as opposed to the max capacity which is Very similar.

if you could somehow perfectally predict all the behaviours, the cell that will have 2000ma capacity , and the other cell that will ALSO have 2000ma capacity at that TIME of use would be matched. good luck trying to keep up with that.

 

[SilverFox]

Hello Ray,

VidPro has outlined some ways to initiate high self discharge rates. The cell consists of a positive electrode and a negative electrode. These are separated by a separator. The separators job is to insulate the two electrodes, but still allow the chemical reaction to take place.

When the separator breaks down, the self discharge rate of the cell increases.

When you get new cells, the first thing you should do is to give them a standard charge (16 hours at a 0.1C charge rate). Once the cells are charged, let them sit for an hour or so and measure their voltages. Set them aside for a week, then measure their voltages again. If you see a big drop, or an uneven drop in voltages, return the bad ones for a refund or exchange.

Now you can run a capacity check to see if the cells live up to their capacity rating. One week of self discharge with healthy cells at room temperature will only deplete around 5% of the total capacity, or less. If you see a large drop in capacity, put them in the return pile.

As a general indication, new cells, when discharged at 0.5 amps, should have around 90% of their rated capacity.

Once you have done these tests, you can be assured that your cells will perform the way you think they should.

Tom

 

[chrome-addict]

Thanks for the detailed explanation. I guess I just assumed that since they were new, they were automatically good cells. I may just give up higher capacity numbers for more reliable/stable cells. Maybe it's time I make the switch to Enloops.