Interesting look at self discharge (yikes!)

[MrAl]

Hello,

Recently i picked up some Energizer 900mAh NiMH AAA cells (a 4 pack) with
a coupon i got from them after complaining about the bad self discharge
of their 2500mAh AA NiMH cells. Before i used them i labeled them 1 to 4
with a magic marker and of course charged them and recorded the time to
charge each one.

That was about two months ago, on 5/14/07.

What else i did was put three of those cells in a light that takes only
three cells. I used cell numbers 1, 2, and 3. I kept cell #4 in the plastic
pack that they came in and it stayed at about 70 to 80 degrees for all
of the two months.

Now today, the light seemed a little dim, but checking the open circuit
voltage they all read about 1.27v, which doesnt tell me much because
open circuit voltage doesnt always tell you anything, but because
the light was dim i knew at least some of those three cells, 1,2 and 3,
needed a charge, so i charged them one at a time and noted the time
it took to charge each one:

cell#, mins:secs
#1, 5:30
#2, 5:50
#3, 8:15

Remember that these three cells have been used in the light over the
period of about two months.

My intended use of the four cells in a three cell light was as follows:
Cycle1: 1,2,3
Cycle2: 1,2,4
Cycle3: 1,3,4
Cycle4: 2,3,4
This way over 4 charge cycles all cells get used three times.

I also know that NiMH cells have rather high self discharge, so i decided
to again charge cell #4. The result was this:

cell#, mins:secs
#4 , 6:55 (this cell wasnt even used yet!)

It's amazing! Cell #4 wasnt even used yet, yet it required more time
to recharge than two of the others that *have* been used.

To recap:

cell#, mins:secs
#1, 5:30
#2, 5:50
#3, 8:15
#4, 6:55 (this cell wasnt even used yet!)

It's quite amazing, that this last cell required more time to recharge than two of the
others in the pack. It lost as much charge as they did, even though they were
being used and that last one wasnt being used at all.

 

[lumenal]

From the looks of your numbers, I'm sure you're using a 15 Min. charger.

Interesting numbers though...

Perhaps these particular AAA cells have yet to be formed properly?

 

[SilverFox]

Hello Al,

Often new cells have difficulties during charging.

Your results are interesting, but I believe they would be more meaningful after the cells have around 5 charge/discharge cycles on them.

Tom

 

[TorchBoy]

Al, they would be even more meaningful if you could use a Maha MH-C9000 (or "similar") to discharge them before charging to see how much capacity they do have left.

 

[MrAl]

Hello again,


lumenal and Tom:
Yes ok, i'll continue to do the tests as the cells age and see if anything
changes. I was pretty sure that the self discharge was lowest
when new though, which tells me it will only get worse. Then again,
two months isnt too bad i guess, and eventually i will be going over
to Eneloops (or clones) anyway.

TorchBoy:
Well, all the cells were charged up before the first use and the #4 cell
was not used at all during those two months, yet it required just as
long as two of the cells that were used (used maybe one minute per day
over that two month period). I'll keep testing however as the cells
get charged and discharged over and over too see if anything significant
changes.

 

[koala]

Hello Al,

I'm not sure if the Energizers 900mAh AAA you have are the same as mine here downunder. I am very happy with their performance, they do work very well. Their rated capacity is actually very close to the actual capacity. They don't seem to have very high self discharge compared to the 2500s, I have not tested the AAAs self discharge rate.

 

[MrAl]

Hi again,

I guess the only conclusion i am arriving at is that self discharge in standard
NiMH is the same as using the cell for light duty applications over the
same time period. I'm not really saying anything is wrong with these cells
(yet) and i will do more tests as time goes on and i use these cells the
way i normally would.
It's good to know this, however, and here is why i think so...
If, when i first bought the cells, i only charged cells 1,2 and 3, and not
charged cell number 4, that would have used up one cycle of life on only
cells 1 2 and 3. But instead, i charged all four, which meant i used 1
cycle of life (approximately) on all four cells. Then, when i charged the
second time (all four cells) i used up another cycle, so that's two cycles
on all four cells, so i am not saving any cells by using only three, when
i should be. On the other hand, if i only charged 1 2 and 3, then when it
came time to recharge, charge only 1 2 and 4, then 1 and 2 would have
two cycles and 3 and 4 still only one cycle! The purposed useage again
looks like this:
1,2,3
1,2,4
1,3,4
2,3,4

But now i see the date the cells are charged is important too, because
after 60 days of sitting the odd cell out needs charging again anyway.
So this would be the new schedual:
Charge 1,2,3, use 1,2,3
Charge 1,2,4, use 1,2,4
Charge 1,3,4, use 1,3,4
Charge 2,3,4, use 2,3,4
and now after four complete cycles each cell has been charged only three
times (instead of 4) and used only three times (instead of 4). This is
an advantage because the cells will last 4/3 times longer.
Funny thing is, if i end up only having to charge every 60 days and i get
100 cycles (at least) out of each cell, they would last for 16 years
as long as the basic chemistry holds up that long (ha ha), which brings up
another interesting point...how long does a cell last if it's only charged
up once every 60 days? Does the cycle life limit the end of life or does
the time itself do something to the chemicals?

Eventually when i switch over completely to Eneloops or clones i will be more
happy because they fit the way i use most devices i have.

 

[SilverFox]

Hello Al,

NiMh cells store best in a discharged state.

The chemistry is most active when the cells are fully charged. The cells tend to oxidize from the inside, so storing them in a less reactive state should prolong their life.

If you were able to squeeze 500 cycles from them you might be displaying their performance in a museum...

Tom

 

[TorchBoy]

Tom, when are you going to do an AAA Eneloop test? I'm handing out for it.