Nimh battery not as full as alkaline's?

Hi all,

I just bought a two sets of 4 Energizer 2500mAh batteries yesterday. One
set came with a charger. I used that charger to charge both of the sets.
It took nine hours to finish charging each set (as indicated by the
turning off of the LED light on the charger and the warmth of the
batteries). Then I used a small, handy battery tester to see how "full"
the batteries are. Sadly my tester says they're just above "replace". I
remembered I also tested one of the sets right off the package and I got
the same indication. It seems like the charging has no effect on the
batteries. I also put a pair of the batteries into a flash light. It
turned out that the light was dimmer than that came out with the pair of
the no-name alkaline batteries originally sitting in that flash light.

Is there something wrong with the charger? Shouldn't my tester have its
pointer moving all the way across the "good" area of the tester, just
like any new batteries will make it to and show that the batteries are
full? Do I need to re-charge the batteries?


Thanks in advance for your input.



JC

On Sun, 11 Feb 2007 19:30:09 GMT, JakeC wrote:

> I just bought a two sets of 4 Energizer 2500mAh batteries yesterday. One
> set came with a charger. I used that charger to charge both of the sets.
> It took nine hours to finish charging each set (as indicated by the
> turning off of the LED light on the charger and the warmth of the
> batteries). Then I used a small, handy battery tester to see how "full"
> the batteries are. Sadly my tester says they're just above "replace". I
> remembered I also tested one of the sets right off the package and I got
> the same indication. It seems like the charging has no effect on the
> batteries. I also put a pair of the batteries into a flash light. It
> turned out that the light was dimmer than that came out with the pair of
> the no-name alkaline batteries originally sitting in that flash light.
>
> Is there something wrong with the charger? Shouldn't my tester have its
> pointer moving all the way across the "good" area of the tester, just
> like any new batteries will make it to and show that the batteries are
> full? Do I need to re-charge the batteries?

There's probably nothing wrong with the batteries. Fresh alkaline
batteries start out at a voltage just above 1.5v and just charged
NiMH (and NiCD) batteries measure slightly above 1.2 volts. So
your tester and flashlight are both showing you the expected
results. But as both battery types are used, the voltages will
drop, and if you have two flashlights running simultaneously, one
powered by alkalines and the other by your NiMH cells, before too
long the alkaline powered flashlight will become much dimmer than
the one powered by the NiMH cells. When used in devices that use
very, very small amounts of energy, both battery types will last
about the same amount of time. Cameras, on the other hand require
moderate to large amounts of power, and for these battery hungry
devices, alkaline AA cells are completely outperformed by high
capacity NiMH cells, usually lasting twice as long in some recent
cameras, and the difference can be much greater in older cameras.

I have a battery tester app. that runs on my Palm PDA, and it's
configurable, allowing several different battery types to be
selected. Same goes for some electronic devices (usually radios)
that have a switch to allow you to set it to "alkaline" or
"rechargeable". This allows them to show a more accurate battery
status than your voltage tester, which apparently is designed only
for alkaline batteries. Some testers are cruder than others. For
example, I have a small, inexpensive tester that uses the same
resistive load for all battery sizes, AAA, AA, C and D. Because the
smaller batteries have higher internal resistances, fresh, smaller
batteries won't move the needle as high as fresh, larger batteries,
even though they should appear the same. Better testers have ways
to present different resistive loads to different size batteries,
making them a little bit more accurate. The bottom line is that
your battery tester can provide misleading results unless you know
more about how batteries really perform. If you're interested,
there are some pretty good charts on the battery data sheets that
can be downloaded from Energizer's website. They're PDF files.