Hi again,
NiMH cells can take quite a bit of current during charge, although they heat up during the process.
For example, a 2500mAh cell can take 10 amps for a very fast charge, at least one fan is required to keep the
cells cool during this time. More typical max current settings would be 1.25 amps for a 2 hour charge,
and 2.5 amps for a one hour charge.
The charge time actually has to be slightly more than the exact calculation...for example a 2500mAh cell
charging with 2.5 amps theoretically should actually take about 1.1 hours to complete, not 1 hour...just something
to keep in mind.
If the cells appear to get too hot then use a small fan to cool them and that should do it.
You may want to look for a better PNP transistor that can handle the required current.
If you were instead talking about the tolerance of the cell as purchased as to the ACTUAL mAh rating vs the
published rating, well that varies by quite a wide margin. The more important thing i have found in the past
is not so much the rating itself, but what has come to be known as the "Self Discharge" of the cell.
The self discharge is a big pain in the neck, because we charge the cell up fully one day and the next week
we find it only has as little as 80 percent of the full charge left already. The week after that it can be as little
as 50 percent of full charge or even less simply because the cell discharges all by itself even without any load
whatsoever connected to it.
The best (and probably the only) way to get around this is to purchase cells that are made to have low self discharge.
The biggest name brand that has this feature is called "Eneloop" and they come in AA and AAA sizes. There are
other brands too however, like Kodak and i think even Rayovac makes these types too now.
The rating itself is a bit lower, like around 2000mAh, but they work so much better it's almost unbelievable.
They really do keep their charge for a long time too, like regular alkalines, and i did a long term test myself to
verify this (i dont take any published ratings on cells for granted anymore after my experiences in the past with
various types and sizes).
These cells are especially good in devices that draw little power, as they dont die down anywhere near as fast as
the 'regular' NiMH cells.
BTW the low self discharge cells have something to that effect written right on the package so you know that they
are low self discharge. Some of them say, "Low self discharge", and others say something like, "holds charge longer",
or something to that effect.
The worst cells for self discharge were apparently the AA Energizer 2500mAh cells. I've heard many complaints about
these cells from many people over the past few years or so. I dont know if they improved them by now or not,
but back when myself and many others tried them we found that they discharged so fast they they were almost
not useful for anything that did not have to eat up all the power the very same day they were charged.
I wrote to the Energizer company and they sent me a coupon for 10 dollars, which i used to buy 4 AAA size NiMH.
Even better yet are the Li-ion cells, which need a different charger but hold their charge for a long time and always
seem to work well, and also have a more predictable discharge curve so you know when the battery is getting low.
Of course their terminal voltage is a bit higher, so the device they are to be used in has to be retrofit a little to get
going. Some devices are very easy to modify, while some are not so easy at all. LED flashlights that use two AA cells
are very easy to modify for example.