On Wed, 26 Dec 2012 08:52:11 -0500, wrote:

These I bought at Harbor Freight. 1500mAh are Chicago Electric Power
Systems (in my battery pack) item # 90149, and the 2500mAh (I just bought) are
Thunderbolt Magnum item # 97864.

Links would be helpful.

http://www.harborfreight.com/pack-of-2-nimh-rechargeable-c-batteries-90149.html
Those are AA cells wrapped in excess cardboard in order to fit in a C
cell package. You can tell with a magnet. If it sticks to the
outside of the cell, it's a real C cell and I'm wrong. If it doesn't
stick, it's cardboard. Welcome to Harbor Fright.

http://www.harborfreight.com/pack-of-2-high-capacity-nimh-rechargeable-c-batteries-97864.html
2500 ma-hr is probably a real C cell. However, it's a the low end of
the capacity scale for an NiMH C cell, which should be 3000 to 5000
mah-hr. I would be concerned about quality.

I am aware of partially charged batteries and not to charge
them at the same length of time as charging fully discharged batteries.

Between the changing capacity with age, the unknown state of charge,
and the rather odd cell capacity specification, you have a moving
target.

I may consider rechargeable lithium batteries in the future, but I will do
some research first to learn about their issues.

Forget everything you know about charging NiCd and NiMH. Lithium Ion
charging is very different and difficult. To do it right, you need a
coulomb counter fuel gauge, that counts electrons going in and out,
and adjusts the charge accordingly. You might be able to get away
with something less (as the RC model people suggest). Light reading:
http://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteries
Note that a warm and fully charged Li-Ion battery self-deteriorates.
Spend some time reading here before diving in:
http://www.candlepowerforums.com/vb/forum.php
http://www.candlepowerforums.com/vb/forumdisplay.php?107-Smoke-and-Fire-Hot-Cells-and-Close-Calls-The-dangerous-side-of-batteries
How to make a 18650 battery and flashlight into an accidental rocket:
http://www.candlepowerforums.com/vb/showthread.php?280909-Ultrafire-18650-3000mA-exploded
I've had an Ultrafire (cheap brand available on eBay) go bad on me,
but not explode. That's because there's a mess of electronics under
the negative terminal in the battery that's suppose to prevent shorts
and bangs.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558