Generally, as long as the battery pack doesn't get hot, the charger probably
isn't causing any damage to the cells.
When the charge is completed, if the battery pack was warm, it should cool
down to ambient after it's been on a trickle charge rate/cycle for a while.
If the battery remains much warmer than ambient while trickle charging (an
hour after a full charging cycle), the trickle current is probably a bit
excessive. Trouble is, ya can't do much about it, as far as modifying most
The trickle charge level only needs to be high enough to cancel/eliminate
the self-discharge rate of the cells (both NICAD and NIMH), although some
new NIMH cells claim to have zero or very low self-discharge rates.
See the Maintenance Charge section
There are many different grades of high quality cells, and some are designed
to withstand a constant, but reduced charging current, rapid charging and/or
rapid discharging. Trouble is, ya don't know for sure which grade of cells
are in a battery pack unless you selected and installed them yourself from a
quality manufacurer such as Panasonic or Sanyo.
Some high quality cells can cost over $24 per cell (1.2V per cell x 10 cells
for a 12V pack), so you can understand why so many folks experience
rechargeable battery problems.
Poorly designed chargers and low grade cells result in short battery pack
lifetimes, even if the batteries are neglected and/or abused.
Trouble is, when most cells get weak, there isn't any way to rejuvenate
them, so replacement is the only practical option to fully restore the
battery pack's capacity.
Since many new battery packs cost almost as much as a brand new tool with
new batteries, landfills continue to fill up with discarded tools, and
groundwater-contaminating/poisoning battery packs.
Some chemistries of battery packs, such as cell phone and laptop batteries
can be restored with the proper, really expensive equipment (Cadex).
Most consumers don't have any way to gauge the condition of their battery
packs. Most battery tester/checkers with a meter, or a few LEDs don't show
the condition of the internal cells. Batteries that test good on these types
of testers can go dead in a matter of seconds.
Some chargers made for the Remote Control hobby users are somewhat
sophisticated, in being able to show mAh capacity during a controlled
discharge. Some models allow the user to select charging rates, etc. I have
one model that does these functions, the MRC Super Brain 977.
There are more advanced analyzers that connect to a PC and plot charging and
Rebuilding battery packs isn't complicated for handy-type users with
soldering skills, if they're willing to buy good cells with tabs, to avoid
soldering directly to the cell terminals. There are usually several nearby
places where bad cells can be dropped off, so they can be recycled or
disposed of properly (incinerated?).
Many universal chargers switch from the charge rate, to a trickle rate when
a delta/volt level is sensed, so many models can be used for NICAD or NIMH.
There is always the possibility that battery packs can be overcharged or
undercharged, and most users won't know.
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