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 sophisticated chargers.
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
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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 discharging rates.
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.