A charger’s simple red/yellow/green light usually isn’t the best way to judge battery health.

The main signs a NiMH cell needs replacing are:

• it no longer holds a useful charge for long
• its internal resistance has increased noticeably

As NiMH batteries age, their internal resistance rises, which hurts performance under load. A good rule of thumb is that if internal resistance has roughly doubled, the cell is nearing end of life. Some battery analyzers/managers can measure this and may also offer conditioning/rejuvenation functions.

In practical use, if a set of batteries starts running down much faster than before, it’s time to retire that set. It’s also smart to keep batteries in matched sets and always use/charge them together. Then when one set stops performing well, replace the whole set rather than mixing old and new cells.

So yes—there are better methods than the charger light: use a battery analyzer/manager, or judge them by real-world runtime and retire matched sets when they no longer hold charge well.