A Tale of Rechargeable Batteries: Updated

Since posting A Tale of Rechargeable Batteries, the PowerEx charger has earned its keep.  Not only does it restore NiMH batteries that were no longer taking an adequate charge from a regular 'quick charger', it also allow me to accurately determine the capacity of NiMH batteries so that they can be matched into sets such that one battery does not become 'the weak link'. 

The PowerEx charger also appears to be useful in diagnosing the 'charge lifetime' of batteries.  I used 6 AA NiMH batteries of matched capacity in a label printer that gets intermittent usage.  When the printer stopped working, I did a normal recharge cycle.  Since the 6 batteries were in series, they should all have discharged by the same amount.  However, the PowerEX reported varying recharge amounts from 500 mAh to 2000 mAh.  Given that all the batteries started with the same capacity, my suspicion is that some were 'self-discharging' at a rate higher than normal.  All NiMH batteries will lose their charge over time - quality issues during manufacturing or breakdown of the internal battery components can accelerate this process such that the batteries quickly discharge even if not used.

In this case, I may have matched the 6 batteries in terms of initial capacity, but clearly not in terms of their ability to hold a charge.  Of the 6 batteries, the PowerEx charger results suggested that four should either be relegated to recycling or only used in high-draw devices where self-charge is less of an issue.