NiMH Automatic Charger: Use the Real Battery Capacity

This Tenergy automatic NiMH charger is typical of the breed: pick a charging current to match the cell / pack capacity, then stand back and let it determine full charge.

The instruction sheet reads thusly:

• For battery pack between 1100 mah and 2100 mah, please use the low level switch — charging rate: 0.9 A
• For battery pack over 2100 mah, please use the high level switch — charging rate: 1.8 A

Pop quiz: what charging current should you use for a battery pack made from nominal 2300 mAh cells?

I thought so, too, but consider this graph (the full post is there):

The actual capacity is more like 1600 mAh, not 2300 mAh. Do you set the charge current based on the wildly overoptimistic cell rating or the actual measured capacity?

As you might expect: charge based on the actual measured capacity, because that’s what the battery can handle.

The higher rate actually worked with new cells, but as the packs aged the charger would sometimes grossly overheat them. Bad for the packs, not to mention a bit scary.

The lower rate worked perfectly, although it took me a while to figure that out.

For what it’s worth, this is the charger I hacked a magnetic grapple onto the thermocouple. Much more convenient and considerably more durable than ptui tape.

Memo to Self: One careful measurement is worth a dozen optimistic ratings.