Actually, the battery being attached certainly does NOT explain the 20V reading! I can't imagine getting that kind of reading with a good battery, more like around 9-10 volts maximum. The higher the voltage is, the more the charge current goes up, and it's not a linear function!
Here's a little test with a 3.2v NiMh battery, the principle and ratios would be exactly the same for the 8.4v NiMh battery, just multiply the voltages by 3.5 as this is a 2-cell battery and the 8.4 volt one is a 7-cell battery.
You can see that only .6 volts more across the battery raised the charging current from nothing to almost an amp! Had I cranked it up another few tenths, it would have shot up even faster! I didn't want to ruin the battery, so I didn't increase it, I think this makes the point.
Using my 3.5 multiplication factor, the first voltage would be 8.54 volts, the second voltage would be 9.345, and the third would be 10.85.
In order for the voltage to actually make it to 20 volts, the current would have to be lots of amps! Since the board has a 1A regulator, that ain't happening, so something is amiss with the 20 volt reading or the battery is certainly bad.
