Battery

[Grahamm]

There's a big difference between a flat battery and a dead one.
No amount of charging, no matter how fancy the charger is, will revive a battery that has knackered cells.

But a dead one can also *appear* to be good, because even if it has a dead cell, it can still read full volts.

The only way to check is to put a drop-tester on it to see if it's actually holding the charge.

Even new battteries can turn out to have a duff cell