[marknorton]

The voltage on a fully charged battery will be aound 4.1 - 4.2v but quickly drops to a nominal 3.7 - 3.8v in use - the higher charger voltage is required to "force" the charge into the battery. Li-Ion batteries are far from constant voltage sources however.

The Pentax charger will be programmed to respond to a particular charging characteristic of the battery - how quickly the current falls - and if this is not what the Leica battery is doing, it will not correctly sense the end of charge. For safety, most chargers give up after several hours, though yours appears not to.