[marknorton]

Charging Li-Ion batteries is a much more involved process than charging lead acid batteries or even Ni-Cads. They need to be taken through a particular charging cycle which is established by the controller chip in the charger, normally programmed by setting component values.

Towards the end of the charging cycle, the battery voltage will be fixed and the charging current will decline over time; when it reaches about 1/10 of the initial current, the battery is effectively fully charged and the charger should switch off.

That voltage needs to be set quite accurately to +/- 0.05v; the Leica battery will charge at about 0.7A to begin with. It may be the charger is getting hot just because the battery is drawing more current than the charger is capable of supplying.

If you can, I recommend measuring the charger voltage and charger current and take readings every few minutes when charging the Pentax battery from flat, then do the same for the Leica to see what difference there is.

Either way, you are correct to treat the Li-Ion batteries with respect - that can explode when misused.