Hi Bilbat,

I’ve seen this before – sometimes the battery terminals on the meter get slightly bent so they apply too much pressure to the ends of the battery when you screw the bottom back on. When this happens, some AA batteries will short internally and discharge themselves, getting quite hot in the process.

I recommend removing the batteries and seeing which one is dead, then adjusting the battery terminal (they can be bent by hand) so that it does not apply quite as much pressure. Then put in a fresh battery.

I hope this helps, please let me know how it goes and sorry for the trouble.
~James