AVC POWER BELTS,  POWER PACKS, BIG PACKS, MULTI PACKS have a built-in thermal-detector. The thermal detector is a bimetal switch, which interrupts the charging line at a temperature of 50C/70.

NiCad batteries have built-in the 50C thermal detector.
NiMh batteries have built-in the 70C thermal detector.

Do not charge in the sun and do not keep the battery on a heater whilst charging.