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 50°C/70°.
NiCad batteries have built-in the 50°C thermal detector.
NiMh batteries have built-in the 70°C thermal detector.
Do not charge in the sun and do not keep the battery on a heater whilst charging.