Thermalution® Green Grade Plus it's fixable to swap halfway between small and big, don't need to worry if one battery is danger. Upper back core heating is sufficient to provide enough warmth if you dive within 30m.
Heating time too short!
Try latest model Green Grade Plus. It parodies up to 8 hours time with 2 large batteries.
One battery can operate it.It is suitable for people who don't head to dive in extreme weather condition and depth over 30m but cost minimum to have a power heated undersuit to warm you up.