The Battery Master is not a charger it is just a small power unit to keep a fully charged battery fully charged. The unit is left in the mains and connected to the battery all the time while the car is not in use.
Batteries in cars left unused will drain power as a natural discharge, and if an alarm is fitted the drain could be worse. Over time the alarm may not function properly and tour car may not start.
This is a small unit the size of a normal power adapter which connects straight to the battery via two crocodile clips. It can not be used as a charger and must be disconnected before starting your car.
it has a 230v-50Hz AC input and 12vDC/300mA output,
Not just cars can be kept topped up but also caravans, boats, standby batteries or anything that uses a car battery. So if you need the car to start first time instead of the panic when it don't, This could be for you.
the only problem is you really need a garage to use it or carry the battery inside.