The booster has its own built-in battery. However, its capacity is very limited. The capacity is 8 ampere hours. Normal car starter batteries have about 60 ampere hours. This means that this battery can supply 12/8 = 1.5 amps over one hour at 12V voltage (absolute ideal value). That is very little (8 watts) and hardly enough to run anything sensible. It can be used to charge a mobile phone or operate an LED lamp, for example. A camping fridge with 30 watts can be operated for a few minutes.