A booster is essentially a battery charger that operates on 12 (or 24) volts: the input voltage may fluctuate, but the output charging voltage will always be correct for charging a second set of batteries. The variable and often insufficient voltages in Euro6 vehicles make it impossible to properly charge a second set of batteries. A booster ensures you receive the correct voltage to charge the batteries properly and efficiently.
When charging lithium batteries from a starter battery, a booster must always be installed. In this case, in addition to providing the correct voltage, the booster also prevents the lithium battery from being overcharged. It provides extra safety.