Originally Posted by black_jack

The 240v ac from the mains is converted to about 12-14v dc and about 4 amps (depends on the charger)

This is the equivelant of a battery (a bit like connecting another battery)- so should be safe to connect without disconnecting the battery,

the battery can act as a "damper" to protect the cars' electronics

a basic charger, without a cutoff, can eventually "boil" a lead acid battery, modern silver calcium batteries (smart; charge) operate/ are safe at higher voltages than "normal" lead-acid, the corresponding circuts on the car must be too

a modern charger charges the car at high (ish) amps (eg- 4 amps) then, when it reaches a pre- selected level, it cuts this right down to "trickle" mode, (a tiny amount of amps), they can also detect faulty batteries and can shut down automatically - this is safe to connect to a battery connected to a car -

It may be in the instructions to dissconnect the battery from the car whn it is being charged - this may be for legal/ health and safety reasons (to cover their backs)

Mechanics connect powerful boosters (many times more powerful than a domestic charger) to car batteries without disconnecting them