The safest method of charging a car battery is to remove one, or both, of its connections to the car -- but you may then lose the settings, such as for the clock etc.

The old fashioned charger produces a voltage of about 20 volts or more which is pulled down to the battery voltage when it is connected to a good battery and charging current flows.

If you use one of those and leave the two battery connections in situ, you need to know the battery has not failed by going open circuit before you apply the charger leads ---- is there still a clicking of the starter relay when you turn the ignition key, for instance? Will it light a lamp placed across it.

Because, if has gone open circuit, you will be applying an excessive voltage to the car electronics, [which you may get away with but it is still a risk] because the battery is not then able to control the voltage to a safe level.

The battery connections should also be making good contact with the battery lugs, because the charger will usually be connected to the leads rather than the battery lugs themselves -- ideally you would clip on to those lugs not the leads but that is not necessary if the connections are good.

The old fashioned chargers also leave the user to switch them off when the battery is charged. Otherwise the over-charging causes a lot of gas that can dry the cells out and do damage and also care must be taken not to have a spark near the gas as it can explode.

The modern electronic chargers are much better controlled than that and will stop before over-charging the battery and, because they use less voltage, there is not usually a need to remove a battery lead.

From time to time modern ones appear in Lidle. About £12 (?).