In the hope this avoids damage to vehicles.
Do not ever connect the earth from battery to battery as this can cause damage to the Engine Management system!!
Always consult the vehicle manual.
The AA recommended method is:
Before connecting any leads make sure the vehicles are the same voltage and that they're parked with their handbrakes on and ignition off. The vehicles must not touch as this can cause sparks or an explosion.
Use the red jump lead to connect the positive terminal of the donor vehicle's good battery to the positive terminal of the flat battery.
Then use the black lead to connect the negative terminal of the good battery to a suitable earthing point on the engine or chassis of the other vehicle. This earthing point must be away from the battery and fuel system.
With both leads connected wait three minutes for the voltages to equalise before starting either engine.
Start the engine of the donor car and allow it to run for a minute then, with it still running, start the engine of the other car and leave both running at a fast idle for ten minutes. Do not remove the jump leads while the engines are running as this can cause serious damage to the electronics on either car.
If the jump leads get hot, then avoid a possible fire by switching off both engines and allowing the leads to cool.
Turn off the ignition on both cars and then disconnect the leads carefully in the reverse order to the way that they were connected. Be careful not to touch the clips against each other or against the car bodywork.
