Jump-starting

My query is prompted by jumpstarting advice (not vehiclespecific) given to a reader of a national newspaper and I should be grateful for your views. The advice was as follows: “With the starter car’s engine running, connect both positive battery terminals. Then connect the negative terminal of the starter car to a negative earth metal body part of the car to be started. Remove the cables from both cars before switching off either engine. Switching off the starter car while they are still connected does the most damage.”

This advice seems at odds with normally accepted procedure (p74 of CM, June 2018). Could the advice be correct for vehicles with smart-charging systems – or is it incorrect for any system?

Phil Cooney

This advice is wrong for any system and definitely not the procedure for a vehicle with a smart-charge system. Connecting or disconnecting jump leads with the engine running will cause a surge to the system that can cause damage.

The smart-charge system monitors the voltage constantly; the sudden variation from the disconnection or connection of the leads with the engine running will cause a spike in the voltage produced.

Connecting the leads with the engine running will suddenly decrease the voltage on the jump-starting vehicle. At this point the system will push a surge of power through to boost the battery, causing a spike of voltage that could damage electrical components on either vehicle.

Equally, removing the leads with the engine running will suddenly decrease the voltage being registered at both vehicles and will cause a surge that may cause damage. I have read reports that advise removing the jump leads while the engines are running and this would have been okay 20 years ago, but fitting or removing jump leads with a running engine on a modern vehicle is not something I would advise.