A friend at work had a flat car battery. I was about to connect jump leads when he pointed out that the manual said I must connect to the chassis - and not directly to the battery.

I did this, but wondered afterwards - Why?

I found the same advice on a 'car' website, ie "Some cars need to be jump-started in non-standard ways (a few modern models, for instance, require you to use contact points on the car chassis rather than the battery itself)"

There is a rather chunky cable connecting the battery directly to the chassis, so what difference does it make which end I connect to?

The only reason I can think of might be to keep sparks away from the battery.

It certainly stops sparks when you connect to a likely looking bolt, only to discover that it goes through a rubber mounting. This prevents sparks by insulating the bolt from the chassis!

Connecting jump leads should be done in a sequence that minimises risks. The last connection that should be made to complete the circuit is the -ve clamp to the dead vehicle's body. This last connection will complete the circuit as the dead battery's earth is already strapped to the vehicle body.

The best reason for doing this that I know of is to minimise the risk of explosion from any venting cell, as hydrogen gas may be given off and - as you state - this method will keep sparks away from the battery.

Thank you.

It looks as if the comment "a few modern models, for instance, require you to use contact points on the car chassis rather than the battery itself" is simply the manufacturers catching up on good practice - rather than some new technical requirement.

According to Google, for some car models the manufacturers say "don't jump start". That one I could accept, as the electronics in modern cars might not like the transients caused by connecting batteries together.

a local haulage firm had a small steel bed tipper, when jump leads were put onto the battery from a fresh charged battery out of the workshop it blew a hole in the bed of the truck and took one of the drivers eyes out, I saw both after the event, not nice.

Andy

hmmm, interesting