Correct way to jump-start a car

I always thought the reason it was difficult to start with the lead on the engine is the earth straps from the engine to the body and then to the battery are often damaged or just crap from new. I double up the leads on cars I keep for long periods and as most of them are over 20 years it seems to help not only with jumping but general running sometimes too.

A lot of mine are old BMW's with the battery in the boot so the option to use an earth point over the battery is often not possible.
 
Both the BMWs I have owned have had "jump points" under the bonnet to which the +ve and -ve booster cables may be connected.
 
They dont always start great from the remote pos and neg terminals so I guess the same problems apply.

The only problems my old BMW's have with draining the battery is for the SI circuit, if the SI batteries are dud then it will draw off the battery, Either disconnect it, sort the SI batts or use a conditioner.
 
DaveHerns said:
I'm more concerned these days with the risk of spiking ECUs etc. I'm not sure I'd volunteer to use my car to start another. You read about turning on lights , HRW etc to lessen the risk but I'm not sure.
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Last time I helped someone I just charged their battery for 5 minutes with my engine running. Their car was not touched with the leads connected
 
Mottie said:
Many years ago, I connected jump leads to a battery the wrong way. I had an old Austin J4 van with the battery behind the passengers seat. It occasionally overcharged so had a fair bit of gas around the battery area. As I made the final connection to the good battery......Bang! The gas exploded and all 6 caps blew off the battery and were ricocheting around the cab like champagne corks. How I didn’t get a face full of battery acid I’ll never know. It definately does happen.
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Sorry didn't spot that. The battery I blew up shattered the entire battery into dozens of pieces.

I should have said, in a Michael Caine voice, 'you're only meant to blow the b100dy caps off'.
 
If the donor car has jump points under the bonnet and battery in the boot, do you use those to charge the dead vehicle battery?
 
+1
I keep my engine running at maybe 2000 RPM for at least 10 mins or whatever if the casualty is a diesel.
John :)
 
Just an idea!

How about one jump cable in two halfs so when the final connection is made, its away from both engine bays in the area between the two cars.
 
Nothing wrong with your science Mr. Spanner........but it's the crocodile clips that are the weak spot with all jump leads - some get quite hot showing resistance!
The fewer the better I think. Mine are suitable for trucks and need two hands to open against the spring!
Regards
John :)
 
I have had batteries explode on me, mainly in Algeria i.e. a hot country, and some rather dramatic. Never lean over the battery, and try to make final connection away from the battery so a spark will not ignite any hydrogen, blow at the battery if you can first also to disperse any hydrogen, batteries in the boot are a bigger problem as not so well ventilated, however my wife's Jag has a special point for jump starting in the engine bay, so no need to go anywhere near the battery in the boot.

Running the engine of doner car before trying to start can put some charge into the battery first, but you should turn the engine off before trying to start the other car, as the extra high current on cranking can damage alternator on donor car.

Connecting the negative last does mean often not connecting near the battery, and discharged batteries are less likely to be gassing than fully charged batteries. My jump leads have a braid between the jaws so high current even with a poor connection does not go through the spring, or hinge, often jump leads have well under sized cables, and poor quality clips, so the idea is to transfer some power first.
 
The reason why you want to keep any spark on the terminals of the dead car is simply this: at this point you have one running car and one dead car. If you do this and it goes wrong, you end up with 2 dead cars.

You can buy surge suppressors which you attach to one end and its job is to monitor surges and eliminate them. I have them for 24V commercial stuff as the risk is greater at this voltage, but car ones are obtainable. Sealey sell them. Cheaper than a new battery.

One small point that I would add in contradiction to someone else's point: if you start the dead car and then disconnect straight away, one of the alternators will be working hard to fill the battery (it is not always obvious WHICH one as due to feedback the other will not be working so hard). If you suddenly disconnect with a significant difference of say +3V then you run the risk of popping the diodes in the regulator. I think the bloke who suggested connecting the leads, providing the dead car with some power and then disconnecting before starting the dead car has the best idea.

HTH.
 
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