Jump starting

[Baxlin]

My daughter flattened the battery trying to start her car yesterday, and phoned me to ask me to jump start it. As I've had a few lecky type problems with the Volvo recently, I didn't want to risk it, so I took my rapid charger and after about 20 minutes there was enough in it to start the car.

This made me think though, with all the many wires and cables attached to car battery terminals nowadays, is jump starting a good idea on modern cars? (Couldn't even get to the -ve terminal on her Peugeot, I had to use a nearby earth point)

[DaveNP]

I give no guarantees and it's only what I do-

With all of the extra electrickery in modern vehicles if I'm jump starting something, I have the side lights on on both vehicles before I hook them up, supposedly this acts to prevent the sudden change in voltage messing up the computer etc.
Some cars are more temperamental than others and if you've got the drivers handbook have a good read first.

Using an earth point away from the battery is recommended by some.

[Clan]

Originally Posted by Baxlin

My daughter flattened the battery trying to start her car yesterday, and phoned me to ask me to jump start it. As I've had a few lecky type problems with the Volvo recently, I didn't want to risk it, so I took my rapid charger and after about 20 minutes there was enough in it to start the car.

This made me think though, with all the many wires and cables attached to car battery terminals nowadays, is jump starting a good idea on modern cars? (Couldn't even get to the -ve terminal on her Peugeot, I had to use a nearby earth point)

No harm at all in jump starting .. battery to battery the car is only 12 volts it cant do any harm .. a tip would be to leave the supplying cars engine running for 10 minutes to get some charge into the other cars battery before starting it .

[GreenBrick]

Contrary to what a lot of people do, you should stop the engine of the "donor" vehicle before cranking over the dead car, otherwise you are putting a big strain on the donor's alternator, as the starter is like an almost short circuit. So use the donor to charge the dead battery for a bit first, then use the good battery in parallel with the dead one.

The order the jump leads are connected is purely to stop sparks igniting gases from the battery. Hook up the +ve leads first, then the -ve to a ground a way from the battery, so the spark won't cause an explosion. Same but in reverse, especially as the battery may have vented some gas while being charged.

As for soaking up a sudden change in voltage, the dead battery will work like a big capacitor, so I cannot see that being a big problem, but I guess someone must have had a problem, otherwise people wouldn't mention it so often.

[NigelDay]

That's interesting about the strain on the alternator. It's a practice I have always followed to switch off the donor engine at the point of starting the dead engine, but the advice in the manuals and on-line is the reverse !

[green van man]

A mate calculated that to stall a 130A altinator on a landrover would take an electrical load of 900A. The altinator will only give it's rated output and the battery acts as a shunt anyway. The battery supplies the load the altinator charges the battery, if the battery is putting out faster than the altinator is putting in the battery will go flat. This is why you run the engine even if not driving the car when whinching.
I used to connect jump leads battery to battery but with the increasing popularity of canbus systems now use the red terminal found around the engine for the positive and a good ground on the bodywork or engine block for the negative. For older cars with no red conection point and exposed battery terminals I still go battery to battery.

Paul.

[Welton]

My mechanic showed me a bin full of dashboard binnacles where modern cars had sent up to 18 volts to the 'dead' vehicle and ruined its electrics.

Ford 'smart' alternators are capable of 18 volts in cold conditions, I'd be very wary if you didn't know exactly how each car functions.

[canis]

Originally Posted by Clan

No harm at all in jump starting .. battery to battery the car is only 12 volts it cant do any harm .. a tip would be to leave the supplying cars engine running for 10 minutes to get some charge into the other cars battery before starting it .

Exactly what I do. The actual contact surface of the clips is very small, expecting a lot of current is expecting too much, I think. Ten minutes charging can make a huge difference, I have found.

Like others have said, I usually stop the donor engine first, but only because I'm told to. I've never had any problems not doing so, so it could be one of those myths.

Has anyone here ever blown a known-to-be-good alternator by jump starting another car?

[Baxlin]

Originally Posted by canis

Exactly what I do. The actual contact surface of the clips is very small, expecting a lot of current is expecting too much, I think. Ten minutes charging can make a huge difference, I have found.

Like others have said, I usually stop the donor engine first, but only because I'm told to. I've never had any problems not doing so, so it could be one of those myths.

Has anyone here ever blown a known-to-be-good alternator by jump starting another car?

No, but I think I'll continue to bottle it, as an extra ten minutes or so using my rapid charger removed any risk. Provided of course there's access to mains power.

However, thanks for all the comments, tbh, I can't remember the last time I started a car using jump leads, certainly I've not owned any since I moved to this house, nearly 40 years ago.

[Craigtange]

Originally Posted by Baxlin

My daughter flattened the battery trying to start her car yesterday, and phoned me to ask me to jump start it. As I've had a few lecky type problems with the Volvo recently, I didn't want to risk it, so I took my rapid charger and after about 20 minutes there was enough in it to start the car.

This made me think though, with all the many wires and cables attached to car battery terminals nowadays, is jump starting a good idea on modern cars? (Couldn't even get to the -ve terminal on her Peugeot, I had to use a nearby earth point)

I left my interior lights on and now the battery is dead. Can I jump start my truck without a problem? If not what's the alternative.