Using my D2 to jump start another car
 

Hi all

Been asked to help a friend get a car running in the morning - it's been sat for nearly 2 months in their garage with a flat battery! It's a weedy little Rover so shouldn't be too tricky for my truck battery!

Are there any precautions aside from the usual of not allowing the cars to touch I should be aware of?

I presume I can plug the jump leads straight across from my battery in the boot to theirs without any risk of damage to the electrics on mine?

Can't recall seeing jump start points under the bonnet to use instead?

Cheers all

Adrian

I would be cautious using my car to jump-start another car, with a very dead battery. It's one hell of a load, there's the risk of damaging semiconductor components in your car, especially if you're using non surge-protected leads.

I paid dearly with my B6 A4 3,0 Quattro. I needed a new ECU and when that was fitted, it was discovered the instrument cluster had taken a hit as well.

An expensive lesson, one I have no desire to repeat.

Scary stuff, Nollywood. Hope it ain't to late for you to back out Adrian? I'll never jump start again. :o

I have had no issues when using my 8 for jumping and have done a few times now, that said i don't know of the condition of the batterys in the other cars.

The owners manual gives instructions on what terminal to connect to first and for the life of me can't remember, think it was positive first.

You're right Adrian there is no points under the bonnet that i'm aware of

I've done it without problems (from the battery in the boot), but I don't have the newer battery management stuff to worry about.
I think the basic rules are:
1. Cars don't touch
2. "donor" car should be running
3. connect +ve first

HP, I think you should add to the basics "jump leads must have built-in surge protection." Like the ones used by the AA and RAC.

It doesn't always cause a problem, but it does happen. A very dead battery ideally needs to be charged externally before attempting a jump start. It's like a massive short across your battery terminals otherwise. A car with an electrical short could have catastrophic impact upon the electrical system of the jumper car...

Adrian, I would be really cautious. 2 months is a long time for a car to be sitting with a flat battery. A flat battery can freeze in this weather, and any attempt to jump start such a car could have dire consequences. A frozen battery can burst (I remember the same happened many years ago, in my BMW 1602). Scary stuff. All that was left of my battery was just the base, still held by the clamp. I was splattered by the battery acid, luckily it was too weak to do more than sting a bit.

I have a healthy respect for lead acid batteries now. :rolleyes:

Adrian, if you really can't get out of jump starting this car (I personally wouldn't) I would suggest, at the very least you connect your leads with your engine idling, and leave it in this state for at least 30 minutes to charge the dead battery.

This way, it will reduce the load on your electrical system when the other engine is cranked. It's really cold out there, and trying to jump start without at least getting some juice into the dead battery will draw a lot of current from your own battery, and place a hell of a load on your electrical system.

Good luck.

I have to confess I've never done it with a "really dead" battery, just a "clunker".
(I've always put long- unused ones on charge first)
And yes, batteries are much "weaker" at lower temperatures; it's -2C here at present.

Thanks folks - got to the car, which turned out to be a low miler MG ZR, and it appears an interior light was left on when it was last used hence why it was flat as you like. Got the leads on and left it for quite a while, then experimented with the ignition and the fuel pump primed with plenty of enthusiasm. Gave it another 20 minutes with mine idling and then upped the revs - it started 2nd turn of the key and purred away quite happily.

Left them to drive down to Halfords and get a new battery - although it was quite a funny thought that they were all going to have to get in via the drivers door as the passenger one and boot were both still deadlocked! So that's baby into baby seat, pushchair on back seat, and a climb across the seats for one adult lol

If it's anything like my father-in-laws old small Rover 400 (sold last year, he no longer drives) the drivers manual tells you how to resync the deadlocking after restoring battery power.
(had to do it a fair few times in the past, takes only seconds)

Adrian. Glad you got it started and everything is OK with yours.

This thread raises some issues, and as we A8 drivers would like to display our Samaritan qualities, when called upon, it would be good if we could get to the bottom of the risks involved in providing that vital spark to a fellow motorist.

As far as I can understand the risk in jumping to a dead battery is to the leads, they melt. So it makes sense to connect and run the donor for a while to get some juice into the recipient before turning it over.
If you could connect the running car to the dead one with massive leads, then surely the draw from a piddly Rover turning over would be loads less than the A8's own startup.

There must be some other reason why jump starting can mess up the ECU etc. Anyone know why this might be?

Incidentally I believe the connection sequence is. Get donor car running. Connect the + on each car first, then connect the - to an earthed (not - pole on the battery) part of the donor car, followed by the other end to an earthed (not - pole on battery) part of the donor.
Keeping the earth connection away from the battery reduces the risk of igniting the hydrogen/oxygen given out by a charging battery.

01) Voltage spikes.
02) Current surges.
03) Short-circuit in dead car.
04) Short-circuit or high-resistance in dead battery, due to damaged plates.

I would invest in a set of jumper-cables with built-in surge protection. A lot safer for the donor car's semi-conductor electronics. I had a set, lent them out to a buddy, and never got them back. They cost me £60 at the time.

Whilst I can understand the advice to minimise sparks in the vicinity of possible explosive gases, if you connect the jump leads to anything other than the battery directly you will increase the chances of passing high current through something sensitive. Connecting battery to battery allows your battery to directly supply current to the point where their battery would provide current for starting, without going through any other circuit on your car, or theirs.

Unless you have had a battery on charge for some time in an unventilated area the chances of a build up of significant quantities of explosive gases I would say will be minimal and can easily be countered by opening your boot and their bonnet for 5 mins before attaching the leads.

Good quality leads are a must, heavier gauge the better.

I took the practical view that when the ignition was switched on there was clearly enough juice getting through for the fuel pump to prime as normal

As for connecting to anything other than battery terminals, I'm of the view that if it was necessary then a point would be provided. I know on some Japanese and German cars it is specified in the manual

My neighbour recently started his rover 75 using his dads Mazda 6 - both V6 - and that had sparks etc flying about with cheap jump leads. Since mine was clearly not straining to provide power I didnt feel too worried - I did also turn off all internal electronics I could to reduce load and risk

I'm usually more worried about frying immobilisers on dead cars from past experience

My leads are very chunky for 12v use - unfortunately the clips are mainly plastic so 1 broke when I dropped it on the floor.....

I've had a truck battery blow up on me when jump starting another so I do understand care is extremely important when dealing with batteries

Need to find some surge protected leads - never seen any! Mind you this is the 1st time I've used mine in 10 years.....

If you're going to spend that sort of cash on a piar of leads you'd probably be better off with a portable starter. Like or similarhttp://www.maplin.co.uk/portable-jum...mpressor-49038

On the D3 it is HIDEOUSLY IMPORTANT that you use the earth point above the battery and not on the battery itself, or it will kill the power management unit.

Arrgh! I was just going to say that I've always ignored the explosive risk thing, and just connect to the most convenient point, usually the battery pole. Then Psyche frightens me with that.

I bought my leads for jumping trucks. Trucks being 24v and the electric fork lift we jumped from being 36v. They had to be good to carry the current.

Always force clamp jaws closed against the lead battery pole if poss and work them around to dig into it. (Unless your jumping a truck, where you just jam the last lead on as fast as you can, eyes closed and extinguisher ready to put out your hair) :ROFL:

Thanks for the heads up Psyche +++