Jump starting

**woofy** · 25-04-2012, 09:39 PM

I have had a strict rule for many years of not jump starting any of my cars, nor providing a jump for anyone else. I threw away all jumper cables years ago.

My wife has declined doing it for people at work, RACV/Q are setup better for this these days and don't take too long to come. I've had new batteries put in from flat battery to driving away within an hr.

So today I had a guy who has detailed my Skoda before and came to do our new Hyundai Santa Fe. He had been here all day in the freezer cold in our garage and at night when he was done and had to get to the other side of Melb quickly his car was dead. Turns out it had happened a week before and he has a charger at home as its happened before.

He knocks on the door and asks for a jump. I was going to say no as per usual and ask him if he had RACV, but the guys car is at my front door, and I'm guessing the answer is no. You can sign up on the spot as I did it once before years ago, but despite that I said ok. So we are in pouring rain and I bring the SKoda over, of course its terminals are covered in that blue stuff so we give up and use the Hyundai. Car starts fine and off he goes.

Now I did say to him, do you know how to connect them and right order and connecting the negative to the block rather than the neg terminal etc, and said I don't like doing this. He connected them in the right order, but I realised with both cars I left the engine running on mine as we connected them. Had he given me a few seconds I would have realised that, its been over 15 yrs since I'd done it, so wasn't thinking.

Overall I'm not feeling too good about that fact, there are so many issues with power spikes and electronics, I am getting a little paranoid that I've probably killed something, or spiked something in my cars I shouldn't have. Am I being paranoid?

Both my cars started fine and ran fine when I put them away after he left, we saw no sparks and we disconnected his car almost straight away....but stlll that lingering thought of what if?

---------- Post added at 09:39 PM ---------- Previous post was at 09:34 PM ----------

Might add that we took off the cables in the reverse order while both engines were running, also supposedly a no no, but I've always known to do it that way years ago...again before ECUs though...so god knows what lucky dip damage I might have done...

**popeclement** · 25-04-2012, 11:00 PM

Done it many times with all my cars and no side effects. However have always left the donor vehicle's engine off till all connections are made, then started and let it run at a fast idle (more than 1,000rpm) for about 30s for voltages etc to stabilise and then attempted to start the recepient vehicle.

**woofy** · 26-04-2012, 08:43 AM

Yeah the engine running while connecting wasn't the best move, I actually have no idea why I did that-it has honestly been at least 15 yrs since I've done it and for some reason I thought you did. If he wasn't there trying to connect them so fast and get out of there I would have thought about it more. It was only idling so wouldn't be creating much voltage, I think the danger is more getting the lead dropped into a belt or fan if it's running.

I guess if something got a spike and is unhappy it would show up sooner rather than later.

**h100vw** · 26-04-2012, 09:03 AM

Originally Posted by woofy

Yeah the engine running while connecting wasn't the best move, I actually have no idea why I did that-it has honestly been at least 15 yrs since I've done it and for some reason I thought you did. If he wasn't there trying to connect them so fast and get out of there I would have thought about it more. It was only idling so wouldn't be creating much voltage, I think the danger is more getting the lead dropped into a belt or fan if it's running.

I guess if something got a spike and is unhappy it would show up sooner rather than later.

You should hook the cars together with power off as the safe option.

The reason for connecting the ground on the flat vehicle to the engine rather than the -ve terminal, is to prevent a possible explosion, if the battery was gassing (producing hydrogen). The extra distance from the possible source of hydrogen, reduces the likelihood of the big bang.

My preferred method for jump starting would involve having the 2 cars connected, with the donor running for 5 minutes. making sure the flat car is turned off. This would bring the voltage up in the flat battery and if the leads are the weedy supermarket aluminium type, when you do go for the start, they won't have to handle quite so much current.

No spikes will get through the car if the ignitions are off. Think about it, you can't produce more than battery/alternator voltage in the donor car. All you can get is a drop in voltage in the donor, which the alternator would regulate once the engine was running.

Gavin

**woofy** · 26-04-2012, 10:28 AM

Nothing I can really do now, I just tend to stress about things after the fact.

**Spyda** · 26-04-2012, 11:24 AM

Honestly, it doesn't matter and your being paranoid for no reason.

Ive questioned the engineers at work about this and they just laugh.

Most people who believe in the old "put your hazards on so your ECU doesn't blow" or "make sure your car is off" are the same people who don't understand parallel and series. I've watched a mechanic hook up a car in 24v because he was more worried about having the hazards on, thats how you blow **** up.

You have to remember that most jumper cables are only 8awg or less with a mov to protect from surges. They will only handle about 30-40amps before the voltage drops off.

If your car is on it will make it easier to start as you have 14.4v and not 12v from the battery. You also have to remember that when your battery is full it can be up to 14v so having the other car on doesnt matter, its just makes more current available for starting.
Also remember that current is drawn, NOT supplied, so you cant over supply current unless there is something wrong like a short.

When an alternator fails it will produce somewhere like 17v so your electrics in your car are usually set up to handle such things.

The spike I'm guessing people are talking about is the sudden jump from a flat battery to a full battery but its the same thing as replacing the battery or starting the car.

The reason you earth to the engine is because its easier for the start motor, again because your jumper leads are crap.

I made my own 2m of 0awg (300+amp) jumper cables so i don't have to wait for the battery to charge off one car before i start and i don't get voltage drops and i can jump start from a distance if you cant get the cars close enough.

**kapowww** · 26-04-2012, 01:57 PM

Originally Posted by woofy

Am I being paranoid?

Yeah...

**woofy** · 26-04-2012, 10:10 PM

Good.....I used to jump things all the time, but since the mid 90s every second car around me blew CPUs, and I haven't had to jump a car since, so not sure whats fact or fiction anymore.