jump starter battery

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The whole point of the precautions (other than the remote connection of the final grounding connection) is to minimise the amount of surge that occurs when the 'dead' vehicles charging system suddenly sees a flat battery, is running at a fast idle following a cold start so is capable of a high output, and consequently spikes the ECU.

Leaving the vehicles running and connected as long as practicable achieves two things. Firstly, the 'dead' vehicles battery will have gained charge significantly. Secondly, the 'dead' vehicle will hopefully have warmed up enough for it to have dropped to idle. In both cases, surge is likely to be reduced.

It is worth remembering that modern cars have very high output alternators, and many of them use some form of Ford's 'Smart-charge' system, where the charging voltage can be higher than older systems.

I would be less likely to expect this to happen had I not been aware of two vehicles that I know it happened to.

The first was a relative's VW T4 that had not been started for a couple of weeks. The owner jumped it, and quickly pulled the leads off. It immediately stopped, and wouldn't restart. The garage diagnosed ECU failure, and had to fit a re-manufactured one.

The second was a neighbour's Pug diesel. The owner ran it out of fuel, and whilst attempting to bleed it (they are notoriously tricky) ran the battery down. He jumped it, and it eventually started, but died when he pulled the leads, and wouldn't restart. I couldn't convince him that the fault was unrelated to the initial lack of fuel, but after a week, he sent the ECU away for testing. It was dead; a re-manufactured unit had it running again.

It doesn't matter. If there is any hydrogen, and you can increase the distance to a source of ignition, why on earth wouldn't you do so?

They would vent exactly the same amount of gas as previous batteries. What do you think would happen if they didn't?

It's not a law, it's common sense based on the misfortunes of others. Read the recent post from steve robinson.

Chris

Reply to
Chris Whelan
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I'd say this advice dates back a long long way. To the days of dynamos and batteries that expected to be topped up regularly.

Modern charging systems and batteries simply don't gas to the extent which was once common.

The explosive element is hydrogen. It is lighter than air, so would normally just waft away in any case, if the cars are outside which is most likely with a jump start.

Battery explosions used to be common. Usually in a garage charging area where the battery was removed from the car. Poor ventilation, and a batteries left on charge until gassing freely on a crude charger. Any spark under those conditions could be a real problem.

Reply to
Dave Plowman (News)

I was charging a tractor battery in a shed a few years back and rather stupidly decided to start a welding job. As the MIG started there was one almighty bang and the battery split in half as it exploded. I was deaf for three days.

Reply to
Aloysius

Quite. Automotive electronics use higher specced regulators than is common on lesser stuff. And of course the car battery has a very low internal impedance so it takes a lot of current to produce a high voltage spike.

Unless that battery is knackered. Or disconnected. And if disconnecting from a running donor car produces this 'spike' how come the donor car always survives?

Reply to
Dave Plowman (News)

Yup. Hydrogen can collect under a roof or whatever. But most cars will be jump started outside. And modern car charging systems are far better than many older bench chargers at keeping gassing to a minimum.

Reply to
Dave Plowman (News)
[...]

That isn't a reason not to do it.

As I keep saying, the way to minimize the risk, no matter how small, is so trivial that there is no reason *not* to do it the recommended way.

It just means the last connect you make with the jump leads is to something away from the battery.

It might not waft so readily where the battery is in the boot, or under the driver's seat however.

Chris

Reply to
Chris Whelan

That's assuming anyone who doesn't understand things can be bothered writing it down and keeping it for future reference.

I really couldn't count how many times I've jump started cars over the years without problems. Important thing is not to mix up the positive and negative. That is most commonly the reason for explosions. With the person swearing on the bible they didn't afterwards. ;-)

Reply to
Dave Plowman (News)
[...]

How do you know it does?

I know of two cases where the 'dead' car's ECU was spiked without any realistic doubt; it doesn't mean it hasn't happened to the donor car in other cases.

When the problem first came to light, AA patrols had unwittingly 'killed' a few cars. As an interim measure, surge protection devices were issued. Jump- starting without using one was a disciplinary offence.

Now, all their equipment has that protection built in.

Chris

Reply to
Chris Whelan

In message , "Dave Plowman (News)" writes

As well as making sure to connect the positive (live) terminals first, I always make the second negative connection by first by only touching it for the briefest of moments. If there isn't a massive 'splat', I connect it properly.

Reply to
Ian Jackson

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