jump starter battery

That too, you shouldn't earth from battery terminal to battery terminal, ideally you should earth from the body or engine block minimally on the recipients vehicle , again it helps with the loading and power surges , less chance of frying ECU's, alternators and other electronics.

Infact some manufacturers say you shouldn't jumpstart another vehicle or jumpstart off another vehicle, should use a donor battery only

Bring back starting handles :)

In message , steve robinson writes

I don't get that. Electrically, the engine block/body should be at the same potential as the negative terminal. If they are not, you have a high(ish) resistance negative connection between the two.

I would have thought that the low impedance of the batteries would iron out any surges, spikes etc. Although jump starting is not something I do very often, so far I have failed to kill anything.

My 1953 Ford Prefect 93E had one, as did my 1963 Austin A40 Farina Mk2. At the time, I was sure that not having them on later cars was a bad idea - but I've since got over it. ;o)

And broken thumbs?

If you need more than 20 attempts to start an engine then clearly there is something else wrong.

It started a 2.4L Diesel from stone cold last Sunday evening. That was straight out of the box from Amazon - the pack itself was showing itself as fully charged. There was just enough juice in the main battery to operate the central locking and interior light but it couldn't turn the engine by itself.

If the battery of the dead vehicle is both flat and old in particular, the surge at disconnection can fry the ECU.

Ford have been advising in their owner's handbook of the best technique to avoid this happening for at least 17 years.

The AA's jump-starting equipment has built in surge protection.

I know personally of two vehicles that needed ECU repairs after incorrect jump-starting.

Chris

The cables on mine are also fairly skinny, but they are also very short and very flexible. Unlike a proper pair of jum pleads they only need to be long enough to reach the terminals with the pack resting on top of the battery.

I thought the idea of that was to have a contact on the recipient vehicle that is at a distance from the battery itself. If you disconnect this first then any spark should be a reasonable distance from any hydrogen emitted by the recipient battery.

In message , D A Stocks writes

Yea, I suppose that does make sense - but I feel that the perceived hazard is more in the realm of over-zealous H&S than real.

It seems these things are the go nowadays.

F Murtz wrote

Yes, must get one.

Bit superficial, doesn't even say why its ranked them that way.

They look like a good bit of gear but expensive if you never use such a thing although they are more useful for many things than lugging lead acid or nicad around.

Yep.

There are documented cases of batteries exploding in such circumstances, and as the risk is so easily mitigated, why take it?

Chris

My understanding (and experience) is that you only get significant gassing from a battery when it has been overcharged and is still being overcharged - a situation that is unlikely to pertain when you are faced with vehicle with a flat battery.

Also, I would think that with the present-day so-called 'sealed for life' vehicle batteries, the normal release of hydrogen will be minimal. Even if there is some hydrogen kicking around, with an open bonnet it is likely to be present only in almost 'homeopathic' concentrations. Yes - there is a theoretical hazard from a few molecules of hydrogen, but surely there is no real chance of it igniting?

In the situation where a battery is overcharged, there is clearly perceptible gassing; it doesn't mean in other circumstances there would be none.

The accepted procedure for jump-starting involves leaving the vehicles connected for a minimum of 10 minutes before attempting a start, and as long as possible after a successful start. (Unless other surge protection measures are taken.) The donor vehicle's battery may well be gassing to some degree at the end of that time.

'Sealed for life' is a misnomer; they are only sealed WRT removing the ability to top up electrolyte. They still have vents for gases, and the amount of hydrogen released would be the same.

I reiterate - it's such an easy precaution that no matter how insignificant the risk, it is better to use the method recommended by vehicle and battery manufacturers, motoring organisations, et al.

Chris

In an ideal world yes, however, how often are battery earth leads, terminals corroded, oxidized fraid etc,

An old or faulty battery, or a faulty alternator control module which allows the battery to overcharge effectively cooking it and drying out the electrolyte can leave enough gas in the battery to cause an explosion, had this happen on a Renault traffic years ago showered me in battery acid i was fortunate the wife was there and had the presence of mind to turn the hose on and wash me down

Which is essentially what I said.

Yes - when you are jump-starting you are not so much trying to start the recipient vehicle directly from the donor vehicle's battery, but instead you're first charging the recipient battery from the donor vehicle's battery and alternator (engine running at a fast tickover) - and when it has been sufficiently topped up, starting the recipient vehicle from its own battery. However, it's debatable whether you always require10 minutes before you attempt a start.

I can't see the point of this. Once the recipient vehicle has started, there's little point in keeping the two batteries connected.

In my ignorance, I feel that the problems of 'surge protection' might be overrated. Vehicles must have pretty ropy control systems if they can't stand a paralleled battery being disconnected. But yes - I can see that in certain circumstances (eg if problem with the recipient battery is that it has gone high-impedance) where there might be a sudden rise of charging voltage - and this might do something nasty.

More than when it is charging normally - ie when the vehicle is being driven?

I've just had to replace my Focus's battery - and yes, there is an inconspicuous vent tube on the side. [You have to remove a bung, and fit-it-yourself.] However, I'm sure I've had other batteries with no obvious vents, so possibly they vented slightly through the 'sealed' screw-in bungs along the top.

To be honest, while 'the law is for the obedience of fools, and the guidance of wise men', I can't really disagree with taking precautions (even if some are essentially only rituals) - especially if they are easy to obey.

Its not the battery that's the issue its the Alternator, when you disconnect, the charge from the alternator has to go somewhere, if the battery can't damp it out it will follow the path of least resistance, this could mean diodes being taken out or transistors being overloaded. That's one of the reasons your advised to switch on your heated screens and lights , these circuits draw a fair bit of power between them and will absorb the charge.

Quite. So a claim unlikely to be tested.

Yes. This is usually where a jump start pack works. The original battery still supplies a fair bit of the current needed. The real test would be with a *totally* flat battery.