Wait a minute, what’s the difference? I thought if your car doesn’t start, battery’s dead and you jump it. When/why would you jump start a battery that’s not dead? How would you know? Educate me please.
I agree my wording was bad. Let me try again.
If your car doesn’t start, and it’s because your battery is very discharged or dead, you have two options:
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Jump start it.
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Put the battery on a charger.
#1 is risky. After you jump it, the alternator is given the job of charging a very discharged (or dead) battery. This is hard on the alternator, especially the internal diodes. #2 is the best and safest option.
The only time you should jump start a car is when it’s inconvenient or or impossible to charge the battery using a charger. Otherwise, you should use a charger.
Got, it thank you. Ignorance exposed, fought, and defeated.
That’s by design. Most jumpstart packs don’t provide current until they detect a voltage potential at the alligator clips indicating connection to a battery. Otherwise it would be too easy for bad accidents to happen, not the least of which would be the clips touching and short-circuiting the unit’s battery. So they won’t work with a totally discharged or very weak battery. The only one I’ve seen that doesn’t do this is one of the big old-fashioned kind with a lead-acid battery, which had a manual switch instead. I suppose this also eliminates the chance of a spark, but I would still not take any chances and connect in the recommended manner.
If you can hear the starter relay clicking, there’s still some charge left in the battery, even if not enough to start a cold engine. Weak but not dead. If there’s not even enough to click the relay, it’s completely dead or nearly so.
It’s very hard on a battery to be completely dead. If it gets down to clicking the relay, stop trying and go get a jump or a charger.
This definitely needs repeated.
If you have a lead acid battery that is completely discharged, the clock is ticking very quickly. You need to get it charged ASAP. Leaving a lead acid battery in the uncharged state for even 24 hours can damage it.
I worked with an ex-mechanic who’d had a battery explode on a spark. He was still pissed off that his then boss had accused him of sparking the battery to see if was charged. Boss obviously believed that a battery would never explode just when adding or removing cables: my man begged to differ.
(And I use the towing lug under the car front if I can’t find anything easier)
And I have seen on many occasions where the Mechanics that work on the buses I drive will hook the ground lead of a large 60 plus Amp charger to the lug nut on the front tire!!!
Of course its a Lazy mans way of doing it and we have several buses needing towing because of “Front Wheel Bearing Failure” ! Really!!!
A lug nut on… a tire? What kind of tire has nuts?
I would assume he meant “wheel.”
Most people use tires and wheel (or rim) interchangeably.
It makes a difference here, though. A tire’s made of rubber, so connecting anything to a tire is going to be pretty useless as far as grounding is concerned. With a lug nut on a wheel, though, you’re mostly dealing with metal, aside from a thin layer of lubrication between the chassis and the axle, so, while not as good as something welded to the chassis, it’ll still work.
Why would it cause wheel bearing failure, though?
This thread has been very informative. I’m sure I remember reading that the reason you don’t go negative to negative is because this will cause current to flow from the strong battery to the weak one, thus potentially ending up with 2 useless batteries. But the fact no-one has yet mentioned this suggests this is incorrect - can anyone confirm?
I’d just leave the hood down and call road service. :paranoid
In the ideal case, there is no metal-to-metal contact in a wheel hub assembly - there is a thin layer of lubricant between all surfaces. In reality, the bearings probably make contact to the spindle in a few locations, but those small contact points might overheat when high current is passed though them. It wouldn’t take much damage to wreck a bearing - a small weld spot on one roller would pretty rapidly ruin the race, and trash the entire bearing.
ETA: You DO go negative-to-negative, just not at the terminal - at the frame. And, current is supposed to flow from the strong battery to the weak one - that’s what makes it a “jump start.”
No doubt this is true, but I can imagine 99% of people are thinking “Ain’t nobody got time for that!”
I bet the majority of people don’t even own one.
To be fair, it’s hard on most things to be completely dead.
tiny electrical arcs between the bearing rollers and races, which pits the surfaces and leads to rapid degradation. Some cars have grounding straps between the struts and the body to prevent premature wheel bearing failure.
Thank you both.
I realise that, I thought the potential problem of connecting the 2 negative terminals was that you then create a continual flow of current from the live battery to the dead one while they are connected, whereas with the recommended method, current is only drawn from the live battery when attempting to start the car. However, now I think about it, this seems logically inconsistent (i.e. if it were true that current was not drawn from the live battery to the dead one if both terminals are not connected, then connecting the final lead to earth would not then allow you to successfully start the car), so I assume I have just been misinformed. Thanks for helping me to clear that one up.
It is true that if you have a really, really dead battery (several cells shorted), connecting a good battery to it might damage the good battery (or melt the jumper cables), or in the best case, prevent the engine from starting.
Fortunately, it’s pretty rare that the bad battery is that bad.
Because you wouldn’t be flipping a switch, you’d be grabbing it with both hands and putting your weight on it. The switch would have to be rated for at least 400 amps for a small car to over a thousand for a large truck. It would make for a bulky, heavier jumper set that would cost at least twice as much because switches cost more than copper wire.
You could use this switch…
