How long should a 12v Car Battey last?

I've always felt these massive leads (which I've often found to be a bit of bell wire with a whopping amount of plastic insulation sleeve around it) just encourage people to connect up and hit Go right away, expecting the leads to carry 500 amps through a few tiny points of contact for whatever nut the clamps are on.. Or if they do have good contact, overstressing the alternator of the donor car..

Naw, they should be more like yours Harry; slim, easy to store, twice as long as they are typically sold and with an irremovable tagged warning to leave them connected 15 minutes engine off before attempting a start on the dead car.

Not that muppets can read of course, but..

Starter motors draw a lot of current. Depending upon the car (ie. large or small) you can expect several hundred amps to pass through the jumper cable to get the starter to turn the engine over fast enough for a start. A diesel engine would likely draw more current for the equivalent-sized petrol engine due to its increased compression.

In terms of the appropriately sized jumper cable that's required (massive or otherwise), a quick check of the cable that runs from the battery to the starter on your car will give you a good idea of what size cable is required. Good-quality jumper cables tend to be heavy and often quite long. The flat battery may not be easily accessible. The set I have is long enough that I can hook up when both batteries, even if they are located furthest away from each other in each car. The thicker cable minimises resistance.

We have traction, leisure, and vehicle flooded batteries, and we also have AGM or VRLA batteries, all lead acid, but also very different. I have not waded through 5 pages of answers, but the charging has also changed, from the third brush dynamo to the engine management control charging, one can't group them all together and give a single answer.

Also when one considers a battery is dead, I have 2 AGM 90 Ah batteries ex-car running emergency lighting without a problem. They will not crank the car, but will run lights for hours, not far from the 90 Ah rating with such a low load.

In the main, we don't look after batteries, years ago we would give batteries an equalising charge every couple of months, the problem is the car engine does not run long enough, if we make an error and leave lights on, so we have a battery half charged (assume 100 Ah), and put it on a smart charger to re-charge, the charger will run flat out say 5 amps for ½ hour, then it will start dropping through the stages, 3 amp for say 10 hours and the 0.8 amp so add it up 2.5 Ah, then 30 Ah, and it needs 50 Ah, so 17.5 Ah to go, so another 20 hours with loses at least before fully charged, so does not matter size of charger, it takes around 24 hours at least to recharge a half charged AGM battery, at 2 hours driving a day, it will take at least a week, and since not recharged straight away, longer as the sulphate gets harder, so more like a month with 2 hours a day driving. Someone like me, 16 miles a week, it will never recharge fully.

So putting a charger on the car once every two months for 48 hours, will extend the battery life by a huge amount, assuming using a smart charger. Or use a Lion battery if the car can use one.

Ron0z said:

In terms of the appropriately sized jumper cable that's required (massive or otherwise), a quick check of the cable that runs from the battery to the starter on your car will give you a good idea of what size cable is required.

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True, if all the current, needs to come from the doner battery, but thinner leads are easier to carry for emergencies, and work just fine, if some charge can be transferred to the original battery, with the doner car's engine running/alternator charging, for just a few minutes.

Just connecting battery to battery, without the alternator running, to boost the voltage, will do little, to any charge in the failed battery.

robinbanks said:

And manual window winders. Very handy for getting out of submerged cars with failed electrical systems!

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Yes, that's always happening to me

Ron0z said:

a quick check of the cable that runs from the battery to the starter on your car will give you a good idea of what size cable is required

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No. You don't need cables as thick as the wire that goes from the battery to the stater motor because jumper cables are not supposed to carry all the current necessary to start the dead car directly and instantly from the good car

Battery terminal clamps look like they do (a round hole, slightly tapered, clamped around a round battery terminal, matchingly tapered) to ensure a large surface area of contact because that's the way it needs to be to reliably pass hundreds of amps

Jumper lead clamps do not afford you the same level of surface area contact and form a weak point in the connection that is comparatively much higher resistance than your whopping cable. You'll also note your jumper leads need to be much longer than the cable from battery to starter, which is typically very short (or very chunky if the battery is in the boot)

All in, most of the effort on starting a dead car should come from boosting the battery of the dead car, and as a result if you're looking for some indicator of how thick jump leads really need to be you should be looking at the wire from the alternator to the battery, considerably smaller than the wire from battery to starter..
..and hence one should be waiting for the good car's to charge the dead battery, not expecting the good car to be the battery for the dead car; all that's going to do is risk damaging something on the good car.

Don't forget that, for example when you crank a working car normally, the car's alternator/charging system is not active and not seeing seeing the massive load from the starter and not providing any power into it. Connecting a dead car to a good one that is running, and instantly cranking, asks the good car's charging system to supply part of a massive load that it never normally sees. In the interests of protecting it, it is better if you give it some minutes to "slowly" charge the dead battery at a lower rate that it is better designed for, then cranking so the was-dead-mow-charged-a-bit battery provides most of the effort

To this end jumper leads can be much smaller than you expect, and the relatively poor connection of the clamps is a vital safety feature, limiting the amount of current that can be pulled through the jump leads in the interests of not stressing the donor vehicle

Nah, you're wrong Robin. You make some valid points, but the guy who left the headlights on last night and has a stony flat battery this morning and has a job interview first up wants the car started as soon as possible and as easily as possible. There's only one solution: big fat copper wires in a set of quality jumper leads. The bigger the better if you can afford it.

Here's another thought for people to chew over. If you were to call roadside service to help you out, what's the bet they would have some heavy-duty setup that would start a V8 engine.

More haste, less speed. Happy to leave you to shag up your own charging system being a Good Samaritan but I don't think I'll offering you a boost any time soon!

Ron0z said:

Nah, you're wrong Robin. You make some valid points, but the guy who left the headlights on last night and has a stony flat battery this morning and has a job interview first up wants the car started as soon as possible and as easily as possible. There's only one solution: big fat copper wires in a set of quality jumper leads. The bigger the better if you can afford it.

Here's another thought for people to chew over. If you were to call roadside service to help you out, what's the bet they would have some heavy-duty setup that would start a V8 engine.

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The real answer is somewhere between the 2.

robinbanks said:

More haste, less speed. Happy to leave you to shag up your own charging system being a Good Samaritan but I don't think I'll offering you a boost any time soon!

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Call out a recovery guy for a 1 off emergency start and it will be big fat leads off his 2nd battery, or more likely a heavy duty battery pack with big fat leads.

If it's a non emergency and especially if it's more than a 1 off then leaving clips attached and smaller leads is probably a better option

But the real issue there would be another underlying fault.

carmanmemoranda said:

Call out a recovery guy for a 1 off emergency start and it will be big fat leads off his 2nd battery, or more likely a heavy duty battery pack with big fat leads.

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Indeed, the RAC will have dedicated jumpstart systems with half the number of clamps, better training and a narrower use case

We don't all have to upgrade to RAC levels of technological provision to give a little old lady a boost on the way to the hospital.. 99.9% of people should just carry some cheaper/mediocre leads and use them appropriately in the best interests of their own car's health

robinbanks said:

Indeed, the RAC will have dedicated jumpstart systems with half the number of clamps, better training and a narrower use case

We don't all have to upgrade to RAC levels of technological provision to give a little old lady a boost on the way to the hospital.. 99.9% of people should just carry some cheaper/mediocre leads and use them appropriately in the best interests of their own car's health

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Id actually suggest on modern cars and with little knowledge the best idea is not to carry any at all but join a breakdown club

Ron0z said:

Here's another thought for people to chew over. If you were to call roadside service to help you out, what's the bet they would have some heavy-duty setup that would start a V8 engine.

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Then they will connect them, run their engine, and suggest you wait a few minutes while it puts a bit of charge in your battery.