How long should a 12v Car Battey last?

[robinbanks]

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 points of contact, or 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..

 

[Ron0z]

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.

 

[ericmark]

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.

 

[Harry Bloomfield]

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.

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.

 

[JohnD]

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

Yes, that's always happening to me