A general question and not make/model specific. {choice now dropped - DD}
So, for example, if you take a 32 Amp battery and you have a 65 Amp alternator, would it charge it from flat in half an hour? Unlikely I'm thinking, so how quickly can a battery charge?
I realise an alternator has to be capable of running the lights, heated rear window, wipers and everything else plus charge the battery. How much charge can an alternator produce and how dependant is it on engine speed? Anyone know?
Edited by Dynamic Dave on 24/10/2007 at 22:53
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Chris
An alternator's maximum output is irrelevant in determining how long a car battery takes to charge.
Any conventionally regulated alternator raises and maintains the voltage across the battery's terminals to around 14 volts. It it then entirely up to the battery how much current flows.
If the battery is large, warm and hungry; it can be about 40 amps - but not for long. It doesn't take long for the battery to start putting up resistance; the current quickly drops to under 10 amps and then tapers to almost nothing.
It would take over 80 hours to trickle the last 15% of charge in and a very flat battery will never be fully recharged by an alternator.
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Thanks Screwloose.
So if you wanted to charge a large, warm & hungry battery, what difference, if any, would it make driving in a lower gear at say 3,000 rpm, instead of 2,000 rpm?
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I think you are possibly confusing amps with ampere-hours.
AH is the measure of a battery's storage capacity. Roughly speaking, a 65 AH battery could deliver 65 amps for 1 hour, or 1 amp for 65 hours. In practice it's not quite as simple as that, but you get the general idea.
Charging time works the same, approximately, only in reverse.
How much of its theoretical capacity a battery will actual hold depends on its condition, and the sophistication of the charging device. An alternator works reasonably well and copes with all ordinary circumstances if the battery is in good condition. But as it ages it increasingly will never attain full charge, and what charge it does accept can quickly leak away.
So the short answer is: 10 minutes with a very powerful and regulated charger on a new battery. Indefinitely with an ordinary alternator and an old worn out battery.
If you mean, how long does a car have to run to put back in what was drained in starting it,
Discharge: 200 amps for 5 seconds = 0.3 AH
Charge: 0.3 AH , 20 amps = 1 minute
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An alternator's output, in amps, is neither constant nor linear.
I recently bought a replacement alternator, specified as a 100A unit. Enclosed was the computer printout of it's test results.
At 1800 rpm (alternator, not engine) it gave 76A, 2000 > 85, 3000 > 108, 4000 > 115, 5000 > 119 and 6000 > 121
In this case the rated output is reached at 2500 alternator rpm, which is about 1000 engine rpm.
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I remember reading a Ford TSB a few years ago (Sierra actually). Ford stated that it would take 8hrs of normal daylight running to fully charge a totally flat battery.
I can't remember alternator output or battery capacity of the relevant vehicle, but I would guess that the 8hrs would apply to the whole Sierra range.
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how long's a piece of string! all the previous replies are right,there are loads of variables condition and capacity of battery condition and out put of charging device,temperature etc.
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