Totse.com is no longer available!
This is a static archive of what the website used to be.
Check out the new articles and visit the community at Zoklet.net

Log in

View Full Version : How long does it take an alternator to charge a battery?


DarkSkye
2009-01-14, 03:24
self-explanatory. I have a 96 mustang and *something* keeps pulling the battery down. It takes about 3 days to do so, so about how long would i have to drive for the alternator to charge the battery back up?

Thanks!

frinkmakesyouthink
2009-01-14, 10:17
I'd give it a good 2 hours of solid driving. That's a bit of a guess though.

You're better off finding what's draining the battery than just recharging it all the time, because car batteries aren't designed to be discharged completely and once they've gone flat a few times they really start degrading. Eventually you'll kill it.

In the meantime you can use a trickle charger to keep it topped up, or at least disconnect it so it's not discharging through the car.

DavidThePyro
2009-01-14, 11:30
It depends on how discharged the battery gets... If it needs a jump to start it, I'll usually run it for 20 min or longer. It doesn't take forever to recharge, unless you've got a weak alternator or are running a high amp load at the same time as recharging...

But, yeah, fix the actual problem, or at least use the trickle charger band-aid approach, or that battery will be dead probably within a couple months.

MunkeyQ
2009-01-14, 12:11
Normal lead-acid car batteries don't like being discharged past 70% of their capacity - the plates sulphate over, and the battery degrades very quickly. If you keep on discharging and then charging it again, you'll kill the battery.

Charging a large lead-acid from about 40% will take roughly 3 hours to 70%, and then a bit less time to take it to 100%. Lead-acids cannot be charged quickly - if you feed 'em a high current, the energy which cannot be stored is simply given off as heat.

The reason a battery will appear charged after only 20 minutes or so of high-current charging (as opposed to a plug-in home unit) is because the high current causes only the tops of the plates to hold charge - called a surface charge. This energy can be drawn off immediately as long as you don't give it too long between charging and using power. However, this surface energy is only a small fraction of what the battery can hold.

If a battery has a surface charge and you leave it to stand for an hour or so, the charge will spread to the rest of the plate, giving it a lower nominal voltage.

DarkSkye
2009-01-14, 14:21
thank you guys.

Ive taken the damned ting to 3 different mechanics , all of which charged me over 50 bucks and still havent found the problem. :(

MunkeyQ
2009-01-14, 14:28
Battery drain problems are bastards to find the cause of.

You can start my connecting an ammeter in line with the battery and removing all the fuses except one. If you then try every fuse in turn, you can work out which circuit is drawing that power.

citizenuzi
2009-01-14, 15:38
Man that's happened to me with a pathfinder and a mercury. The mercury was classic shitty ford wiring by the firewall, and the pathy was a bit more obvious with the previous owners' hackjob of foglights.

DavidThePyro
2009-01-14, 23:45
Battery drain problems are bastards to find the cause of.

You can start my connecting an ammeter in line with the battery and removing all the fuses except one. If you then try every fuse in turn, you can work out which circuit is drawing that power.

Or just connect a test light (any 12v light) between the positive post and battery cable, it will light up if there's a current draw, then remove fuses one by one until the light goes out. Then at least you know what circuit the draw is on.

Rocko
2009-01-15, 00:39
Or just connect a test light (any 12v light) between the positive post and battery cable, it will light up if there's a current draw, then remove fuses one by one until the light goes out. Then at least you know what circuit the draw is on.

Not the best way to do it, parasitic draw can be small enough that it won't light up the test light. Better to use an ammeter so you have actual numbers to work with.

DavidThePyro
2009-01-15, 03:03
A test light will light up if the draw drains a battery in 3 days. And if it doesn't light up, obviously it won't work, but you spent all of 5 seconds figuring this out. Definitely worth a try if you don't have a ammeter on hand. And you don't exactly need real numbers, it's pretty much a current draw or not.

knows2nose
2009-01-15, 08:32
UH, two things....

1. The original question still stands. How long does it take for an alt. to charge a battery- never.

An alternator only maintains a surface charge. If you drain it and you dont charge it properly, it will stay dead, or close to it. If your old enough to know what a generator is, it would charge a battery.

2. If you hook a test light to the positive side of the battery expect it to stay lit. If you want to test with it put it on the negative side.

carry on.......

MunkeyQ
2009-01-15, 10:20
UH, two things....

1. The original question still stands. How long does it take for an alt. to charge a battery- never.

An alternator only maintains a surface charge. If you drain it and you dont charge it properly, it will stay dead, or close to it. If your old enough to know what a generator is, it would charge a battery.

2. If you hook a test light to the positive side of the battery expect it to stay lit. If you want to test with it put it on the negative side.

carry on.......
Sorry, but you're wrong on both counts. I'm sure everyone here can back me up. Allow me to politely explain. ;)

1. An alternator does not only maintain a surface charge. It kicks out something like 50 amps via a charge regulator, which is more than capable of charging a completely dead battery. An example - how do you think jump starting works? Once the engine is running, the alternator supplies enough current to run the engine and charge the battery. An alternator is a generator - the latter is an old-fashioned term normally used for a dynamo. In very basic terms, an alternator is different to a dynamo in that is produces AC which is rectified to 12v DC, whereas a dynamo produces DC straight off. Alternators are better are producing power at low RPMs, hence why modern cars use them.

In short - an alternator is perfectly capable of charging a battery. If you think otherwise, you're not very educated to say the least.

2. Current in a circuit is the same whereever.. Basic electronics here my friend. :p If something is drawing current fron the battery, it does not matter where you put the light in that circuit...it will still light up. Conversely, if nothing is using any current, no current is flowing and no matter where you put the light, nothing will happen.


Please don't become the new Howard Radford. If "your" old enough to know how an electrical system works, then I would have thought you can spell.