Car battery charging question

I left my lights on this morning (it had been raining) and ran my battery down. :smack:

I got a jump and got going again.

I seem to recall someone telling me that just revving the engine (with the transmission in park or neutral) will not charge the battery as well as actually driving the car.


Slight correction: not revving the engine, but idling the engine instead of driving.

If you ran it flat ( and it sounds like you did ) idling the car won’t properly recharge it. It would probably be prudent to either take it to a shop and have it recharged properly or go for a nice 50 mi. drive on an interstate (100 mi. round trip ) where you can get it up to some good revs for a substantial time.

I recommend this from experience :wink:

ps. If it started after you boosted it, thats a good sign. Take the drive anyway.

Why is this? What if I just put it in neutral and rev it up to highway speeds?

Perhaps GaryT will tell me I’m wrong, but IIRC, an automotive alternator will max out production at 1000~1500 RPM. Therefore, you can let it run on fast idle and charge the battery. At $2+ per gallon a charger is a better choice.

That’s a load of BS. Running your car will charge the battery. The actual time it take to charge the battery will depend on how discharged it is. But 20 minutes should be more than adequate.

Here’s a question. I’ve heard that if you run your battery flat, trying to charge solely off your alternator puts undue stress on it and you can risk blowing your alternator. Apparently, you really only should charge an absolutely dead battery with a charger. Any truth to this, or more car hooey nonsense?

I wouldn’t say undue stress, but think of it this way. Battery in car A is dead. Car A is jumped to car B. As soon as car A starts, and the jump cables from B are disconnected, the alternator in A goes max level. Should it work? Yes. Would I do it? No. I’d prefer to put the battery on high level charge for 30 minutes and verify that the cells are full.

In terms of alternator output, danceswithcats is essentially correct. Alternators do not produce at maximum capacity at idle speed, but at roughly 1500-2000 rpm they’re charging as much as they’re going to, and additional engine speed won’t help. This is different from the function of DC generators that were used in decades past, which didn’t reach maximum output until much higher engine speed.

There’s no magic to driving the car as opposed to revving it in park or neutral at high enough rpm’s. However…

…alternators are designed to maintain a battery’s charge and replenish what was spent to crank the starter. They are not designed to charge significantly depleted batteries. Trying to do so puts extra stress on the alternator, likely shortening its life. It’s also entirely possible that the battery will never get fully charged from just the alternator. Do your alternator and your battery a favor and charge the battery with an actual battery charger. And do it at a low amperage setting.

Lots of bad information here.
Simply put the alternator in a modern car has two functions[ol]
[li]Supply electricity for all the electrical items in the car when the engine is running[/li][li]Keep a fully charged battery fully charged, or to fully charge an almost fully charged battery[/li][/ol]
A dead battery is not almost fully charged. :wink:
The alternator acts like a trickle charger for the battery. It will only put a few amps over what the electrical loads in the car are. I know that some cars have crazy big alternators, but that is for the loads that are incurred while running, not for charging the battery.
If you were to take a dead battery, jump it and then run the engine for say an hour and then check the state of charge, you would find that it is no where near fully charged. I’ve done this in class to prove a point to non-believers.
My suggestion is if you run the battery flat, put it on a battery charger.