Dear Car Talk:
Back in the old days (i.e. 1964), we used to think that if you had a low or dead battery, it would charge faster if the engine was running faster. So we would drive it in a lower gear at high rpm's, thinking we were helping the battery charge faster. Was that true then? Is it true now? Was it ever true?
Yes, it was true then, and it's true now. The battery will charge faster if you rev the engine faster. Why? Because the faster the crankshaft turns, the faster it turns the belt that runs the alternator. And the faster the alternator turns, the more electricity it produces to run all the electrical stuff in the car -- and recharge the battery. But we don't recommend that you use it that way for a weak or dead battery. The alternator is fine for replenishing the battery after you start the car, and for keeping it topped up. But it's an expensive and inefficient substitute for a non-automotive, plug-in battery charger.
Think about the economics. If you want to charge your battery from a dead or low state, you're going to spend $8 on gasoline driving 50 miles to nowhere (actually, you'll spend more if you're driving in low gear at high rpm!), and, at the same time, you'll be wearing out your alternator, which costs hundreds of bucks to replace. Whereas if you bought a $29.95 battery charger and plugged it into the wall, the battery would charge fully overnight, and it'd cost you 31 cents.
In fact, when we buy a new battery to install in a customer's car, there's usually a warning sticker that cautions us not to use the alternator to charge up the battery; they want us to plug the battery into a charger, and avoid quickly turning the new alternator into an old alternator.
Plus, if you have a dead battery and the cause isn't obvious (like, you know you left your lights on or your in-car espresso machine was running all night), you won't know what the real problem is. Let's say the problem is that your battery is faulty and won't hold a charge. If you just drive around to charge it up, you'll wear out your alternator, and then you'll need a new alternator AND a new battery.
Or what if the reason your battery is discharged is that the alternator is on its last legs? Then you go on that 50-mile drive to charge up the battery, and the alternator dies when you're 25 miles from home, leaving you with a nice, fat towing bill.
So my advice is: If your battery fails or falters, have the battery and the charging system checked out. And while it's being checked out, any reputable shop will know enough to put your battery on the charger for you, and give it back to you all ready to go.