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 rpms, thinking we were helping the battery charge faster. Was that true then? Is it true now? Was it ever true?
-- Chris
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 nonautomotive, 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.
DEAR CAR TALK: We have a Subaru Outback and a Toyota Prius. As soon as there are more charging stations around, we plan to get a Tesla Model S electric car. We have a pop-up trailer from which, when not in use, the battery is removed and hooked up to a battery charger. To my surprise, the charger has kept the battery charged very well. When we get the Tesla S, the Subaru would be used only for winter driving and for vacations where we would have to tow the trailer. My question is: If I take the battery out of the Subaru for roughly eight months of the year, would it negatively affect the electrical system or anything else on the Subaru?
-- Vinnie
No, Vinnie. If you take the battery out of the Subaru, the only real danger is that you'll trip over it in the garage. So, alternately, you can just disconnect the battery and leave it right in the car.
And I wouldn't bother keeping it on the charger for eight months -- you'd just be wasting electricity. If you have a good charger, you can just hook it up the day before you need the car, and the battery should be fine by the next day.
That also reduces the risk that something would go wrong with the charger and it would burn down your house while you're gone.
When you reconnect the battery, there are a couple of inconveniences you'll have to deal with, besides the obvious stuff like losing your radio presets. Most modern cars have computers that "learn" and adapt the engine and transmission to your driving style. So those settings will all be lost, and the car will have to "relearn" them. That means you may notice the car running a little poorly or shifting a little roughly at first. But it'll go away quickly.
The other thing to keep in mind is that if you have a new or freshly "back from the dead" battery, you won't be able to pass a state emissions inspection until you drive the car for several drive cycles.
During those drive cycles, the computer is checking various components of the emissions system in your car. And only when it has enough data will it report a passing grade to the inspection shop. Typically that means you'll have to drive the car for a few days before getting it inspected.
My brother and I used to recommend all kinds of other things to people who were storing vehicles for months (squirting oil in each cylinder, covering the tailpipe and intake manifold), but we gave up on that, because nobody ever bothered with it. And in the vast majority of cases these days, the cars start and run just fine after being stored, anyway.
So just disconnect the battery, and don't forget to roll up the windows. Good luck, Vinnie.
Ray Magliozzi dispenses advice about cars in Car Talk every Saturday. Email him by visiting
cartalk.com
HomeStyle on 12/20/2014