Running the car and the battery

  • 4 CYL
  • 10,000 MILES

I know it would be better to drive the car but does running the car idle for 20 or 30 mins help to recharge the battery at all?

Do you
have the same problem?
Friday, November 7th, 2014 AT 6:27 PM

1 Reply


Yup, but all generators are very inefficient at low speeds. Generators work with a magnet, a coil of wire, and most importantly, movement between them. That's why the belt has to spin the magnet, (electromagnet, in this case). The amount of current they put out varies depending on the number of loops of wire in the coil of wire, the strength of the electromagnet, and how rapidly the two are moving in relation to each other.

We can't change the number of loops of wire, at least not while the unit is operating on the car. It is not practical to speed up the engine when you need more electrical power to run the heater fan or headlights. The only way to adjust the output to meet the needs at any given time is to adjust the strength of the electromagnet. Fortunately that is real easy to do and is the job of the voltage regulator.

Up until 1959 there was only the common DC generator which was very inefficient. They did not charge the battery at all at idle, and at higher speeds about the best you could hope for was 30 amps. That wasn't enough to run all the electrical systems under all conditions so it was common practice to have to charge the battery at home every few weeks.

The AC generator was developed by Chrysler and first used in 1960. They copyrighted the term "alternator", but everyone uses it. Their next improvement was the electronic voltage regulator used in 1970 that added a pile of reliability to the system. Every manufacturer uses their own variation of this design today, and even the worst designs outlast the old DC generators.

The smallest alternator, electrically-speaking, that I'm aware of developed 33 amps, but it was hard to find because if you ordered any optional equipment at all, you got the larger 55 amp unit. Those were also Chrysler alternators. GM's most common ones developed 65 amps and were a pretty nice design too. Each one had their own advantages.

All AC generators are much more efficient, and current models are capable of as much as 140 amps. Typically you'll find 90 to 120 amp alternators on vehicles from the last fifteen to twenty years. To finally get back to your question, we still need that movement between the coil and electromagnet, but where the older DC generator didn't produce any usable current at low speeds, the newer AC generator does. Under a "full-load" test, one of the requirements is to raise engine speed to 2,000 rpm, or a little over double idle speed. Because of one of its inherent design characteristics, an alternator is physically incapable of developing more output current than the rated current it was designed for. That can be accomplished well before 2,000 rpm, but that engine speed is the industry-standard for testing on all cars. In practice, the typical 90 amp alternator will still be able to develop perhaps as much as 40 - 60 amps at idle.

It takes about 10 - 20 amps to run your electric fuel pump, injectors, and ignition system. Add in the dozens of computer modules, instrument cluster, and radio, and there's another 10 amps. If the electric radiator fan turns on intermittently, that's another 15 amps. Pretty soon there's nothing left to recharge the battery from what it lost from cranking the engine.

The battery will only accept about 20 amps when it's partially run down and trying to charge back up. If that isn't available, it will just take longer to become fully-charged. Current never drops all the way to 0 amps. When it drops to around 5 amps, the battery can be considered fully-charged.

That's probably a little more than you wanted to know. If you're considering storing the car for the winter, a better alternative is to simply disconnect the battery. I have to be careful when I say that because a few manufacturers have purposely designed in tricks that force owners to have their cars towed to the dealer after disconnecting the battery to have numerous computers unlocked or reprogrammed. According to some national trainers, Hyundai, Toyota, and Chrysler are the top three in the world for customer-friendly business practices, so it's unlikely disconnecting your battery will result in any problems.

There is no advantage to running the engine periodically. 99 percent of engine wear takes place when the engine is still cold and warming up. That's when some parts haven't expanded yet to fit properly. Oil doesn't flow well when it's cold, and the contaminants that build up in it don't vaporize well so they can be drawn off and burned. Those contaminants cause sludge buildup. If you run the engine for 20 minutes per month for six months, you will have caused two hours worth of engine wear. If you ran the engine for two hours straight at normal speeds, you will have done less than 20 minutes worth of wear. Basically you'd be stopping the engine just as the wear was about to decrease.

If you just want to keep the battery charged up, a better alternative would be to connect a small solar-powered battery maintainer. Unless the manufacturer specifies differently, like Cadillac, the industry standard is all the computers on the car can't draw more than 35 milliamps, (.035 amps), to maintain their memories. At that rate, Chrysler says a good battery will still be charged enough to crank and start the engine after sitting for three weeks.

Even if you do disconnect the battery, it's a good idea to charge it every couple of months or use a charge maintainer. It's normal for a battery to self-discharge slowly, and when that happens, the acid turns into a higher concentration of water. If that freezes it will bulge the sides of the case and distort the plates until they short together. There's no fix for that other than to replace the battery.

Without going into a pile of boring detail, home battery chargers do not put out as clean a charging current as an alternator does. It's current goes real high, then to 0 amps, then real high again 120 times per second. That will get the job done, but that pulsing current vibrates the plates in the battery. As any battery ages, the lead flakes off the plates and floats to the bottom of the case. When it builds up high enough to touch multiple plates, that cell becomes shorted, and again, the only fix is to replace the battery. To avoid that, always charge a car battery with a home charger set to the lowest setting. A good battery that is fully-discharged will be almost fully-charged after about an hour to an hour and a half when charged at a five to 10 amp rate.

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Friday, November 7th, 2014 AT 9:06 PM

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