Lights on for extra battery charging?

October 1, 2013 | By Jim Richardson


Back when vehicles had generators, more than one garage mechanic told me a faster way to charge a dead battery while driving was to turn on the headlights. They all said turning on the headlights would boost the output of the generator. Dumb me was thinking this would only serve to drain any power being built up. While I was never able to prove or disprove this theory, neither did I run out the battery’s power by doing so. If that were true and I think it was, would it be true today with alternators?

I ask this because some models of new cars are prone to battery drain while sitting in the car lot. As a driver of dealer trades for a local dealer, I have had to jump-start many such vehicles. Once leaving the dealership, I have to remember not to shut the car off when I stop to get gas somewhere down the road. A few times I have forgotten this and needed a jump to get going. If turning on the headlights would indeed boost the output of the alternator, I might escape trouble the next time if I were to turn on the headlights. What say you?


Turning on the headlights will not help as far as charging the battery goes. An alternator or generator is designed to charge the battery at 14.2 to 14.5 volts on a car with a 12-volt system no matter what the demand of additional accessories might be. And batteries for six-volt systems get charged at 7.2 to 7.5 volts whether the lights are on or not. That’s because if a battery is charged too quickly it will shorten the life of the battery. Also, when a car battery is charged rapidly, hydrogen gas builds up and the battery can actually explode.

In the ’20s, cars didn’t have voltage regulators. Instead they relied on cutout switches to prevent over-charging. But the problem with them was that on a long highway trip, sometimes the battery could become over-charged and ruined anyway, so people would turn on their lights during the day to prevent that possibility. Also, with older, generator-equipped cars, the battery will be discharged more quickly when the engine is just idling if the lights are on, because the generator won’t be turning over fast enough to charge the battery completely and keep the lights burning brightly.

voltage regulator location
The voltage regulator on most modern alternators is under the electrical clip at the back of the alternator. On earlier cars it is mounted separately, usually on the firewall.

Older cars with generators used a separate voltage regulator with contact points in it to monitor the rate of charge and to prevent over-charging. Later cars sometimes used an external regulator, but more often, a solid-state regulator is built into the alternator and is employed to do the same thing. In any case, turning on your car’s headlights will not help you charge the battery more quickly.

The best way to keep the batteries charged on stored cars is to first make sure the battery terminals are clean and then use small trickle chargers to keep the batteries at their peaks. Of course, driving the cars every few days will do the same thing, and will keep the engine, transmission and brake seals wet so they won’t leak.