I had a battery drain on my 1986 Porsche 944S which left the battery completely drained (solved, it was the radio). I jumped it successfully and took it on a drive to charge the battery up. As I was driving I flashed my brights and was surprised when the engine seemed to almost completely cut out for a fraction of a second. As I drove around more I kept flashing my brights and this dip in power got less and less severe until it disappeared and flashing my brights made no impact.

I checked the voltage with a multimeter, when running I am seeing about 14.3 volts DC. My question: is this typical behavior when running on a flat battery, or is it a symptom of a problem?

  • Ideally, you'd charge your flat battery on a battery charger, not your car's alternator. I suspect your "problem" of the engine cutting out when turning on your headlights was related to driving on a flat battery. When you say "battery leak", do you mean your battery is leaking fluid? If so, get it replaced.
    – cory
    Feb 22, 2017 at 15:35
  • I meant a battery drain @cory, not a leak as in leaking acid. I've edited to correct it.
    – GdD
    Feb 22, 2017 at 15:57

1 Answer 1


To put it simply;

Your alternator was working very hard to charge the battery while you were driving. The additional load of flashing your high beams added to the amperage draw which in turn added to the load on the engine turning the alternator.

As the battery became charged, it was able to help buffer the amp draw until the charging was minimal and the load on the alternator was normal again.

It would be ideal to charge the battery with an external charger rather than put that much strain on the alternator. But in this case, it is most likely normal behavior and not any problem (since you know what drained the battery in the first place).

mechanical load of an alternator
alternator operation

  • I would like to have been able to put the battery on a charger, but needs must in this case. Thanks for your good answer.
    – GdD
    Feb 23, 2017 at 10:40

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