Can a car keep going with a dead battery but working alternator?

I'm a bit rusty on the theory, and could be completely wrong, but I believe an alternator needs a bit of current from the battery to 'get going' by using the current to generate the beginnings of a magnetic field which is needed to generate electricity. Therefore a dead battery may result in a dead alternator unless there is enough residual magnetism to get the generating process started.

"Rob Beattie" wrote

I drove a Citroen xsara at least 70 miles with a dead battery. It just died without warning, I got the car jump started and drove home, when I got home I switched off and then tried to restart but it was no go. The next day I got a new battery and asked them to test the old one. There was not enough charge to register on their meter. So it would seem that the alternator provides all the current a car needs.

pb

The answer is that an alternator does require a smal magnetic field to get it going. This is usually provided by the current going down the ignition light.

An alternator normally will kick off itself but it wil require very high revs to get it going. You know if the alternator is OK if the ignition is as bright as it normally is with ignition ON and the engine not running, and fully off when the engine is Running. If the ign light is slighly dim when the engine if stopped or running then the alternator is shagged.

NEVER run the engine without a battery connected as the volts produced by the alternator can be high enough to reverse bias the diodes and blow them up. Running with a shagged battery is usully OK - unless it is open circuit. My guess is that you have a shagged battery. So change the battery. If you must, get a garage to test the alternator - it should only take 15 minutes. Without the garage test, if the car starts OK after a week of using the new battery then the alternator is fine!

I ALWAYS get a heavy duty battery - even for a small petrol engine. You can get them for about £50.

Good luck.

Been there, done that! The alternator will usually have enough residual=20 magnetism to get started, and once delivering power should continue to=20 do so until the output drops low enough to stop the engine. In my case,=20 while slowing down to exit the motorway to Birmingham at 5pm on a Friday=20

- I was not popular! The alternator was fine later, with a new battery. Note that with modern cars with ECU's there might be a danger of voltage=20 spikes damaging the electronics; I haven't tested this but wouldn't care=20 to risk it unless proved safe. Dave

Correct. The battery acts as a buffer to supply any brief higher current demands that the alternator alone cannot supply.

A fautly battery does not load an alternator correctly, so I have found that it is impossible to test an alternator accurately in such a condition. If the alternator was knackered as well, then the engine would die as soon as the jump leads were removed. The initial connection to a boost batery will initiate operation of the alternator. From then on the magnetic field in the rotor is controlled by the regulator using current generated by the alternator itself.

HTH

Anthony Remove eight from email to reply.

Correct. You're completely spot on on the theory. I love watching people trying to bump start cars with completely dead batteries and scratching their heads as they tow them a mile up the road at 30 MPH in

Thanks to all who replied. Very helpful. The consensus appears to lay the blame at the foot of the battery so I've had it replaced. Keeping my fingers crossed and AA card with me over the next few days of motoring.