Alternator voltage regulator: symptoms

Difficult to tell

Only until it went flat.

The battery wasn't flat then.

Assuming the charge light comes on when you turn the ignition on & goes out when you start the car: Measure the voltage with the headlamps on. Then check the obvious things

Thanks for your reply. I'll check the earth leads. No lights on the dashboard come on when I turn the ignition on. Not even the battery low light. There is not enough juice to turn the engine over. Or to work the headlights. Which leads me to suspect that the battery is not holding any charge at all, rather than an alternator problem.

Mike

With the engine running that's ok.

I wonder just how he checked it. The only easy way is to measure the output voltage and current at the same time while connected to a known good battery - without having extensive test gear. I suspect he used a clamp on ammeter - and if the battery had a faulty cell it wouldn't draw the correct charging current.

Try measuring the voltage while the engine is cranking - or the starter switch in use if it won't turn over. Or just turn on the headlights and measure the voltage. With a good battery the volts won't drop by much.

Modern batteries can die without warning - mine did. You'd need to charge it externally then perform a voltage test while it cranks. Alternately any decent battery shop should have a tester - if you can trust them.

Well the batteries definitely flat then. But if it was flat & the alternators not working then that also makes sense. Can you recharge the battery with a mains charger?

Thanks Dave. He just used a multimeter and pushed the probes on to the alternator terminals. He said the output voltage was all over the shop and I needed to replace the alternator.

However, I thought the alternator kicked in *after* the engine was running. And that an OK battery would turn over a small engine like the Micra with no problem.

I'm beginning to suspect that the battery is totally gone. Bruce

Strange measuring it at the alternator output - you normally do it across the battery. A good check at the same time is to measure battery positive to alternator output - that gives the voltage drop along the cable/connectors and also the same between battery negative and alternator body - for the same reason.

You certainly wouldn't see 14 volts with the engine stopped. And yes, you don't need a working alternator for the car to start - that task is done purely by the battery.

Easiest way would be to charge it with an external charger and see if it then starts the car ok. If it does - but subsequently fails to - it could well be the alternator.

Thanks. Some measurement data: Engine turned off. Battery is 12.50 Volts. Engine tuened off. Headlights turned on. Battery is 1.6 Volts

Looks like the battery is very weak.

Bruce

or you have a poor connection

In message , "Dave Plowman (News)" writes

Perfectly feasible to have a battery die like this. I had something similar in a Laguna last year - started up fine in the morning, ran all day with no problems. Stopped at a petrol station to fill up and it would not turn over. Luckily someone was nearby with some jump leads and got it started for me and the next day I got the battery replaced. No problems of that nature since.

Either way it needs charging from the mains

Thanks, Duncan. But if I charge my battery up fully, then start the car successfully and then the next morning the car refuses to start. Does this mean: a) The battery is knackered and can't hold a charge; or b) The alternator is knackered and won't do its job of topping up the battery It seems like diagnosing such a problem isn't as straightforward as I thought. Bruce

Start the car & then with the headlamps & the demist on watch the voltage, at idle it should, with a charged battery be over 12V, rev it to about

Assuming it's not a bad connection somewhere - it's not weak but fooked.

If it *is* reading 12.5 v off load it is charged. But fooked.

You're into a world of boredom or expensive multimeters though before you can actually say that. At which point you could actually measure if it was charging. Over 12V with no load from a flat battery isn't unusual.

I unscrewed the battery cells. It looks a bit dry in there. And I haven't topped them up in living memory. Is it worth topping them up at this stage? Bruce

If they're dry enough to make a differnce then the odds are the alternator is dead & it's taken the battery with it. Topping up with distilled/deionised water won't hurt though.

Never on any I've tested. With a high impedance voltmeter. If you're getting 12.5v off load and the battery won't even light the lights it means a cell or two has gone high resistance.

I reckon I'll need an new alternator + battery. Battery is about 50 pounds. Any ideas on prices for a reconditioned alternator? Thanks Bruce