Can a screwed alternator burn a battery?

GTS ("GTS" ) gurgled happily, sounding much like they were saying:

Yes. Make sure you've got a multimeter to hand, and check the voltage at the battery once the car's running.

It shouldn't be much over about 14v. If the voltage reg in the alternator's knackered, then it could get to about 17v or so, which will boil the electrolyte in the battery quite nicely.

Thank you for the quick response. I will have to get a multimeter from somewhere - or I may just replace the alternator as it has not been charging the battery properly for a while - hence having to charge it myself. If it is giving out too high a voltage, would it cause the electrolyte to overheat so fast? It started smoking / steaming within a minute... Graham

Well the other option is that it's dicharging it too fast, but normally something else would get hot. IT normally takes more than a minute though, even with a fully charged battery, try starting the engine with the headlamps on & the rear window demist, that'll limit the current out.

Course it bloody can. If the reg fails and the voltage rises and falls according to engine revs (upwards of 50v is possible) then it will boil, or worse explode a battery with surprising ease.

Tim..

GTS ("GTS" ) gurgled happily, sounding much like they were saying:

They're only a tenner from somewhere like Amazon, Maplin or Screwfix.

They're so damn cheap and useful, I don't understand why every home doesn't have one as a matter of course.

Quite possibly.

Pretty rare at idle though, that takes a lot of current.

Hell's bells! I won't touch it again then until the alternator is replaced. I was going to do this myself - but having looked I can't see any way of getting the alternator in or out without much dismantling - there is just not enough space. (Rover 220SLi 1994). So off to a garage I guess. Aways the same dichotemy with old cars - spend more than their 'book' value on repairs, or scrap them and spend even more on a replacement... Graham

Almost unbelievably, I bought a good Gunsons' multimeter from Halfrauds during the week for £8..

Mike P

GTS ("GTS" ) gurgled happily, sounding much like they were saying:

Don't panic. He's massively exaggerating. There is no way on this planet an alternator will chuck out that much, and it will take a LOT of abuse to boil a battery completely dry.

He may well be laying it on a bit thick however unregulated or faulty alternators can chuck out high voltages

Thats one of the reasons they tell you not to run the engine with the battery disconnected , alternators put out ac voltage which has to be rectified

ac voltage is the average between phases o the cycle ,

Assuming the battery is ok I seriously doubt it would get near 50 volts. The internal resistance of the battery is too low. I had an alternator turn itself hard on once and the voltage was just over 17. Of course this was only a 75 amp one - larger ones might manage a wee bit higher.

steve robinson ("steve robinson" ) gurgled happily, sounding much like they were saying:

I know. I'm used to cars with external VRs which regularly go south. Highest I've ever seen (smelt?) is about 17v.

I've seen 16.5v on a 2CV with a duff VR. That made the battery bubble nicely :-)

Mike P

Accompanied by the sound of a chisel on slate Mike P, managed to produce the following words of wisdom

One of the limos I used to drive overcharged the battery until the case bulged and cracked open. Blew all the electrics in the rear, but the interior lighting was something to behold for a few brief, very bright seconds. The opera lights on the outside were like something from Close Encounters.

I've seen an alternator raise battery voltage to over 17V at idle, and that was on a system with a standard output alternator (80A IIRC), with three batteries to absorb the output (two 70Ah vehicle batteries, with a 120Ah auxilliary battery). We didn't try revving it once we checked the voltage at idle...

Yes. I suffered from faulty voltage regulator on my Citroen AX GT a few years back which for a while gave me candle effect head lamps until it decided to kill the battery. The battery had literally melted inside. It had to happen around midnight, in the middle of nowhere and pouring it down with rain as well!

Andy

What are the lights on the outside about? Is it to make sure the privacy glass works?

Ok I got a multimeter, and started the car with headlights, rear window heater, heating fan all turned on. For the first few seconds all was good - just under 14v. But then you could hear the engine / alternator change pitch - and the voltage went up to 17.5 volts! So at least it is now certain that the regulator is knackered. It looks like the regulator / brush assembly could be removed with the alternator left in place. Is this worth a try before replacing the whole thing? Graham

Yes - I replaced the reg. on my SD1 after the same fault some 10 years ago and the alternator's still fine.