So battery Volts - 12.10V , with Engine Running - no load - 13.95V , Nav, Lights, rear demist on - 13.5V.

I read somewhere alternator op should be 14.2V to 14.7V ? and battery should be 12.6V ?

Have a look at this https://www.clubpolo.co.uk/forums/topic/335902-testing-your-alternator/

If your not having any issues don't worry about it.

I have had a flat battery a few times, RAC checked it and still said battery was viable and charging system was working. But it's really on the nose of my de-mist light ! As stupid as it sounds I have OCD and like everything just so. So the light not working is driving me bananas ! Last thing I had to do was pax door lock mechanism. But will check as per niges post and see what's what re the alt, does seem to be low and perhaps explains the battery going flat. She has done 184000 !!

Clean contact surfaces of the earth connection from engine to chassis/battery negative.

Where is the earth ? Resting battery voltage was 12.06V , no load alternator was 14.10V dropping to 13.36V with everything on. Rear demist does work but still no light. When starting the car there is a screeching noise from alternator area (not a belt slipping).

Battery voltage with no load and engine running should be 13.7v-14.2v. If it's anywhere between that it's charging ok. If the battery is old it could hav a partial dead cell causing cold crank amps to be down. You need a load test done on the battery to diagnose that.

What is the mileage on the car? If its a Valeo alternator expect it to be ready for the bin, I replaced mine with a Bosch.

 

Why don't you hook up another car via some jump leads and with both cars running try your heated rear window and if it lights up as normal you know you need to start looking at the battery/alternator

Edited 15 January, 20179 yr by SuperbTWM

Yeah I'll give that a go tomorrow. She's on 184000 !

Yeah I'll give that a go tomorrow. She's on 184000 !

I bet you a mars bar you will end up putting a new alternator on it to fix your issue

From what you've said it seems like it's charging ok, and points more to a battery that's had its day. Mine went after 12 years on the car.

From what you've said it seems like it's charging ok, and points more to a battery that's had its day. Mine went after 12 years on the car.

battery is irrelevant really (althogh i'm not saying its in tip top shape), when the engine is running the voltage is well over that of a good battery which means all the current comes from the alternator. The only way power is taken from the battery is when the voltage drops below that of the battery IE when the engine is off.

Edited 15 January, 20179 yr by SuperbTWM

battery is irrelevant really, when the engine is running the voltage is well over that of a good battery which means all the current comes from the alternator. The only way power is taken from the battery is when the voltage drops below that of the battery.

But if a battery has a dead cell the alternator can still charge at the correct level which th OP says it is 13.95 with no load, but only 12.1 with engine off sounds like a dead cell or partially dead cell. With all accessories on and charging at over 13 it is more likely the battery in my opinion.

But if a battery has a dead cell the alternator can still charge at the correct level which th OP says it is 13.95 with no load, but only 12.1 with engine off sounds like a dead cell or partially dead cell. With all accessories on and charging at over 13 it is more likely the battery in my opinion.

Yes, I can't disagree with that. A new battery would be a cheaper place to start anyway.

 

My mars bar is still on the alternator though  :D

 

What about a borrowing a better battery to try first?

New battery fitted (charged before installation) holding 12.66V , still no light ! Voltage with accessories on still 13.56V with engine running.

New battery fitted (charged before installation) holding 12.66V , still no light ! Voltage with accessories on still 13.56V with engine running.

What light are you referring to??

Rear De-Mist switch

You need to check volts drop on earth and supply circuits when under load, there will be you-tube guide on this that will show you better than I can explain!

 

If I was you I'd be checking the fuses which sit on top of the battery(part of the cover?).........poor connection/high resistance here will be obvious if the plastic has started to melt/deform.

Rear De-Mist switch

Have you replaced the switch??

Yes , twice ! Was intermittent now nothing. Demist still works.

New battery fitted (charged before installation) holding 12.66V , still no light ! Voltage with accessories on still 13.56V with engine running.

 

Have you tried another car running with jump leads attached yet?

Not yet, the wife's S-MAX was going to need major disassembly to get at the battery.

I have OCD and like everything just so.

Try some OCD on how to work your voltmeter and you'll have your answer!

Pray do tell JohnMack ?

Ok.

Do volts drop tests under load. You're looking to have minimum drop.

Check voltage at battery doesn't drop under 9v when cranking, it should remain above 9v for a period of 10 seconds easily.......you'll need to pull a fuse or put a live to the solenoid to do this. If voltage drops under you have battery issues........ You quoted 12.1 volts in first post.......this IS low as someone else mentioned but if starting ok then I suspect the battery is good but just discharged.

Did you check the alternator fuse at battery?.........is your car petrol or diesel?