Battery not charging

Battery on my cab is going flat over time and 'obviously' the alternator is not charging it for some reason.  So, swapped out the alternator with a 2nd from local scrap dealer, but still no charge.  I say no charge meaning I'm not seeing ~14V across battery terminals with my multimeter.

With ignition on and engine not started, battery warning light comes on in dash, and goes out once engine started.

Have I simply bought a 2nd duff alternator from the scrappy!?!?  …but if this was the case wouldn't my warning lamp come on?  What else could be wrong

Ta,
Chris

Car is:
MK1 Golf Cab.  1.8 carb

How old is the battery? If the battery is stuffed, no alternator will charge it.

Check your wiring loom from the alternator - after 20+ years that goes and it won't allow charge through to battery.

Battery is ~3 months old so fine.

Voltage test between neg battery terminal and main alternator +Ve output pin read ~11V, which is the same voltage I'm reading when connected across battery terminals..

cocjh1 said

Battery is ~3 months old so fine.

Voltage test between neg battery terminal and main alternator +Ve output pin read ~11V, which is the same voltage I'm reading when connected across battery terminals..

There's been a number of people on here (I guess its the time of the year) who have had similar experiences, and found that their "new" battery is in fact faulty too. 11V across the battery terminals is pretty flat. I'd suggest that the new battery wasn't stored correctly and was supplied to you faulty (ie had been discharged, not checked, then left, and an irreversible chemical process happens on the cells which basically makes it faulty).

Also I'd not put much faith in a secondhand alternator from a scrappy, unless they were able to test it properly before removal and declared it healthy.

I'm pretty sure the battery is fine.  It only reads 11V now because it's flat.  Normally it holds charge and has been off the car a few times now to recharge when it eventually goes fully flat and not able to turn the engine over..  I always trickle charge it over a couple of days etc..

But failed battery or not I'm not seeing any charge going into it..  In the absence of any other problems maybe it's a 2nd duff alternator, but shouldn't the battery warning light on the dash come on?

Ta,
Chris

The dash light is either on or off, doesn't really give much info.

Fully discharging a car battery normally knackers it…..

Freaky - I was just about to post the exact same problem…as Paul says, it seems to be a bit of a common issue at the moment!

If I charge my battery up I can start it and then the voltage meter inside slowly drops over about 5-10 mins. I first noticed it on Boxing Day when out on a long drive so had to just nail it home and hope for the best, however didn't end up using it again as I was out of the country for most of Jan so the big freeze totally drained it. Just hope it hasnt killed the battery altogether as it was new on Xmas Eve for ?70!

There's a local company near to me which has got a bench set-up to test alternators so I'm going to take it down in the week….

I replaced the plug and the wiring today so can rule that out on mine (used vehicle wiring products as recommended on here - top service) - will let you know how I get on in the week Chris

The way to check would be to completely disconnect the battery (ie positive terminal), then monitor the voltage over a number of days or weeks.

Would you expect a decent battery to hold the full charge?

(It's forecast snow for most of next week so I'm assuming the sub zero temperature will speed up the discharge?)

Appologies for hijacking the thread Chris!!!

MikeyBoy said

Would you expect a decent battery to hold the full charge?

Yes! For months, if needs be!

As paul said, a fully charged working battery will hold charge for months, although as he previously said you should really trickle charge them, probably at least every two weeks.

Maybe you have a faulty radio or sometimes alarm, also sometimes the radio can be wired up in permanant mode, rather than memory only, which also can effect battery life. What I mean by that is that they have 2x +12v lines, one that should be connected to permanant live and the other switched live via the ignition, what some people do is wire both to permanant live, some radios draw a fair old current(relitivly) even when off this way.

If I'm not getting ~14V across the battery with engine on idle, does that mean the alternator is knackered?  What else could cause this other than what's been discussed so far on this thread??

- Worn brushes in alternator (easy to swap)
- Poor terminals or wires, basically anywhere in the circuit including the earths, gearbox-body earth strap etc.

Thanks for the help on this thread guys.  It's much appreciated.  

Anyway I fitted a brand new alternator last night.  I left exciter wire off alternator and battery light came on in dash to test wiring and bulb.  Reattached exciter wire and still not seeing ~14V across battery.  However with full-beam headlights on I can see the lights dim & brighten with the engine revs, so conclude the alternator is doing something.

Going to give it a week and see what happens.  Maybe it is just a knackered battery having been fully flat a few times now..

You might not see ~14v until the battery is fully charged.

People get confused with this, the alternator and battery are connected together directly, whatever voltage the alternator has, the battery has and visa-versa.

Therfore if you have a low chargedbattery the voltage you measure will be the same at both points (Alternator + Battery).

The only thing that will make a big difference is if you have a large resistance, or open circuit (i.e the wiring between the two is shot).

Mine was a faulty voltage regulator on the back of the alternator