Strange battery goings ons

[Captain Furious]

So I was wiring some towing electrics last night and all was going well, until it came to testing the supplementary socket for caravan supply

 

As you may probably know these are controlled by a voltage sensing relay so that certain circuits are only powered on when the engine is running, the relay only energises when it detects above around 13.2 volts.  Even with the engine running it was only getting 12.4V

 

The immediate thought was that the alternator might be knacked.  it seemed strange, because there has never been a problem starting, infact it always turns over very enthusiastically and often needs to be cranked for long periods of time if it hasn't run for a while, which the battery never fliches at.  Its also near enough a brand new battery and a bloody big one at that.

 

My mate had a look in his garage of many things and found another alternator we could substitute, so we fitted that, fired it up and exactly the same, 12.4V.  at this point I either have two identically knackered alternators or some other electrical fault, so I checked the voltage out the back of the alternator and it was 14V as you'd expect. (probably should have done this first but hey ho)

 

So we spent the next 3 hours tracing wires, testing resistance, looking for some cruddy connection or bad earth that might be dropping the voltage somewhere, nothing was obviously untoward so I took a slightly different approach.  I disconnected the cars wiring and connected the battery via some jump leads to another car which I know to be good, 14V measured at the jump leads, as soon as you connect them to this battery, 12.4!

 

Likewise, connecting the battery of the other car to this, gives a solid 14V charging rate, and 14V throughout the car, so the cars wiring must be fine.

 

It just seems that this battery isn't producing enough voltage, at rest it sits at about 11.8V, with the engine running 12.4, yet it is most definitely not flat.  It starts it every time, and as I say it turns over with ease, the only thing I have noticed is the lights and electric windows seem a bit dim and sluggish, probably due to the low voltage

 

So I'm not really sure if this is a question, because I think I've ruled the battery at fault, but has anyone ever come across anything similar? 

 

I am going to check the receipts folder to see if the receipt for it is in there, though I have a feeling even if it is, convincing anyone that the battery is at fault and not the car could be tricky.

[Jifflemon]

Couple of q's from me:

Whats the vehicle?

How big is this battery? resting voltage of 11.8v doesn't sound healthy at all. However, the low charge rate when running could simply be the alternator working flat out to charge the battery up.

 

were you getting 14v at the back of the alternator with this battery on and engine running?

[Captain Furious]

Vehicle is a Mk2 Granada, so I am fairly certain there is no electronic shenanigans going on that interfere with the charging system. 

 

The battery is, I believe, an Exide rated at 95AH and 800CCA and yes, I was getting 14V from the back of the alternator with this battery connected and the engine running.

 

Thing is I've put battery's that are stone cold dead on cars and jumped them before and seen 14V almost immediately whilst running.  This one shows no sign of being discharged other than its resting voltage, ordinarily 11.8v would be flat and have no chance of starting a car, but this will crank it over for hours no problem at all.

 

The more I think about it the more it is wrecking my head, because if there is 14V exiting the alternator but only 12.5 arriving at the battery\rest of the car it surely points to something in the vehicles wiring dropping the voltage somewhere or drawing a large amount of power.  Yet with a different battery I get 14V.

[red5]

Sounds like battery to me. Duff cell I'd imagine.

 

However

 

Engine running, multimeter on 20v DC, black lead to battery -ve, red lead to B+ terminal on back of alternator (not battery +ve)

Should show 13.8 - 15.0 volts.

 

Next

 

 

If possible you need to stop engine firing - disconnect coil/etc. 

Engine running, multimeter on 2v AC, black lead to battery -ve, red lead to B+ terminal on back of alternator

 

Should show less than 0.5V AC

 

Then

 

Engine off, multimeter on 20v DC, black lead to battery -ve, red to battery +ve.

 

Check reading, then crank engine and check voltage reading - shouldn't drop below 9.5v

 

Yay.

[Captain Furious]

Thanks for that, I didn't think to check for any AC leakage, I'd kind of ruled the alternator out as the cause when the 2nd one gave exactly the same results.  but i'll give it a proper check over, especially if I'm putting a new battery on

 

I think it probably is the battery too, the clue other than that is when the engine is off (and even when its on to an extent) everything is very dim and things like the electric windows struggle to go up - as if it was a flat battery, then you turn the key and it spins over like a good one. 

[hauserplenty]

Check the ground straps too. Any loose/corroded contacts on the engine or frame can cause similar problems, though obviously replacing the battery is called for.

[danthecapriman]

Vehicle is a Mk2 Granada, so I am fairly certain there is no electronic shenanigans going on that interfere with the charging system. 

 

The battery is, I believe, an Exide rated at 95AH and 800CCA and yes, I was getting 14V from the back of the alternator with this battery connected and the engine running.

 

Thing is I've put battery's that are stone cold dead on cars and jumped them before and seen 14V almost immediately whilst running.  This one shows no sign of being discharged other than its resting voltage, ordinarily 11.8v would be flat and have no chance of starting a car, but this will crank it over for hours no problem at all.

 

The more I think about it the more it is wrecking my head, because if there is 14V exiting the alternator but only 12.5 arriving at the battery\rest of the car it surely points to something in the vehicles wiring dropping the voltage somewhere or drawing a large amount of power.  Yet with a different battery I get 14V.

I get similar things with voltages on my mk2 Granada!

It's got a nice big (Exide!) battery in good order, new alternator and I've tried all sorts of things on mine but the voltage always seems to be lower than what my other cars get.

Yet the car turns over easily and always starts.