Alternator output

[Bigted]

hi all,

Just trying to understand the alternator charging a little better. My beach (140 bmt) always seems to push around 14.8v at both batteries all the time (even after hours of driving) - even when it's just been on hook up. Should the voltage drop as I presume the leisure battery would be full after hook up. My drive battery never seems to be that full (12.3 to 12.5v at rest) even after a long drive.

Any comments / insights welcome!

 

[AlanC]

Hi BigTed,

That is exactly what I would expect.

The way various batteries and charger combinations work is dependent on the chemistry of the battery in question.

You have two lead-acid batteries on your van. A sealed wet electrolyte type (i.e. contains liquid Sulfuric Acid) in the engine bay, which supplies the power for the starter motor initially, and a leisure battery which is a sealed AGM type (Absorbed Glass Mat) and has a solid sulfuric acid electrolyte which means it cannot spill any acid if tipped or damaged.

The charge for both lead acid types is a constant voltage to the terminals of the battery. The alternator will control this, providing a constant 14.5volts approx throughout the charge no matter how long that is. The current drawn from the alternator by the batteries will depend on the state of their charge, starting high and reducing to a trickle of around 0.5 amp when fully charged.

When not being charged both batteries terminal voltage will drop to around 12.5 volts approx at rest, just as you have found.

So all is OK with your battery system.

Alan

 

[Bigted]

Thanks Alan,

My engine bay battery is also AGM as I think they moved to these for stop start function. I seem to be getting a big voltage drop off on my leisure battery when my Waeco cfx28 fridge kicks in (plugged into 12v socket behind passenger seat). It cuts in the fridge battery protection very quickly (set to medium). The fridge is rated at .75 ah/h so I would expect a couple of days running before need to recharge. The max 12v draw is 6.5 amp according to specs.

Any ideas?

 

[Bigted]

Also my lesuire battery reads around 13v at rest after charging

 

[WelshGas]

Thanks Alan,

My engine bay battery is also AGM as I think they moved to these for stop start function. I seem to be getting a big voltage drop off on my leisure battery when my Waeco cfx28 fridge kicks in (plugged into 12v socket behind passenger seat). It cuts in the fridge battery protection very quickly (set to medium). The fridge is rated at .75 ah/h so I would expect a couple of days running before need to recharge. The max 12v draw is 6.5 amp according to specs.

Any ideas?

That is what others, with a Beach, have found using the 12v socket behind the Passenger seat, but not when using the 12v socket in the Boot. Why, I do not know. It could be due to the wiring causing a voltage drop and so the auto cut out activates. Some advocate wiring a separate circuit with thicker wire, fused, directly to the Leisure Battery with a low resistance plug and socket.

https://vwcaliforniaclub.com/thread...-hard-wiring-fridge-to-leisure-battery.17642/

 

[WelshGas]

Also my lesuire battery reads around 13v at rest after charging

Leisure Battery should show about 12.7 v at rest. The variation will depend on the quality of your voltmeter. As long as it is consistent then it might just be over reading.

 

[AlanC]

Also my lesuire battery reads around 13v at rest after charging

It is within spec, so I wouldn't worry about 0.3 of a volt at rest.

Alan

 

[AlanC]

Thanks Alan,

My engine bay battery is also AGM as I think they moved to these for stop start function. I seem to be getting a big voltage drop off on my leisure battery when my Waeco cfx28 fridge kicks in (plugged into 12v socket behind passenger seat). It cuts in the fridge battery protection very quickly (set to medium). The fridge is rated at .75 ah/h so I would expect a couple of days running before need to recharge. The max 12v draw is 6.5 amp according to specs.

Any ideas?

As others have said, the wiring on the front socket may well be rated at a lower current value than the rear one. No idea why this may be. The mystery of VW...!

Alan

 

[Max-Felix]

Thanks Alan,

My engine bay battery is also AGM as I think they moved to these for stop start function. I seem to be getting a big voltage drop off on my leisure battery when my Waeco cfx28 fridge kicks in (plugged into 12v socket behind passenger seat). It cuts in the fridge battery protection very quickly (set to medium). The fridge is rated at .75 ah/h so I would expect a couple of days running before need to recharge. The max 12v draw is 6.5 amp according to specs.

Any ideas?

Same as mine and others - see the topic WG linked - I posted loads in there and won't repeat it all here. In short hard-wiring is easily the best and most efficient way if using the fridge regularly - it does not mean you cannot unplug it but..blah, blah, blah...see link!

https://vwcaliforniaclub.com/thread...-hard-wiring-fridge-to-leisure-battery.17642/