Why does starter battery voltage drop?

[acropolis22]

...... when I switch on all the hab lights? So my batteries were both showing in excess of 14V when I arrived at the storage yesterday. I wanted to run down the leisure battery a bit to see what sort of current would come in off the solar. So I switched on all the hab lights. Leisure battery voltage started to drop and solar current started climbing. As I expected. But I also noticed that the starter battery voltage dropped steadily to about 13.75V. Is this normal? Not what I expected. What is the explanation for this?

 

[TerryL]

My guess. There is a link between the leisure and starter batteries to keep the latter charged when there is an excess of power to the former, such as a good solar input - that's how my system is set up using a battery master but there are other ways of doing the same thing dependant on vehicle.
When you load the leisure batteries this reduces or stops the excess voltage to the starter battery and it slowly reverts to a normal "resting" charge, i.e. 13.75v. I'd only worry if it continued to drop significantly.

 

[acropolis22]

My guess. There is a link between the leisure and starter batteries to keep the latter charged when there is an excess of power to the former, such as a good solar input - that's how my system is set up using a battery master but there are other ways of doing the same thing dependant on vehicle.
When you load the leisure batteries this reduces or stops the excess voltage to the starter battery and it slowly reverts to a normal "resting" charge, i.e. 13.75v. I'd only worry if it continued to drop significantly.

That makes sense. It does seem to stop dropping at around 13.75V. So normal behaviour then?

 

[chatteris]

by unlocking the van you have woken the ecu so will be using power

 

[acropolis22]

by unlocking the van you have woken the ecu so will be using power

It doesn't happen if I leave all the lights and 12V stuff switched off.

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[Landy Andy]

All batteries will drop as the 14v that you noticed on them was the charge put in from driving, and the base level of a 12v battery is about 12.6v and that is what they will rest at. Any time you see higher than that sort of voltage will be due to the battery not yet settled from having had some form of input (EHU, solar, engine power, charger)

 

[acropolis22]

All batteries will drop as the 14v that you noticed on them was the charge put in from driving, and the base level of a 12v battery is about 12.6v and that is what they will rest at. Any time you see higher than that sort of voltage will be due to the battery not yet settled from having had some form of input (EHU, solar, engine power, charger)

Cheers. Another day, another lesson...........

 

[berni109]

What solar controller do u have.
Mine a votronic will only divert charge to the veh battery when hab is completely full. So as soon as u start using hab power - controller switched off charge to veh bat.
12.75 is just about full resting volts for lead veh batt. Anything higher and it's getting some solar input assuming no other charger input.
More or less what Landy Andy said as I was typing but I always thought 12.8v was fully charged

 

[tacr2man]

as here