Hi Guys
Looking for some advice, I have just purchased a 1.9TD Fiat Ducato motorhome and want to fit a spare leisure battery to run my TV and freesat box through a 600 watt invertor.
Problem I have is charging this battery while on the move. I installed wires from pos + and neg - on the vehicle battery to the spare but with the engine running the spare is still showing 12.57 V, The vehicle battery is charging at 13.5.
What am I doing wrong, or am I doing anything right?
Help and/or advice is much appreciated.

TAM H

It is usual to only provide fused +12V feed from the main vehicle battery to the caravan/other battery. The earth connection is usually taken from the vehicle chassis.
On longer feed runs the vehicle chassis has a much lower resistance than wires.

What size wire did you use? (in square millimeters). at the moment is sounds/looks like you have impedance problems and are losing volts.

Try this:

1) Measure and record vehicle battery voltage (at terminals)
2) Same for auxilary battery
3) Aux battery voltage from battery +ve to vehicle chassis

Can you see the voltage loss?

Your explanation is pointing to my wiring, think I need higher guage wire. Also realise the -neg wire is not required to the main battery -neg only to an earth on the vehicle.
Will try that over the weekend.
THANK YOU

TAM H