hi all have 2 batteries main (starting one) and the second (to run all my aux lights)
the thing is can i put a volt meter on the aux battery and read it.
have got a relay that switches on with the ign (with a diode in line to stop feed to main battery).
with the ign on (relay closed) the voltage of both batteries should be the same as high voltage will flow to 2nd battery so not a true reading.
or am i making thing to complicated and just read main battery voltage.
j

You've got it right, you can only check your second battery with the engine off (relay open).

NeedleNose said:

You've got it right, you can only check your second battery with the engine off (relay open).

Click to expand...

Bingo

Your voltmeters will read whatever the highest capable source is - so if your batteries are at 11.5, 12.1 and your alternator is at 14.4 - if all three are tied together both meters will read 14.4v

This is because the batteries are no longer sources in this case - they are considered drains - the battery is suddenly an appliance that's drawing power from the alternator if there's a higher voltage source.

This changes during a trickle charge situation where the charger can't force the voltage up to a regulated high - so a mains battery charger can only usually produce a couple of amps - the display voltage will be quite close to the battery voltage.

An alternator though is a high current source and is voltage stabilised - so you'll always see the alternator output voltage.

thanks all i will still have a meter but will have a push button to test the voltage when the ign is off.
(so it will read just the aux battery)
j

terr said:

thanks all i will still have a meter but will have a push button to test the voltage when the ign is off.
(so it will read just the aux battery)
j

Click to expand...

That's what a lot of camper and have for their leisure batteries, little digital meter with a "test" button.