#1 For any reasonable accuracy of a volt meter, the meter should directly
sense /
be physically attached to the battery banks + & - terminals not at the battery switch. Otherwise you're not reading battery voltage, just random bus voltage, with all the voltage drop associated with the wiring & terminations and this is most often not an accurate representation of the actual battery terminal voltage.
#2 If you want both volt meter to come on when you fire up a 1/2/B switch then you will want a DPST 12V relay that is driven by the "C" post of the battery switch. The relay will
increase the "on" consumption of the digital volt meters and you'll find average coil currents in excess of .075A.
#3 Beware of cheap Chinese voltmeters as the accuracy is all over the map. Remember every 10% of battery capacity is represented by roughly 0.1V change in voltage. For the best accuracy and "
trend monitoring" you'll want a voltmeter that reads to the hundredths position (12.6
5) not just the tenths (12.6). Also, using volt meters to track battery bank SOC is difficult unless using it as a "
stop discharge" guidance point. In other words,
do not discharge your 12V bank below 12.2V at your
average house loads.
#4 One of my favorite inexpensive volt meters is the Blue Sea 1733 DC Voltmeter. It draws just 15mA or 0.015A. If you want two of them this would be 0.03A. If you want two of them, plus a relay to drive them, then you're looking at 0.105A
+.
#5 If you use one voltmeter, as in the diagram below, and you forgot and left it on, consumption looks like this:
0.015A X 24 Hours = 0.36 Ah per day or 2.52 Ah per week
#6 You are better off with one digital volt meter and a SPDT switch (momentary or fixed your choice). There's no dire need to know the voltage of a bank you're not actively using.
#7 Don't forget that each battery lead to the volt meter needs a fuse within 7" of each banks positive post.
The below diagram keeps it simple.