Detecting 12v dc battery power non contact is tricky, there is hardly any electric field and no ac to pick up capacitively, probably the only way to do that is to have a bleed resistor dump some power so you can pick it up with the current meter, that or just bung a resistor divider and a meter or pic/'duino atod input across the power lines.
Silly suggestion:- A 555 could be configured to generate 100 khz rf, this can be injected to the power lines, lead acids tend to be high impedance to rf, then you can detect the rf on any power lines that are switch on, of course the switches would need to have enough isolation when off for the rf not to be coupled.