Melted Volt Meter

I have wired a duel battery system. Very basic with a solenoid (see Diagram). I have also installed a dual volt meter to keep an eye on both batteries. For some reason The Aux battery volt meter circuit board smoked. I'm not sure if it's something I did or was there a failure in the gauge unrelated. There is a Air compressor directly wired to the Aux battery Is it possible this is my Issue? I understand I will only get separate volt readings when the solenoid is in the open position.