Might want to check the specs on your voltmeter to see how much current it draws. Some are in the milliamp range. If that's the case, you can wire it direct (with fuse) to the battery. Yes, over a period of months, it can discharge the battery, but if the bike is sitting idle for months it should be on a trickle charger anyway. If you wire it into some other circuit, depending on the resistance in the circuit and the amount of current being drawn, the voltage indicated may be less than the actual battery voltage. Which is OK, as long as you remember that the reading is artificially low. If the idea of having the voltmeter always on bothers you, you can put a switch in the circuit.