RE: regular voltmeter and onboard?
The on board devices are useless as they don't put a load on the batteries.
The best and most cost effective meter you can use is the Hangar 9 load meter. Its about $40 at the LHS, has a universal connector a 0.5 and 1A load capability and a display that reads to 1mV. It also fits in your pocket.
I read mine EVERY flight on my single pack planes, once every few flights on my larger capacity, dual pack planes...
DP