Just received and installed mine last night. I was concerned when it read 11.9v (engine off), so I found this thread and read the comments about the inaccuracies/normalcies, etc. Based on my understanding of the issue so far, this is to be expected and I have to live with it. BUT since it supposedly can still be a useful tool, I'll need a little bit of hand-holding to understand it/how to use it with inaccurate readings.
My limited experience tells me that when my bike ('08 Ninja 250r) reads UNDER 12.2ish V with the engine off, the battery is dead (won't turn engine over). All my control readings were taken with the same multimeter (Ideal brand from Home Depot, probably $30-40). New battery reads 12.65ish+V (engine off). My current battery is still strong, and I left it on a Battery Tender last night for this test. With engine off, but key in 'on' position, the ADVmonster Voltmeter settles at 11.9v, while the multimeter reads 12.60V. With the engine ON the readings are as following: ADVmonster meter = 13.3V; and multimeter = 14.8V. This SEEMS like a pretty big discrepancy to me.
I realize this was a very inexpensive FARKLE, but had I bought a more expensive voltmeter, would I be seeing more accurate results?