While I'm no expert, I think you'll at least want to run 10 percent, and I've heard many folks say they switch to 15 in the winter. Me, I always use 15 percent, seems like my engines just run better on it.

As for the voltmeter, plug it into any open channel on your receiver and it should work. I had one once and it always toldme I was fully charged even when I wasn't, so don't put a lot of stock in those things.