The Innova units and their ilk will give you a fair idea of your voltage, but it won't actually be battery voltage. It gives the voltage on the circuit it's on, which includes all loads on that circuit along with the draw, though small, of the meter itself.
I have been using the Innova unit for a couple of years, but it always shows lower than an open circuit reading on the battery with a dvom, and it will go into the yellow and red zones much quicker than it should.
I'm not saying don't get one, if you're trying to save a few bucks, but use it only as a general guide and double check at the battery with a volt meter if it leads you to be concerned.
There are a number of after market meters out there that will do a better job, but of course you will spend more for those.
Being the cheap guy I am, and knowing enough electronics to get myself into trouble, I bought one of these:
3-1/2 Digit LCD Panel Meter, PM438 | MPJA.COM
Based on info from a couple of sites I breadboarded a circuit and installed it in a box. I have it set up to give me voltage and amps in and out with a couple of switches. Made cheap wire shunts based on info from another site.
Final testing is almost done. Readings are identical to the DVOM. Looking forward to the final install.