I've run SR77's with V3 a lot. The Earthworks draw a lot more phantom power than most mics... like 10 or 12ma compared to 2-3ma for most other mics. So battery times will be shorter.
For regular evening shows I use 9V DVD batteries, and unless a battery is on it's last leg, it will go 3 hours, even with Earthworks. With DVD batteries the V3 low battery light should never come on, so go by the DVD batteries. I use the DVD batteries because I use the same thing with my R4's, so it's handy.
I've also got a bunch of old 6V SLA's, which I use for festivals where I save the DVD batteries for the R4's. Sometimes in the middle of the day on the side stage there are bands that I really don't give a shit if the battery dies or not
, so I will run the SLA's and push them to the limit. I've had SLA's run for 6 or 8 hours, with AKG's or Gefells, and without the A/D. Once the low battery light comes on I don't panic, I frequently run for another set, and I think I've gotten 3 hours with the low battery light blinking. That's primarily a function of the individual battery of course.
If you run a 12V SLA, the V3 will draw the same amount of current (milliamps) as a 6V SLA, and the extra wattage just goes into waste heat. 12V doesn't get you anything, unless you have a bunch of 12v batteries and want to use them. Otherwise don't bother. If I used 12V batteries, I wouldn't send it back to Grace and get it recalibrated, I'd just carry a small voltmeter.
I'm probably going to buy a couple more 6V SLA's and rebuild my dead ones. Last time I looked they were something like $15 for 12000 mah. Heavy, but if the bag stays in one place for 12 hours a day, I can live with it.