I don't believe a V3 calibrated for 12v would even run on a 6v batt. What serial # is your V3? Some of the early units shipped mis-calibrated: the low batt light was calibrated at too high a voltage so it would trigger too early, say 6v (under load) instead of 5.8v. The auto-shutoff was still properly calibrated for a 6v SLA (at whatever voltage that is, don't recall off-hand).
There are instructions for re-calibrating on the Grace Design page, or you can ship it in and I believe they'll do it at no cost. Also, if it's an early serial # (1-100, I think), you may want to check to see if it has a DC offset. I think the first 100 units shipped with a DC offset. This, too, they'll "fix" at no cost.
Here's a
post from Michael Grace re the DC offest issue on the Oade Bros board.
first of all wasn't sure where to post this. move to remote power if need be. I'm using 6volt eco's (bp50) I've had the batterys for 2 and half years. used them with my p1 and the v3 I had since aug. I've never ran them all the way down. with the old v3 I NEVER saw the battery light flash ever. now with the new one it goes on after 2 to 2 and 1/2 hours. the v3 is supposed to be 6 volt. any chance it's 12? would it work with 6 if it was 12?? I know my batterys could be loosing juice but I never had an issue untill the new v3. happens almost every show now. I thought it was only one battery but it's not. happened with the "good one" last night/