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12.7 minus .3 equals 12.4 which equates to shot battery which you proved with the new battery. But put that new battery on a maintainer to get it up to full charge. That is if your maintainer will do it. I have one that will not charge a battery that far down already. But a quick fast charge then switching to a maintainer works every time. Batteries and maintainer are kinda crazy. But you should be okay with yours even with my weak maintainer.