Yikes.

That's beyond my scope of understanding. Can you hook up a multimeter in between the battery and the tank and see how it's drawing? Maybe there's some bizarre drain.

I know with lipos, even though they are rated a 3.7 volts per cell, it you run them to 3.5 or so, they are like 90% depleted. I mistakenly ran a 2 cell down to 6.8 volts (3.4 per) yesterday and when I charged it, it took 817 mAh, when it's only a 850 total. 3 volts is death and full is 4.2 per cell.

In my experience, electronics either work or they don't, they don't decided they way yours seems to be doing. The only outside influence is power.

But again, I'm just reaching here, I'm no expert by any stretch.