Where are you checking current draw, at the AC motor (AC amps) or at the fuse (DC amps)? Is it blowing constantly (Ie when you are on site) or randomly/intermittently? I would check battery voltage during a range of operation from fully charged to fully discharged, sounds like a bad cell or cells to me. DC current at the fuse will rise in inverse proportion to voltage drop for a given load and it may happen too quickly for the BDI to react before the fuse blows. As long as the relief valve is operating at the correct pressure you can discount the mast assy entirely.