IMHO Fuse #7 melts because it is already close to amperage limits in the best of circumstances, and fuse blade/terminal corrosion that is a very real threat for DeLoreans due to battery location pushes it over the edge.

There is a venturi tube that pulls battery gases into the passenger rear wheel well that does a very good job of keeping outgassing under control, but it only works when the car is moving. I suspect that is why DeLoreans that sit for protracted periods of time have so many more fuse box issues than DeLoreans that are driven regularly (I'm talking thousands of miles per year, not regular trips of a mile or two apiece).

Simply starting the engine and letting it idle while the car sits still definitely does absolutely nothing to vent battery gases.