My math may be completely off here, battery capacity and runtime may not be as simple as this.

My Tandy 1100FD has a 6v 1.8ah gel battery. It claimed to get about 3.5 hours of life from this battery pack or about 500mah consumption.

That seems REALLY low for 80's hardware IMO.

If I substituted that with, say, a 5v/2 amp/3600mAh battery pack, would that be sufficient voltage to power the laptop? I know the built-in battery life estimation would be shot to hell. I have found one that fits the battery compartment fairly well.

Would most USB battery packs have built-in reverse charge protection? I had planned on using a diode to prevent reverse-charging the battery pack, but that may not even be necessary...