[johnmyster]

I don't think a fuel pump draws more current at lower voltage.

If your concern is running your system at less than 13.8 volts...

Car audio guys used to install 7 cell battery systems (a 3 cell golf cart 6 volt, and a special 4 cell 8 volt battery in series) in order to have 14 nominal volts. They'd charge this one of two ways to around 15.5 volts. Either they'd use an inverter to step up from their 12 volt system, or they'd have a secondary alternator set up to run the higher voltage. Extra voltage into the amplifiers meant they could push extra power out of them.

Nowadays, car audio classes aren't based on amplifier rated power. Thus, the stimulus to do such things no longer exists. If you could find the needed batteries to run a 14 volt setup, you could run battery only and not have your "low voltage" concerns.

Or if you're doing two batteries anyhow (no weight concerns) then get a semi truck battery. They'd handle your commute with no voltage sag at all.