But it's also the MOTORS. It generally is not good for motors to be run at too low voltages. If nothing else, that increases the damage to brushes and commutators
When voltage goes down the amperage goes up to perform the work. This is OHMS law.
So when you operate the top motor you should have the engine running @ 1200 RPM so the charging system is up to full voltage, 14.5 or so. This is much easier on the electrical system than with engine off and battery voltage dropping the whole time the top motor is running. Same with power windows.