In reality not more than 2 min, but for design will say Max time is 5 minHow long is the turbo on, average, inporportion to total driving time? A little ancient history [1950-1963] : Instrument trucks used two battery systems, 12 & 14V, using 6V batteries with connecting straps cut on one to make 3- 2V cells. Charging via 12V & 14V alternators. I modified some by converting to straight 14V systems, one alternator, worked fine, nice bright driving lights & quick starting; all 7 then converted- but that was before every thing was electronic. If turbo time is relative short, a 2V battery , 50Ah, could be switched in via relay's and charged with buck converter from 12V system.