I didn't even have a generator so I bought two 12v Honda Civic batteries. They are available everywhere and weigh about 22#'s each & are $50-$70 each.
Both fully charged at home. Battery one would easily take care of tech, qualifying and 2-3 rounds of eliminations. If I was still in the program, switch to the second battery which was fully charged. That would take care of the rest of the day. If I was worried, I would charge the first battery on a buddies generator. With terminals attached to each battery, swap takes about two minutes. If that. When racing out of town I'd charge the batteries at the hotel. Yes, in the room.
This is a SS car with no alternator. I finally bought a Honda suitcase generator and one battery was good for the entire day.
I am changing to 16V.