I would be willing to wager that less than ONE PERCENT of launches are performed by someone walking to their launch site unless on ones OWN PROPERTY. Thusly, it is an insignificant MOOT ISSUE discussing use of a car battery and it's weight. Nobody caters to the fringe 1% of their clientele unless it is for extreme luxury goods with giant profit margins per sale (Such as Ferrari automobiles or Patek Philippe watches).

I prefer to do it RIGHT (With a 5Ah+ Wet Cell with 12+V) or not at all.
Li-Po cells offer a lighter yet still high-current alternative. I wouldn't use 4AA cells for anything over a low current ELECTRIC MATCH.

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When in doubt, WHACK the GAS and DITCH the brake !!!

Yes, there is such a thing as NORMAL, if you have to ask what is "NORMAL" , you probably aren't !
Failure may not be an OPTION, but it is ALWAYS a POSSIBILITY.
ALL systems are GO for MAYHEM, CHAOS, TURMOIL, FIASCOS, and HAVOC !