Cessna 120-140 Association

my current cg is 12.8 and if i stay at that cg and add a starter (9.2 lb) and battery (pc 680, 15 lb), to maintain original cg, the battery would be placed 22.5 inches. correct or wrong?

Not enough information. How much does the plane weigh? Do you currently have a starter and battery you are removing? How much do they weigh, and what is the arm for each? Will the arm for the new items be the same?

non electric from factory, c85, so starter should be around 24" from datum. trying to figure where the battery will go, to find battery cable length and maintain original cg

The starter is forward (-25). The light weight starter is probably a few inches farther forward)) so the battery should be aft to balance. Original battery box is +56. Using these arms and your weights your CG will shift slightly aft to about 13.1". Putting the battery at +25 will put you about 12.6" but you'd be sitting on it... 13.1 is well within the acceptable range.

thanks, i'm thinking under the seat. i have the spring seats instead of the cushion type, so there is plenty of room with battery laying or mounted on it's side