The power available to turn the starter depends on the voltage available. What many people don't consider is the internal resistance of the battery. About the worst setup to use is the gelcel, which has a fairly high resistance, plus a pretty short life. They do work well as doorstops, so not a total loss.

I built my first cordless starter over 25 years ago. Just a couple RC car packs in series of only 1200 mAh which was plenty for a weekend. I would select batteries today based on what you have on hand to charge with or what you want to buy. Some chemistries tend to self discharge.

Regardless, keep the voltage high, the internal resistance (# cells x mOhm/cell) low, and the wiring heavy duty, short and clean.