I run a 1956 Johnson 30 HP with electric start several times a week at a lake in summer. Only for starting, no other accessories as motor does not recharge the battery. I just use a good quality car battery in a size that also fits one of my old cars. I believe I have used both 24 and 27 size. I recharge about every 3 weeks or when starter cranking speed sounds a bit slower. The charge usually lasts for about 3 weeks. Then I take the battery ashore and recharge. Done that for years, works very well.
