Very good.
My 50 Olds had room for a second battery, but it would have been a 2nd
6-volt battery. When you only have 6 volts, you often need a secodn battery, but I never got one. One December night it wouldn't start and for some reason I called AAA or something, and they couldn't start it either.They sold a device that would rearrange the connections of the two batteries. Never had one but I think it went from parallel for charging to series for starting.