I use 3 group 29 deep cycle batteries in my boat. 2 run 24 volt trolling motor and other runs everything ( lights , electronics , pumps , ect ) and starts big motor . The reason I run all deep cycles is because running everything off starting battery could pull it down too low on a full day trip .
My question is did the starting battery short out ? It failed to start motor and arched when I tried to jump off one of my other batteries . I had to disconnect that battery and hook up leads to one of my other batteries . Went and bought a new replacement as soon as I got back home .