Actually both are important. But a good battery is where you start. A good battery design is key IMHO. From there you need a decent charger that can trickle charge the battery and not over charge it or over heat it near the end of the charge cycle.

I like the Blue Top Optima Deep cycle Batteries which I have had on my boat now going on the 8th year for my starting battery. The Deep cycle Optima I used for my trolling motor is still working but I took it off the boat and use it for accessories like fishing lights for night fishing. It's still accepting a charge and stays fully charged for 9 months.

I put a new bigger Optima Blue Top Trolling Battery on my boat for last season but didn't get to fish much last year. I just checked that battery yesterday and it was still fully charged. My older Optima needed a quick top off on the charger but it was showing fully charged in less than five minutes on the Minkota Automatic battery charger.

But in truth you need a good battery and a good charger to keep the battery charged up.