My bass boat came with an on board charger. It is totally automatic.
It is very light as it doesn't use the old heavy transformer like old chargers use. I believe it is called a inverter type charger. It stays hooked up to the batteries all the time and it has a cord that I plug into an outlet. It has two banks and charges my trolling and engine battery at the same time. The electricity it produces is cleaner than old charges and it slows it self down to almost nothing when the batteries are charged. It gives you a green light when it is done and you can leave it on all the time if you wish. I also have a portable one like it that I bought at Lowes and use it on all of my other batteries. It really works great and charges your battery right every time. Also my portable one has other functions like a battery reconditioner and tester, reverse polartity protection and charging system tester. I believe it was like 65 dollars.