A control device for a battery of a marine vessel recovers a power storage amount of a chargeable lithium ion battery while protecting the same. When an acquired power storage amount of the lithium ion battery becomes equal to or less than a first predetermined power storage amount, a switch to connect the lithium ion battery and a line or a load is turned off. While the switch is off, as an electric power balance on the line, a power inflow amount, a first power outflow amount, and a second power outflow amount are acquired. Based on the electric power balance acquired while the switch is off, whether a chargeable condition in which the lithium ion battery is chargeable is satisfied is determined. When the chargeable condition is satisfied, the switch is turned on.