Battery charging
Batteries do best when kept on a charger, called float charging. For a 12V battery the normal charging voltage is about 13.3V, depending on the temperature and battery type. This counteracts the self discharging of the battery and keeps them at the best possible condition to prolong life. Check with the battery manufacture and get the charging specifications, most are available online.There are several failure modes of batteries. Almost all of them have to do with leaving the batteries in an uncharged state for prolonged periods of time. (The others have to do with overcharging). Boat batteries, unless plugged into shore power each night and on a charger, are almost always undercharged. This causes sulfation, plate corrosion, and therefore decreased battery life. A long time on a float charge can also help to reverse some of the sulftaion that is inevitable in batteries.As long as they are not discharged while they are on a smart charger the charger should maintain the proper float voltage and thereby prolong battery life (that’s what they are designed to do). Keep you batteries on a charger year round. Also make sure you check them just as you would over the summer (water level, connections, cleaning). They won’t use as much water because of the temperatures.BTW, I design battery chargers and install high reliability battery installations for commercial customers (UPS, telecommunications, etc.).