I don't have a set amount of time but they seem to go at least 4 years before I start to notice they need to be replaced. They are not both on the same schedule so I am happy just letting them show signs of needing replacement before I even worry. Since I can use 1 or both for starting I just let them go until they don't have enough charge to start the boat effectively.
5 years is about average life for boat batteries if buying better quality ones. Cheaper ones seem to go out around three years or so.
Keeping the terminals shiny clean and very tight helps a lot.
If the style with caps - keeping water level filled ( distilled water ) adds to the life as well.
Biggest thing that hurts is a battery being drained all the way down. Deep cycles can handle this better then starting type batteries.
Air temperature plays a huge role - a weak battery might do OK in 80* temps. but fail when it gets cold.
I replace mine every 5 years - I make sure to check the manufactered date to get fresh batteries. I've seen batteries over two years old for sale in some stores. Try to deal with a store that turns over the stock often - no sense spending $$$ on old battery.
Since I have two batteries, and only use one at a time, I keep them until one fails. I then load test both and replace any that are bad. Since I only use one battery at a time and use it for starting, running the washdown pump, and the radar/depthfinders I can afford to have one battery fail during a trip. Historically I've gotten 5-6 years out of my batteries.
Hybrid Mode
