Boats eat batteries. Replacing disposable batteries is simple, buy spares for all sizes, and the one you don’t have will be the one you need. In contrast, buying the right storage battery can be confusing. A number of different types are offered, at widely varying prices.
If your boat’s electrical system powers only a radio or a GPS when the engine is off, you won’t go wrong by replacing the worn out battery with a marine starting battery of similar size. f your boat has equipment that consumes power when the engine is off, for example, a refrigerator or a microwave, you need a marine deep cycle battery. A battery undergoes a deep cycle whenever more than 20% of its stored energy is consumed before it is recharged. Used correctly, deep cycle batteries can provide 200 to more than 3000 discharge/charge cycles. Starting batteries are not designed for sustained discharge and will last for only 50-60 use cycles.
You need to make three decisions when you buy a deep cycle battery. How big should it be? How long do I want it to last? What type of battery technology is best for my use. The "how big" is easy. Buy the largest capacity battery that will fit in the battery box. Deciding how long you want the battery to last is more complicated. High cycle life batteries cost more. However, the cost per cycle goes down as battery life goes up. You will want to find the best trade-off between price and cost per cycle.
A boat used six months of the year will experience about 60 battery cycles, about one for each day of use. A light/medium duty battery good for 200-300 cycles will last about three years. Use the boat almost every day of the year and you will accumulate about 200-250 cycles. A medium/heavy duty grade battery will provide 500-750 cycles, or about three years use. A sailor off on a world ranging voyage can put 700-1000 cycles on a battery in one year. The price of a 3000 cycle life very heavy duty battery will be a bargain for his boat but will give you a non electric shock. Remember, batteries wear-out even when not being used. Buying one that whose cycle life won’t be consumed for 10 years is not a good idea.
Once you have selected the battery life that’s best for your boat you need to choose the type you want from among the three generally available; flooded cell, gel cell or AGM (absorbed glass mat). Flooded cell batteries have removable cell caps and need monthly inspection of the fluid level in the cells (and addition of distilled water if it is low). To ensure long life these batteries must be recharged soon after each use. Like all batteries, they lose charge over time and need to be recharged about once a month. The gel cell and AGM batteries have no cell caps, only pressure relief valves, and require no maintenance.
When not in use they loose charge more slowly than the flooded cell type and can withstand being left in a partially discharged condition far better than a flooded cell battery. However, they cost more than a flooded cell battery; about 40% more for a gel cell and 80% more for an AGM. Also, neither the gel cell nor the AGM can last as long as the highest grade flooded cell brutes.
Now that you have bought it, keep it healthy: (1) keep the top of the battery clean and dry. A film of dirt will allow current to flow, slowly but surely discharging the battery; (2) avoid consuming more than about 50% of the stored energy from the battery before recharging it. For most types, the battery will be 50% discharged when its voltage drops to about 12.2 volts; (3) recharge promptly, no battery benefits from sitting around in a semi discharged state.
Consider your answers to these three questions when you buy your next battery: (1) are you willing to service it (2) will you recharge the battery after each use cycle (3) your battery budget. Regardless of the type of battery you choose, a battery that will last about three to four years is a reasonable trade-off for most people.