How to squeeze more life out of your marine batteries

A fully charged battery will be showing around 12.8 volts, and shouldn’t drop to less than 12.4 volts within a few days, or else it is showing signs that it is losing the ability to hold “lifespan” charge – the thing that results in your boat’s engine starting fine when you drop it in the water, but failing to fire once you’re ready to head home. Yeah, we all know someone who has been there.

Batteries may wear the “maintenance free” tag, but that doesn’t mean they’re a fit-and-forget commodity.

Even when a battery falls to between 12.1 to 12.4 volts, damage is being done. The lack of energy will slowly start to eat into the effective lifespan of the battery, an irreversible process that could be costly both on and off the water.

Letting the battery fall to 12.0 volts is worse, moving into the space where the lower charge’s impact on the reduction in battery lifespan is classed as “severe”.

“This is why charging your battery especially before storing for a long period of time is essential to maximizing the life of you battery” Century Batteries Johnny Kennedy said. “Also keep the battery in a cool, dry place, regularly check the battery's state of charge, and if it drops below 12.5 volts recharge it.”

There are two types of batteries used in on-water applications: a marine battery, and a deep-cycle battery. Let’s look a bit deeper into the differences.

The conditions that marine batteries have to deal with are much harsher than for a car. While a road-going vehicle basically moves in two dimensions, a water-borne one has to contend with the big vertical forces and shock generated by waves.

This means that marine batteries must be more durable to resist fatigue cracking in the outer casing, and the internal components. Using a car battery in place of a marine-rated one runs the risk of changing the distance between the active materials, faster internal corrosion, electrolyte spills and the worst outcome, battery failure.

Car batteries also have to behave very differently to marine ones. The car-based ones essentially need to have enough cranking amps to turn a big, heavy engine to start it, and then hand the day-to-day running of the electrical system over to the alternator.

In boats, not only does the battery need to start the engine, it also has to power an array of instruments, radios and other devices such as smartphones.

The size of the battery you get depends on the electrical load you’re potentially going to have to hit it with. The batteries normally come with a horsepower (CCA rating) that reflects how much cranking amps it can churn out, and “semi-cycling” – its ability to run electrical equipment.

Small 580CCA (cold cranking amperes) batteries will work for, say, up to 70hp boats as long as you’re running no electrics, while a 680CCA battery will have enough power to crank a 150hp engine and run basic electronics such as instruments, navigation lights, a bilge pump and a radio.

Even the highest-rated batteries, made for 350hp-plus engines, can only run limited electronics including a GPS chartplotter, a fishfinder, autopilot or trim tabs.

For anything else, that’s where our second type of battery steps in.

These are the tortoises of the battery world, designed to trickle out their power reserves at a slow, steady rate compared with the marine battery’s rabbit-like explosion of cranking power.

The term “deep cycle” refers to the conditions the battery is designed to operate under, involving a long, steady draw that deeply discharges most of its power. They’re optimised for “cycling”, or in this instance, regular and repeated draining and recharging.

They can come as either “flooded”, "AGM" or “gel” batteries; you can read more about the differences in their characteristics here.

The size of a deep-cycle battery is largely dependent on the amount of electrical load you’re likely to place on it, as well as a 50 percent “depth of discharge” buffer that builds in a damage-minimising safety net.

Make that batteries, plural. If you’re going to be running onboard electronics, then a combination of a high cranking-amp marine battery and a slow release deep-cycle unit will ensure plenty of outboard-starting muscle and slow-release electronics grunt.

The best way to do this is to use a continuous duty solenoid that will allow recharging electricity to flow across both the marine battery used to crank the engine and the deep-cycle battery, but will stop power flowing from the deep-cycle battery back to the marine one. This way, the marine battery remains isolated from the deep-cycle battery, and won’t ever draw on its energy reserves.

“If you’re unsure about what size deep cycle battery you need, visit our website we have a handy calculator that works out the power you need based on the equipment and devices you have on board,” Kennedy said.

“Boaties getting stranded due to a flat battery is one of the most common problems faced by coast guard and marine rescue services, and it’s a lot different to being stranded on land, so we implore you to always check your battery before heading out and make sure you have the right batteries for your power requirements.”