Batteries on a boat

Batteries on a boat are required to be marine batteries. Marine batteries are designed for the harsh vibrating and pounding they will receive on a boat. Marine batteries are therefore usually more expensive than car batteries. Don’t be tempted to purchase a car battery instead of a marine battery. A marine battery will last much longer and will be much more reliable than an car battery.

3 types of batteries for your boat

Marine Starter Batteries

Starting battery on a boat

Marine Starter Batteries provide quick but powerful spurts of energy over short periods of time and are designed to start the engine and be rapidly recharged by the engine alternator.

A starting battery should not be used for trolling motors or for powering appliances.

Marine Deep Cycle Batteries

Deep cycle batteries on a boat

Marine Deep Cycle Batteries are designed to discharge slowly over a long period of time and to withstand several hundred charging and discharging cycles.

A deep cycle battery is the right choice for powering an electric trolling motor and other battery-powered accessories such as audio systems, a windlass, depth finders, fish locators, and appliances.

Deep cycle batteries should not be substituted for starting batteries.

Dual-Purpose Batteries

Dual purpose battery on a boat

Marine Dual-Purpose Batteries combine the performance of starter and deep cycle battery, and are a good choice on smaller boats when there’s no room for two batteries.

While they’re able to perform the tasks of a starting battery and deep cycle battery, they’re not as efficient as separate batteries.

Starter batteries

Starter batteries are also known as cranking batteries. This is because starter batteries provide a large bump of amperes to crank over your engine. So starter batteries are naturally used to start a marine engine and are designed, unlike deep cycle batteries, to recharge rapidly. Deep cycle batteries such as AGMs are designed to deliver fewer amperes but are meant to work for extended periods of time.

Can you use a car battery? I guess you can but it is not advisable. You get what you pay for and this coupled with a simple risk assessment will tell you that spending the money on a quality marine starter battery is worth every cent…

Types of deep cycle batteries

Flooded Batteries

Wet or flooded batteries are the most widespread and cheapest kind of battery installed on vessels. They use a container of liquid sulfuric acid to act as a route between lead plates. When the battery is charged, the electrolyte produces hydrogen and oxygen. This requires vented battery boxes and compartments to let the gas escape safely outside the boat.

Due to the heat and gassing produced during charging, flooded batteries require regular inspection and topping-up with distilled water. Flooded batteries self-discharge at a higher rate (around 6.5% per month) compared to gel or AGM batteries, and therefore require year-round charging. Flooded batteries must be fitted in an upright position and do not tolerate high amounts of vibration. However, flooded batteries cope with overcharging better than gel and AGM batteries.

In my circumstances, my battery bank is under my bunk in the master cabin. So although somewhat cheaper, I think the maintenance issues, the creation of gasses  (under my bunk) and low tolerance to vibration rules them out.

Gel Batteries

The “gel” in gel batteries is a combination of fumed silica, sulfuric acid, distilled water, and phosphoric acid. The gel is quite sticky and prevents leaks if the battery is upturned or the case is damaged.

Charging gel batteries does create a small amount of hydrogen and oxygen to be generated at the plates. However, the pressure inside the cells blends the gases to create HO2.

Gel batteries are also referred to as “recombinant” batteries. This also keeps the battery from drying out due to charging cycles.

Gel batteries charge at a lower voltage than flooded or glass-mat batteries, requiring a boat’s charging system such as alternators and solar controllers to be very carefully regulated to prevent damage.

Absorbed Glass Mat batteries

Absorbed Glass Mat batteries or simply AGM batteries feature glass mat walls saturated with an acid electrolyte between the battery’s negative and positive plates. During charging, pressure valves allow oxygen produced on the positive plate to migrate to the negative plate and recombine with the hydrogen, producing water. AGM batteries have more jolt and vibration resistance than wet or gel batteries. AGM batteries are practically maintenance-free.

AGM batteries also have much lower internal resistance which allows more starting power and charge rates. AGM batteries recharge quicker than other types of deep-cycle batteries. They can accept a charging current, up to 40% of the amp-hour capacity of the battery compared to about 25% for the flooded type or 30% for the gel. This means AGM batteries recharge faster.

Long-lasting, safe, a self-discharge rate as low as 3% and excellent performance make AGM batteries exceptional dual-purpose batteries for yachties who require quick starting power and reliable deep-cycle ability. Naturally, you get what you pay for and you do pay more for AGM batteries.

Battery Tips

Batteries are extremely important. Here are some tips on how to avoid disappointment:

  • Secure your battery well. Use a specifically designed battery tray which can be secured to your boat with bolts or screws. Additionally, use solid straps to keep your batteries securely in their battery trays. This will avoid your battery wandering off during rough weather, reduce vibration and jolting and thus enhancing your battery’s lifespan.
  • Check your battery terminals often: they need to be tight and clean. A good battery will have bolt connectors as a wingnut has no place in this situation. Battery terminals must be free of any corrosion. Clean or replace them if there is any sign of rust.
  • Use a quality AC charger for when you are using shore power or if you are away from the boat for extended periods of time. Mastervolt makes quality chargers as do Victron.
  • Conversely, if you use solar or wind to top up your batteries then use quality controllers. Victron produces a range of controllers to suit any situation.
  • Install positive terminal covers to prevent sparks or existential touching when using spanners or other metal tools.
  • Recharge your batteries constantly and do not let their charge fall below the manufacturer’s recommended percentage. As a rule of thumb, no less than 80%.
  • Maintain the water level in flooded batteries: Lead-acid batteries need distilled water as much as your engine needs oil.
  • Install a quality power management system that will tell you in real-time what your batteries are doing as well as provide historic data. Battery management systems can warn you when your batteries have issues such as exceeding minimum percentage discharge. Have a look at the Victron range of solutions.

Well, the next question may be “what about lithium batteries?” A good question which deserves its own article…

Any doubtful points, disagreements or simply dot down your own experiences with batteries on your boat. Leave a comment!

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