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How to choose the right battery for your boat

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Interesting facts about marine battery or boat battery.

In order for you to be able to fully enjoy your time on your boat or yacht, you need a reliable, constant and safe power supply. The most important things at a glance:

  • Low-maintenance wet batteries for boats are inexpensive
  • Maintenance-free GEL boat batteries can be installed in any position as a supply battery
  • Maintenance-free AGM marine batteries are the right choice for most boat owners. You can use many models as consumer batteries and for electric boats
  • Maintenance-free lithium boat batteries with 5x longer service life are considered the ultimate among boat batteries

Which batteries for boats are available?

The marine deep cycle batteries must be powerful to power important devices such as navigation systems, interior lighting and refrigerators. Depending on the intended use, different battery technologies can be considered for use on boats.

Marine battery technologies in detail

  • The classic wet batteries for boats and yachts

Battery manufacturers have been constantly developing wet batteries since they were invented. Today, the low price is the main argument for buying them. During charging, hydrogen and oxygen are produced, which escape through a valve in these sealed boat batteries. In order to refill distilled water, the battery must be easily accessible.

Current lead-acid batteries require little maintenance and have low gas emissions. Thanks to medium vibration resistance, you can use them on your boat. For optimal durability, wet batteries should only discharge to 50 percent.

Sealed wet batteries can gas during charging. In this secondary process, water is broken down from the electrolyte into hydrogen and oxygen. Depending on the model, it may be necessary to top up with distilled water, with the exception of “maintenance-free” (MF) batteries.

  • GEL Batteries for Boats & Yachts (VRLA)

Due to their characteristics, GEL and AGM batteries are referred to as valve-regulated lead-acid batteries (VRLA).

With a GEL-like electrolyte, they are a further development of closed batteries and do not gas during normal operation. VRLA batteries require no maintenance and you can operate them at an angle. With proper treatment, they can survive several cycles. Strong overcharging leads to irreversible damage.

GEL batteries bind the actually liquid electrolyte with silicic acid, the electrolyte is no longer liquid and is in a GEL-like state. This means that operation in an inclined position is unproblematic. Batteries based on GEL technology are capsize-proof, maintenance-free and cannot leak. The main advantage over AGM batteries is the lower self-discharge.

Water decomposition is almost completely reversed internally (recombination). In the event of overcharging (e.g. due to excessively high charging voltage), recombination is suspended and the integrated pressure relief valve is activated to discharge the gases. Due to the relatively high internal resistance, they are not suitable for starting the engine, so GEL batteries on boats are only suitable as supply batteries “Deep cycle batteries are often used for electric outboard motors because they are perfect for use due to their cycle stability. Deep cycle batteries are virtually the perfect marine batteries and meet all requirements such as low self-discharge and high cycle stability.

What is a agm battery? The closed AGM batteries (absorbant glass mat) integrate modern and contemporary technology. Glass fiber fleece binds the electrolyte, the glass fiber mats are located between the plates and absorb the acid. Therefore, these boat batteries are leak-proof, maintenance-free and can be installed in any position, just like GEL batteries. For safety reasons, overpressure valves are also part of the equipment and are triggered in the event of severe overloading. As with GEL batteries, frequent overcharging leads to an irreversible loss of electrolyte liquid and should be avoided as far as possible by using chargers.

Batteries based on AGM technology can deliver high currents, making them suitable for energy-intensive consumers such as electric windlasses and bow thrusters.

  • Lithium Iron Phosphate Batteries for Boats & Yachts (LiFePO4)

Lithium iron phosphate batteries are in a league of their own, outperforming lead-based batteries in a variety of disciplines.

The lithium-iron phosphate batteries with the latest lithium technology, which are mostly manufactured for cyclic use, have been replacing the lead-acid, GEL or AGM batteries previously used for several years. Selected models can also be used as a drive battery.

The LiFePO4 batteries score with enormous efficiency, ultra-short charging times and an advanced battery management system. Manufacturers such as Accurat manufacture the body batteries (e.g. Accurat Traction T100 LFP supply battery) for boats and yachts, and they also work excellently as service batteries for on-board electronics.

Compared to wet, AGM and GEL batteries, they deliver more power, are much smaller and lighter, and therefore have a significantly higher power-to-weight ratio. LiFePO4 batteries withstand deep discharges of up to 100 percent and up to 5,000 charging cycles – that’s 5x longer than the best AGM batteries! The purchase price is currently still relatively high, but over the years the lithium iron phosphate batteries have impressed with their excellent price-performance ratio. Supply and demand are constantly increasing. Due to their low weight and space requirements, they are also particularly suitable for small and medium-sized boats.

Which boat battery do I use for the different purposes on the boat?

The batteries differ due to the technologies and the respective structure with regard to the possible amount of current, number of cycles, capacity and safe depth of discharge.

  • Pure starter batteries are designed to deliver maximum energy for a few seconds. When making your selection, pay more attention to the cold cranking current specification than to the capacity. Today’s small models deliver higher starting currents than earlier variants. For maximum durability, you should not draw more than 15 percent of the capacity from the battery.
  • Consumer batteries deliver a smaller amount of electricity over a long period of time. You can discharge these to about 50 percent without damage. Consumer batteries are the standard on boats and are important for navigation devices and on-board electronics.
  • Combination batteries can be used as starter and consumer batteries, they are a convincing compromise.
  • Traction batteries drive an electric motor. They survive discharges up to about 75 percent.

How important are the charging cycles for boat batteries?

In the product descriptions or manufacturer information you will find some information on the cycle stability. For many boat and yacht owners, a consumer battery with around 500 cycles is sufficient.

Example: If you spend a day on your boat or yacht about every other weekend per year, that means about 25 charging and discharging cycles. If you also take a tour over two weeks, 14 cycles are added. This results in almost 40 cycles per year and 400 charging cycles in ten years. Models with 500 to 600 cycles at 50 percent discharge meet these requirements.

Long-distance sailors have higher demands on the on-board supply – a loading and unloading cycle takes place every day. As a result, normal consumer batteries reach the end of their service life after about two years. For daily use, we recommend purchasing a boat battery with a cycle life of more than 1,000. Modern lithium phosphate batteries, which can also take part in a multi-year circumnavigation, are ideal here.

How powerful is the battery for my boat/yacht?

An off-grid solar system consists of the following important components: solar panels, solar generators, solar batteries, inverters and other accessories. In addition to the battery technology and the application or purpose, the purchase decision also depends on the required performance. Use the following calculation to determine the required total capacity of the marine batteries:

Consumer power / battery voltage = required current in amperes

Required current in amperes * switch-on time of the consumer = necessary ampere-hours

Required battery capacity = required ampere hours * safety reserve (factor around 1.5 for lead batteries)

Calculation example for an electric motor with 200 watts:

200 watt electric motor / 12 volt battery voltage = 16.66 amps

16.66A * 6 hours operation = 99.96 Ah

99.96A * 1.5 reserve = 149.94 Ah battery capacity

In order to drive a boat with a 200 watt electric motor for six hours without a break, you need a battery with a capacity of 150 Ah.

Repeat the calculation for the different consumers to determine the total power required for your boat.

Also think about the weight

A higher capacity means a higher weight. An AGM supply battery with 150 Ah weighs around 40 kg. Installation and removal as well as transport are correspondingly difficult. Modern lithium batteries with 100 Ah only weigh around 15 kg and provide the performance and endurance of an AGM or GEL battery with 180 Ah.

Note: Chargers for GEL, AGM and LiFePO4 batteries cost about the same.

Summary

If the purchase price is the decisive selection criterion, then invest in a classic wet battery. However, these do not last as long, require frequent maintenance and must not be installed horizontally.

  • GEL batteries are superior to wet batteries, but are only suitable as supply batteries and usually have a slightly longer service life than AGM batteries.
  • AGM batteries are the right choice and versatile for many boat owners due to their installation position and low internal resistance.
  • Lithium iron phosphate batteries offer the same advantages in terms of safety and installation options as AGM batteries and at the same time have a significantly higher energy density and service life. Over the years, the additional investment in a high-quality lithium battery can therefore be worthwhile.
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