Boat batteries and their properties - what to consider when choosing a battery for your boat
There are various electrical systems on a boat, including appliances, as well as systems for power generation, storage, and distribution; all running on DC or AC power or both and are often connected to each other via inverters, battery chargers or accumulators.
Accumulators are nothing more than direct current storage devices with 12 or 24 volts and variable capacity, which is specified in ampere hours (Ah). Each accumulator consists of a series of energy cells, which are arranged in a battery, i.e. connected to each other: By adding their individual voltages, 12 or 24 volts are ultimately achieved. The nominal capacity of a battery indicates how much electrical power is stored in it. For example, a 100 Ah battery can theoretically supply 1 ampere for 100 hours, 20 A for 5 hours or 50 A for 2 hours before it is discharged. In practice, however, this is often not the case, as the total energy that can be drawn from a lead-acid battery also depends on how it is discharged: quickly or slowly, gradually or all at once.
Basically speaking the higher the discharge current, the less energy the battery can supply. Batteries must therefore be sized in such a way that there is no situation on board where a battery has to supply too much current. Otherwise the battery would lose its full power. The power consumed by an appliance roughly corresponds to the supply voltage multiplied by the current flowing through it, i.e. Watt=Volt x Ampere, from which A=W/V follows. To ensure that a marine battery can always provide sufficient energy on board, it is necessary to charge it regularly. This is possible via inverters, separate diesel or combustion generators with 220 volt mains voltage or by direct connection to wind, solar and water generators. These systems require special care and attention. You can find out more about this in our guide to solar power on your boat
What should you look out for when choosing a marine battery?
When choosing what batteries you need, there are a number of factors to consider. For example, the battery types you need on your boat depends on the required voltage, the type of engine to be started, the alternator (if available), the power requirement, the type of systems to be supplied on board, the type of battery technology (free acid, AGM, GEL or lithium) for starting or for deep cycling, i.e. whether it should be possible to discharge batteries to a low charge level. Battery types exist that correspond precisely to the requirements listed above.
For example, it doesn't make sense to install a battery with a current that is higher than the maximum output of the alternator or to use an undersized charger that does not charge the battery properly and ultimately leads to the battery being nominally more powerful but actually less powerful.
Different battery types
The most commonly used batteries on board today are lead- acid AGM or GEL batteries although "open" batteries with electrolyte filling are still very popular both at sea and on land (e.g. in motorhomes). Lithium batteries are the top model for different performance levels and budgets. AGM batteries are more robust, sealed and can be discharged quickly and have a high charging speed. GEL batteries are fully sealed and there is no risk of harmful gases escaping, so they can be placed anywhere on the boat, in any position and even close to areas where people are present. So-called SMF marine batteries (SMF stands for ‘sealed maintenance free’), are completely sealed and maintenance-free.
Non-use of an SMF marine battery results in progressive discharge of the battery. The extent of this self-discharge depends on the temperature (lower in the winter months, higher in the summer months), the technology, the years and the usage. In our part of the world, however, we can assume a self-discharge in the winter months in the order of 2 to 4 per cent per month.
Each battery type in detail
The former shows that it is important to choose the right battery when making a purchase. There are different types of batteries that are suitable for different applications.
Lead-acid batteries with liquid electrolyte
Lead-acid batteries with free electrolyte and with or without a liquid level indicator are very popular among lead-acid batteries , especially in small boats. They are cheap and can be found practically everywhere. They are also used in motorhomes and lorries and despite being the cheapest batteries are perfectly adequate for their task. They are particularly suitable for ‘switch-on’ or peak currents (A) for very short periods (approx. 5-10 seconds) as ‘consumption’ with limited energy requirements. A special VRLA pressure relief valve, which is fitted to the best models, ensures that the batteries do not vent and do not need to be topped up with water.
Lead-acid batteries with AGM technology
Lead-acid batteries with AGM technology (Absorbent Glass Mat) are suitable for many applications, especially for engine starters, windlasses and bow thrusters, as they can be discharged quickly and recharged just as quickly. They are also suitable for powering appliances on-board offer excellent starting performance when required. They are the most versatile in their class.
Lead-acid batteries with GEL electrolyte
Lead-acid batteries with GEL electrolyte are suitable for supplying appliances such as refrigerators, lights, radio systems, autopilots and for all services that require little power over a longer period of time. These batteries can handle fewer excessive charging and discharging cycles and are very sensitive to energy overload and overheating during charging. It therefore makes sense to install a temperature sensor that can stop the charging process if necessary. They are most effective as a ‘back-up’, as an auxiliary or emergency battery for processes that occasionally require very high power. Following the manufacturer's instructions helps to ensure that these batteries also have many charging cycles and a long service life in continuous use.
Lithium-ion batteries
Lithium-ion batteries are the top of the range. They are more expensive, but weigh up to 70 % less than lead- acid batteries, last three times as long (up to 2000 charging cycles) and their capacity does not fluctuate over time. With continuous use, they always guarantee the same performance and are particularly suitable for long journeys.
The service life of a marine battery
The service life of a battery depends on many factors: For example, shocks, vibrations, exposure to dirt, temperatures that are too high or too low and also the charging process can affect a battery and influence its service life. The charging process in particular must be carried out according to precise rules in order to avoid damage and thus reduced performance.
Follow the basic rule of correct charging to ensure a long service life for every battery. Nevertheless: All batteries will need to be replaced at some point. A battery must be replaced when it reaches the end of its service life. To be safe, you should also change after several years of use (after about 5-7 years) or due to changes in the technology and energy requirements. It is perhaps the most difficult thing to judge and determine that a battery has reached the end of its life and must be replaced. However, if you first check things like the integrity of the housing, the condition of the terminals and the voltage (verifiable with a voltmeter or a densimeter, if with test caps) you can then perform further checks on other parts of the battery. For example, if everything is in order, the circuit must be loaded with a large load (according to the procedure proposed by the manufacturer), starting the motor or power on one or more appliances for a prolonged period to check the actions and reactions.
Batteries and their areas of application
A distinction can be made between batteries for three different applications on board: engine starter batteries, batteries that power appliances and lights and batteries for electric drive motors. So the question is, which marine battery is the right one for which application?
Engine starter batteries: These batteries are designed to deliver high currents for a short time (about ten seconds) when starting the motor. These types of battery can be damaged if they are used for too long or discharged too much. In general, the engine manufacturer specifies the characteristics of the batteries required for starting. Normal lead- acid batteries are usually used for this.
Service batteries: These batteries can withstand longer (even several hours) and ‘deep’ discharges and are very long-lasting and stable. They supply everything needed on board for several hours. For continuous, slow discharge and cyclical use, they are connected to consumers with constant absorption such as refrigerator, lighting, winch and on-board instruments and can withstand up to 400 cycles before losing power. They can deliver the required power over long periods of time and are used on sailboats or motor boats, especially when sailing or at anchor. Both gel batteries and AGM batteries and lithium-ion batteries can be used.
Batteries for operating electric motors: These batteries are specially designed for operating electric motors on board. They provide the power and capacity needed to run the electric motor and can be either lithium-ion, AGM or other battery technologies, depending on the type of boat and use. In our guide to batteries for electric outboards we go into more detail about these specific battery types.
Where to install marine batteries on board?
Although each battery, even if it is sealed, usually features a pressure relief valve designed to regulate potential gases, there is still the possibility of malfunction which may result in a minor but potentially harmful gas emission. Therefore, it is always advisable to place batteries in designated locations as required by specific regulations and guidelines.
You should install batteries preferably in well-ventilated compartments and in well-ventilated locations to ensure optimal air circulation and to minimise potential gas concentrations. Do not place the battery in the bilge, where moisture, condensation and stray currents can cause serious damage that could affect not only the battery itself but also general safety on board.
Careful positioning of the boat battery is essential, especially on small boats where every extra kilogram can affect the handling. Therefore, batteries should ideally be placed symmetrically to the sides and within the centre of gravity of the longitudinal axis so as not to compromise the stability of the boat.
In addition to the where the marine battery is physically located, the ambient temperature is also of crucial importance for the performance and service life of the battery. Extreme heat or cold can affect battery chemistry, so it is advisable to store batteries in a temperature-controlled area to maintain optimum performance.
Example of an electrical system with batteries on a sailing boat
The first step in determining what type of battery should be installed in a boat requires an understanding of the power requirements of your battery bank, both for starting and operation of appliances etc. To do this, it is necessary to calculate the start-up current required to start the engine and the expected consumption for operation over a certain period of time, for example 24 hours. By calculating the ampere consumption of the expected on-board devices using the formula ampere-hours = watts/volts, the required capacity for these appliances is obtained. If you add the total consumption and multiply it by the number of assumed operating hours, you can get a reliable and fairly accurate indication of how large the battery bank on board needs to be.
To illustrate this, here is an example calculation with possible appliances on board, including an approximate consumption overview:
- LED navigation lights: 0.5A x 8h = 4A
- Refrigerator: 5A x 8h = 40A
- Radar: 22A x 10h = 22A
- Autopilot: 2.5A x 18h = 45A
- LED inside lights: 1A x 2h = 0.5A
- AIS-GPS: 0.75A x 24h = 18A
- Laptop PC: 2,5A x 4h = 10A
- VHF reception: 0.25A x 12h = 3A
Total consumption in 24 hours: approx. 150 - 160Ah
Our tip: The ideal operating battery bank should have at least twice the capacity of the total consumption of the consumers on board over 24 hours. Conventional lead-acid on-board batteries (liquid electrolyte, AGM or gel batteries) cannot deliver more than 50 % of their nominal capacity, otherwise they may be damaged. In this example, a 200 Ah battery would be fully discharged after 24 hours if the remaining charge were to reach 50 %, i.e. 100 Ah. Using the calculation as an example, the recommended total capacity of the battery bank should therefore be at least 300 Ah.
For an on-board system with 2 boat batteries it is recommended to install an inboard starting circuit. To do this, the positive terminal of the battery is connected to the starter motor. A second cable leads back to the negative terminal of the battery and closes the circuit. A third cable is also connected to the ignition key. Turning the key closes the circuit, activates the starter motor and starts the engine. In addition to the starter motor, other electrical components such as sensors for temperature and oil pressure monitoring also play an important role in the system. There is also a separate battery bank that provides additional electrical energy and is recharged while the engine is running.
Our conclusion
Choosing the right boat batter is essential if you want your time on the water to go as smoothly and safely as possible. With the various types of boat battery that meet individual needs and areas of use, you can be optimally prepared for your water adventure. Whether for engine starting, power supply on board or other applications: the right battery ensures reliable performance and a problem-free sailing or motorboat experience. Using the marine battery correctly and for its intended purpose as well as regular maintenance and checking the service life can help to avoid unexpected failures and a bad boating experience. In our great boat battery test, we hope you can make the right choice, in fact, we know you will!