Boat batteries: what you need to know for cruising


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The batteries on our boat are critically important for navigation and safety, first and foremost, because starting the engine and running the generator that will recharge the batteries themselves when we are not at the dock depends on the battery. Together with Diego Volpi, general manager Power Group, one of the largest manufacturers of batteries for marine use, we put together this guide to boating batteries where we tell you everything, but everything you need to know to make sure you never run “out of power” again and do no harm.

Batteries in boats, how they work

Batteries in the boat? How they work…

Diego Volpi, our interlocutor in this interview, serves as General Manager Power Group.
Diego Volpi, our interlocutor in this interview, serves as General Manager Power Group.

Cruising boats now almost all have separate circuits with at least two batteries. A starting one, dedicated to the engine and sometimes to high-powered utilities such as anchor windlasses or bow-thrusters. A services battery for low but continuous-drawing utilities: lights, radios, plotters, logs, ventilation, bilge pumps, inverters, and whatnot. It is essential to know the characteristics of your battery, its condition, and perform good maintenance to make sure it performs at its best. Most importantly, not to make mistakes that could cost you dearly . Now is the time to get clarity and a good understanding of how a marine battery works, and delve into the various technologies (AGM, gel and lithium).

Boat battery guide

  • Let’s start with “traditional” lead technologies: AGM and gel, what is the difference?

AGM and gel are related technologies; they are both based on the same lead-acid technology and have completely replaced the old liquid batteries that were “open.” AGM and gel batteries are all sealed batteries. The internal structure of the lead plates of these two models has two different features: the AGM is constructed using many thin lead plates. Having a very large contact surface area with the electrolyte, it can discharge very quickly, so it is ideal for generating large peak currents for short times as a motor starter. They can also be adapted to the use of light services. The gel one is a battery that by construction has ideal characteristics for full cycling, that is, supplying current to services that require constant current for long periods.

  • Lately, lithium-ion batteries have been catching on. What are their characteristics?

Lithium battery cells do not allow for variations in construction capable of differentiating performance in cue or full cycle.By its nature, the lithium battery, like that used in our cell phones, is an ideal battery for boat services. The ability to optimize its characteristics shifts to the charge and discharge control electronics, a circuit built into the battery BMS (Battery Management System). Some manufacturers are also experimenting with lithium for starting. Lithium batteries are much lighter in weight and have superior durability and charging performance than gel batteries.

  • How long does a battery last and what are charge cycles?

The life of a battery is related to charge cycles. Discharge-charge cycle means a complete cycle: bring battery to minimum (20% charge for Lithium, 50% charge for lead) and bring it back to maximum (100%). The number of cycles is used to measure the life of a battery. If we compare good quality 12V batteries, the average life of an AGM travels inside the 350 charge and discharge cycles. The gel, on the other hand, can go up to 500, 600 complete cycles. As an average life, a lithium model can easily reach 3000 charge cycles!
We know that in on-board use, full charge and discharge cycles can almost never be guaranteed. Be aware that two partial charge cycles of 50 percent shorten the life of the AGM or gel battery a little more than a single full cycle. The lithium battery, thanks in part to the control electronics that manage the charging of all cells, does not have this problem.

In this table we have summarized the energy consumption (indicative, in amperes) for on-board utilities, whether you use them “overflowing” (not recommended) or sparingly. The major “energy suckers”? The autopilot and the refrigerator… Lighting also plays a role. This is why it pays to switch to LED…
  • Basically, how long will my battery last in the boat?

Battery life depends on the quality of construction, use, temperature, and the care one takes in charging and discharging. This applies to all batteries.
All cycles given by the manufacturer refer to a reference temperature of 25 degrees under ideal conditions. Batteries that live in high temperature environments or work only in summer last less, much less. For starting, we can expect from an AGM battery, 50,000 to 80,000 starts. For that of services, with normal use, we can expect a service life of 6-7 years.

  • What is the memory effect of a battery?

infobox how long does my battery last?AGM and gel lead-acid batteries suffer from what is called the memory effect, like the batteries in early cell phones. Electrically, the effect is expressed in a drop in voltage. In practice, the battery reduces storage capacity as if “remembering” the state of the previous partial discharge. To avoid this, partial charge cycles should be avoided; one should always try to have the battery discharged as much as possible and then recharged.

On lead-acid technologies, the battery performs up to about 50 percent charge. That is why it is advisable, even once a week when in port, to fully charge the battery by leaving it attached to the charger even one or two days. Automatic chargers provide the ideal full cycle, partially making up for the memory effect.

Lithium does not have these problems, you can also completely discharge the battery electronics however protect the battery will report the low battery status as early as 80% to optimize battery life and operation.

  • What is the right battery for shipboard services?

The anchor windlass and bow-thruster act like a starter motor; a high-spark AGM battery is just fine. The refrigerator, lights, log, plotters, radio systems, autopilot, are all services that require little power for a long time. A cyclic gel or lithium battery is definitely indicated for these.

On some boats, yards already introduce air conditioning as standard. These systems draw a lot of energy, and you need to understand how the compressor works and for how long, in order to choose the appropriate battery.

an AGM X-ray battery
In the diagram is the section of an AGM sealed battery for marine use: it is made using many thin lead plates. Having a very large contact surface area with the electrolyte, it can discharge very quickly, so it is ideal for generating large peak currents for short times as a motor starter.
  • How much do batteries cost depending on their technology?

Lithium prices, unlike the expectations of a few years ago, are not falling, due to the extremely high demand for the raw material, which is produced by only a few countries in the world, and the booming demand from the automotive sector. However, at present it can be shown that Lithium battery is competitive with gel batteries, indeed, it is the best solution. Because it costs about three times as much but does five times as many charge cycles. Even with a large initial investment, the lithium battery is a winner in the long run.

On-board battery technology. AGMS. GEL and Lithium compared

  • Is it possible to have one or more batteries connected together?

Batteries on board should be divided by services. The starter battery will do start only and the services batteries will do services only. Each site has its own reasoned construction choices on the installed service design, however The advice is to install as few batteries as possible, and avoid interconnected battery packs. Two 100Ah batteries compared to one 200Ah battery, can be unbalanced with each other and not charge properly, also the weak point of on-board systems are the electrical connections, the less there are, the less problems they will give! And it must be said that two batteries cost more than one.

There are also complex issues about series-parallel connections or 12V battery connections to get 24V, so if you have battery banks on board, always rely on trained electricians or the manufacturer.

  • Can only one battery be installed?

Only on small boats. In that case, the choice will fall on AGM batteries, which, although optimized for engine starting, can very well be used in hybrid mode by powering on-board services. If, on the other hand, the engine on board just isn’t there, as on small sailing vessels, or the outboard starts belting…. here’s that for the services of lights, vhf, phone charging, it will be ideal to choose a small, portable gel or even better Lithium battery that can be recharged at home or with solar panels.

  • One of the advantages of lithium batteries lies in their precise “diagnostics”-what does that mean?

We are already on the third generation of Lithium batteries. Mastervolt’s concept, for example, was to create marine batteries that were capable of replacing all existing batteries, and therefore fully compatible with any of the chargers already on board.

From the beginning to do this, the control electronics BMS (Battery Management System), allows the battery to be controlled in a timely manner during both charging and discharging (to control cell imbalance).

  • How does it work?

The control parameters of the internal electronics, are available in output on the communication bus, are displayed on the on-board management systems. Thus, one always has real-time information about the state of charge and health of the battery, alarms for low battery, exceptional events, and maintenance. These parameters can be controlled remotely for assistance and tuning by the manufacturer.

In fact, it is possible to connect the communication bus to the USB port of a laptop computer, and through a simple wifi or 4G connection, manufacturers can know everything about the status of the batteries and provide assistance on the whole system, or upgrade the firmware to the latest version!

  • For sailors who are at sea more than at the dock…. does it make sense to have a battery backup for essential services (radio, lights, plotter…)?

Having an emergency battery for boats above 24m, battery for radio or other essential independent services is already mandatory.

However, for the yachtsman, having an extra battery is not a popular choice. The real problem is starting the engine. Any boat today is equipped with a parallel battery system: when you are in the situation where starter battery is dead, you can exceptionally use the services battery to start the engine.

An extra, disconnected battery adds to the maintenance problem. Better to concentrate on keeping the main batteries charged and in perfect working order.

  • Where to install batteries in the boat?

Each site makes its own choices. Each battery, although sealed, can still have minimal gas emission; in fact, it has a small valve to compensate for pressure variations.
Batteries should be installed in the appropriate compartments indicated by the construction site, and preferably in ventilated locations. I have also seen DIY solutions of batteries installed under the bed, which are absolutely no good.

  • 12 or 24 volts?

For boats over 12 meters, there is now a tendency to use 24-volt systems.

  • Is it possible to regenerate marine batteries?

Traditional open lead-acid batteries allowed for minimal maintenance. Hermetic batteries do not allow for regeneration, once their cycle is over, they need to be replaced: however, there are small devices that can reduce sulfation.

  • How can they be disposed of?

Very simple, by taking them to the manufacturer. Those who sell batteries have an obligation to take back spent batteries.

  • What happens if the batteries end up in water?

Marine batteries with AGM and gel technology are sealed: in the worst case scenario, if they end up completely underwater, there could be a short circuit to the system, which will be damaged.

This is because the lead-acid battery, internally, has no protection system. The lithium battery, on the other hand, goes into self-protection in extreme situations.

  • How do alarm and self-protection systems work?

The lithium batteries have about ten temperature sensors inside, which monitor the state of all the copper connection plates. If the battery monitoring system detects any abnormality, it generates an alarm and is able to activate the external protection relay that disconnects the battery. On the battery, next to the contacts for the communication bus, there are contacts to which the relay can be directly connected.

  • There has been a lot of talk about lithium batteries catching fire on cell phones, can this happen on boats?

No, lithium batteries for marine use are safe. There are so many types of lithium technologies, with completely different chemistries and technologies. The formulas used for smartphones or on cars should not be confused with the different formulas used for marine use. For boating, lithium-iron-phosphate batteries are used. The latter combination (LiFePO4) has the worst capacity-to-volume-weight ratio, but it has the highest level of safety! Therefore, a 100% safe chemistry and construction technology equivalent to a gel battery is chosen.

For example, the automotive sector favors propulsion-optimized batteries that are lighter and not universally compatible, rechargeable only with the manufacturer’s charging system, and not replaceable.

In the event of a fire, what burns is the plastic coating on the cells: the new trend is to build cells with metal coating, to provide even more safety on the lithium battery.

Luigi Gallerani

Focus: boat batteries, what does the market offer?

AGM and gel: prices around 2.8 to 5.5 euros per Ah.

Lithium: prices from 11 to 25 euros per Ah.

A 10- to 12-meter sailboat, on average, will mount a 12V 200Ah service battery (about 740 euros gel or 2000 euros lithium) and a 12V 70Ah starter battery (about 260 euros),

A motor boat of 8-10 meters, will mount on average a 12V 150Ah service battery (600 euros gel or 1900 lithium) and a 12V 140Ah starter battery (550 euros).

In the case of lithium service batteries, we have priced based on reduced capacity; gel batteries are over-priced keeping in mind their loss of efficiency at 50% charge.

Here is our selection of reliable marine battery manufacturers:

Exide batteriesExide

Offering both AGM and GEL batteries, of note is the Dual AGM series, batteries specifically designed for hybrid use for both starting and on-board services.


Rich Lithium LiFePO4, AGM and GEL range for marine. Lithium MLI series features integrated MasterBus CAN management and communication system, the new MLI-Ultra models also interface with NMEA200 protocol and CZone. With the BMS Battery Management System, new lithium can replace Gel while maintaining compatibility with existing systems.


Lithium LiFePO4 batteries for marine use. Epsilon series batteries have integrated charging circuitry, and can replace Gel while maintaining compatibility with existing systems.


Union Battery Service offers, for boating the Zenith “maintenance free” Series (sealed AGM) and Zenith Lithium (lithium), interesting to choose terminals of different shapes. With dedicated chargers, they can be recharged in less than an hour.

boat batteriesVarta

For marine use, point to the Professional Dual Purpose series AGMs, hybrid starter and service batteries.

Victron Energy batteriesVictron Energy

It produces both AGM, Gel and lithium-phosphate models. Also of interest from this manufacturer are compact AGM batteries for small boats.,



opening photo: Guido Cantini



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