Choosing rechargeable batteries for your boat

The world of energy storage is in full swing with the arrival of new Lithium technologies and the technical improvement of the old lead-acid batteries. A lot of technologies are emerging and have not been tested on a boat yet. We will focus on 2 main families: Lead acid batteries (AGM) and lithium batteries (LifePO4 and Lithium ion NMC).

We will avoid GEL batteries which are difficult to recharge with a motor generator or solar panels, their use in navigation is very limited.

what you need to know

Some concepts to learn

  • « Storage capacity » or Capacity
    It is the quantity of current that a (new) battery in perfect shape, is capable of supplying when it is charged. It is expressed in Amps-hour (Ah) or in KWH.
    For example, a 70 Ah battery in 100 C will really have a current of 1 A for 100 hours or a current of 100A for an hour.
    In reality, since it is not recommended to go lower than a certain percentage of discharge, we can only use part of the capacity: 40 to 50% for a lead battery, 80% for a Lithium battery.
  • The charge and discharge current of a lithium battery is expressed in amps. This capacity represents the current that the battery can potentially supply. The unit is amps A or C (ratio of charge or discharge current / total capacity).
    A 100 Ah battery with a C1 can supply 100 amps.
  • The rate at which a AGM battery discharges also has an impact on its capacity: the faster it gets discharged, the lower the actual capacity of the battery becomes.
    This discharge rate is indicated in the form of: C10, C20, C100…
    For example, a battery of 70 Ah in C 100 will actually have a capacity of 70 Ah if it takes 100 hours to discharge at a current of 0.7 A. If it takes 20 hours to discharge (thus at a current of 2,75 A), the capacity of this battery will go down to 55 Ah, to pass to only 50 Ah in 10 hours with 5 A.
  • « Floating voltage » or « maintenance voltage »
    This is the voltage at which a battery must be permanently maintained to ensure that it is optimally charged.
  • The absorption voltage
    This is the maximum voltage sent to the battery during charging (14.1 volts for an AGM for example). This voltage is then maintained until the battery is fully charged before switching to the floating voltage.
  • Cycling capacity of batteries
    When we talk about a number of “”charge-discharge”” cycles, this corresponds to the number of cycles from before the degradation of the battery is such that it is no longer possible to use it for its initial purpose.
    A cycle corresponds to a full discharge and a full charge of a battery.

    The manufacturers indicate the number of cycles that the battery can perform before losing part of its capacity (often 80%)
    • Very good AGM batteries can reach 500 cycles
    • Very good NMC batteries can reach 2000 cycles
    • Very good LIFEPO batteries can reach 3000 cycles

      We understand that saving 20% on the price of a battery to have half the life span is not always a good idea.

Charging the batteries correctly

To recharge a deeply discharged battery, it is necessary to apply 3 charging phases to the batteries :

  1. A first charge “”Bulk charge”” or “”Boost””, of high current (up to 25% of the capacity in Ah of the battery) with a relatively low voltage (13.5 V for a 12 V battery),
  2. A second “”Absorption”” phase, with low current and high voltage (14.4 V),
  3. A third phase “”Float””, of low current and low voltage (13,5 V).

These three phases are carried out in sequence in order to charge the battery to 100% perfectly.

  • Number of charges per engine alternator
    A 12 V alternator equipped with a standard internal regulator delivers a constant voltage of 14 V; a voltage that is too low to ensure the “”Absorption”” phase (14.4 V are required).
    This results in the batteries not being charged to 100% of their capacity.
  • Charging with photovoltaic panels
    It is very important that the solar regulator respects the 3 charging phases recommended by the battery manufacturer, otherwise they will deteriorate fast.
  • Dock charger
    It is the same with the dock charger. Some old or cheap models are not compatible with all batteries, especially lithium batteries.

Good quality modern chargers also allow you to choose the kind of batteries.

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make the good choice

Which batteries should you choose ?

AGM BATTERIES AGM (Absorbed Glass Mat)

Recommended usage

Auxiliary batteries that can be recharged by solar or wind energy, but also suitable for start up, if enough capacity.

Possible usage for small power electric motorization or on big vessels where you have enough space and where the weight factor is not important.

Manufacturing and safety

Despite their sealing, AGM batteries are pressurized and therefore equipped with a safety valve: When the pressure becomes too high (overload, high temperature etc) gases escape.
They do not like hot environments, which is directly linked to a reduction in their life span.

Advantages of AGM batteries

  • Sealed, maintenance-free
    No maintenance, no need to add distilled water; reinforced security when compared to open lead-acid batteries.
  • High charge and discharge capacity up to 4C
  • Low self discharge rate (1 to 3% per month)
  • Good resistance to shocks and vibrations
    The plates are compressed and securely attached to protect them from shocks and vibrations
  • A good reaction to heel or swell
    The electrolyte liquid being enclosed, they can work in every position.

Disadvantages of AGM sealed batteries

  • Short cycling life
    Most AGM batteries, particularly the cheapest ones, do not withstand deep discharges. For a use at sea or for electric propulsion, you should choose batteries such as Victron’s Super Cycle batteries


A lithium battery is an energy storage technology from the family of electrochemical batteries, whose reaction is based on lithium and other different materials depending on the chemistry used.


  • Lighter and smaller batteries
    This technology currently offers the best specific energy (energy/mass of around 100 to 260 W/Kg) as well as the highest energy density. They are 70% lighter than lead-acid ones.
  • More durable batteries
    Lithium batteries can last up to 15 years. (3 years more than lead-acid batteries)
  • Low self discharged (1 to 5%)
  • Maintenance free


  • Price
  • Needs a BMS

Attention !
There are many options on the market at all prices that are difficult to compare. However, there are lithium batteries and lithium batteries. A lithium battery at a very low price of an unknown brand is suspicious. Since the batteries are sealed, it is quite easy to put in cells of bad quality, low end electronics or components.

Lithium technology is now well mastered but they require a lot of precautions to avoid unpleasant surprises. However, a lithium battery of low quality is still a significant investment and a 5 or 15 years amortization changes the analysis a lot.

Lithium batteries are becoming more and more popular in the nautical world to be used as auxiliary or secondary batteries and are essential for electric motorization because of their weight and their ability to charge and discharge quickly.

There are two main kinds of lithium batteries on the market: NMC Ion lithium and lithium iron phosphate.

Focus on Lithium battery



  • They are the lightest with an energy density often greater than 170 W / Kg
  • Cheaper than other chemistries


  • Relatively low charging and discharging current (0.3 to 1 C)
  • Requires a sophisticated BMS (battery management system) that supports cell information, among other things.

Recommended use

Being very light, they are recommended especially for the electric motorization of sailboats wishing to embark a high energy capacity

They can be recharged by a generator through a charger that will respect precisely the C of charge.



  • Total discharge is possible without damages
  • Fast recharge without damages
  • Excellent ratio weight/capacity when compared to AGM
  • A long life span (3000 to 4000 cycles)
  • Deep discharges without damages


Very high price: 10 to 15 €/Ah
Heavier and bigger than NMC Ion Lithium

Usage recommended

It accepts significant charges and discharges, they can be used on a boat for the ….. pack (windlass, diving unit etc) and for the electric motorization.

They are ideal for hybrid propulsion (48 V generator + batteries)
They are recommended for professional usage.

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