Choosing Your Marine Battery

A battery has several galvanic cells, where each cell has a positive oxide lead plate and negative lead metal. Between the plates is an electrolyte consisting of water and sulphuric acid. During discharge the lead oxide is converted to lead and the acid content decreases and during recharge the lead sulphate is then converted back into lead and lead oxide.

Flooded batteries produce hydrogen and oxygen when being charged so they must be vented to avoid build-up of explosive gasses. They also require maintenance – periodic inspection and top up of distilled water

Starter Batteries delivery short bursts of high amperage power to the starter motor and do not like being discharged.  Starter batteries have thinner and more numerous plates.

Deep Cycle Batteries can handle deep discharges, (up to 50%) and can be discharged and recharged many times. The plates are thicker and the battery bank, as a rule of thumb should be approx. 4x the amount of energy required between cycles.

Hybrid/Dual Purpose Batteries are capable of both starting and deep cycle use.  They are a good compromise allowing bursts of power as well as deep discharges.

Key Terms

Charge Factor – When charging a 12v lead battery the upper limit on charge voltage is 14.4v at 20 degrees. This is because the efficiency of the battery s approx. 80% and which means it must be charges at a Charge Factor of 1.2. 1.2x12v = 14.4v

Discharge Factor –Length of time a battery can be used at a given load.

Cycle – Batteries will only last a number of charges and discharges depending on the type of battery and quality. One cycle is the process of fully discharging and recharging the battery. A good quality wet battery typically lasts approx. 200 cycles.

Plates – The total plate surface determines the maximum current that can be delivered by the battery. The thickness of the plates determines the number of times that the battery can be charged and recharged – number of cycles.

Absorption Time – This is the duration of the 2nd phase of a charge. The battery will be at 80-100% charged during this phase and it typically lasts about 4 hours for a wet battery.


As a rule of thumb your alternator should be at around 30% of your battery capacity and your battery charger should have enough amperage to equal the sum of the DC loads plus a minimum of 10% of your battery capacity.

3 Step Charging – A 3 step charging system can remain connected to the batteries and ensures a longer life span.

Step 1: The Bulk Phase

The output current of the charger is at maximum during this phase and the battery is charged quickly

Step 2: The Absorption Phase (approx. 4 hours with wet battery)

This begins when a battery has reached approx.  80% charge and continues until it is fully charged.

Step 3: The Float Phase

When the battery is fully charged the batteries are maintained in optimal condition

Step 4 (optional) – Equalisation – This phase prevents batteries from aging prematurely and forces the battery to its highest possible state of charge, boiling the electrolyte and dissolving the lead sulphates on the plates.

Tips for Best Performance

  • Shallow discharges lead to longer battery life
  • Do not mix battery types. Mixing battery types (lead acid, Gel, AGM) can result in overcharging Don’t leave batteries deeply discharged for any length of time
  • 80% discharge is the maximum safe discharge
  • Charging system must be able to deliver sufficient amperage to cope with usage and changing the battery bank. As a general rule of thumb the battery charger should be a minimum of 10 % of the size of the battery bank.
  • Never mix old batteries with new ones in the same bank. Old batteries tend to pull the new ones down.
  • Keep batteries in a cool and dry
  • Annual check of battery terminals
  • Top up the distilled water as necessary
  • Charge batteries after each period of use

How to Kill a Battery

  • Undercharging – failing to fully recharge batteries leaves sulphate on the plates and the batteries reduce their ability to perform. Keep your batteries charged and equalise your wet cell battery every 6-8 weeks.
  • Overcharging – repeated overcharging boils the electrolyte out of the battery and the battery can overheat.
  • Excessive Deep Discharge – Do not completely discharge deep cycle batteries if at all possible. Leaving a battery discharged for any period of time will cause the plates to sulphate. It is advised to run your deep cycle batteries down to approx. 50-80% each cycle.


Voltage and Current Provide Power

The electrical voltage if the potential difference of two points in an electrical circuit.

AC – Alternating Current – 230v 50Hz – EU

DC Direct Current – is supplied by batteries and solar panels.

Current (I) is measured in Amps (A)

Current flows through the boats wiring when there are appliances in use and depending of the voltage connected the current can vary. Correct cable thickness is very important. The cable has resistance (R) which is measures in Ohms and causes loss of energy in the form of heat. Voltage loss occurs and if the drop is too great the appliance may not work.

The voltage and current provide Power (P) measures in Watts (W)

Ohms Law V=I x R Voltage = current x resistance

Power (P) = Volts (V) x current (I)


Using the correct size wire on your boat is critical for safety and efficiency. If cable diameters are too small they can create resistance and heat and can cause fires. Lower voltages, such as 12v systems used on boats mean that the current must be much higher to create the required amount of power.

As a rule of thumb for 12v DC systems, 3 amps power per 1mm2 cable diameters applies. Ie. if a battery or battery charger provides a current of 75 amps a cable of at least 25mm2 is required.