Battery Care

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Care and Maintenance of Batteries

The basic principles of battery care are:

  • Keep batteries charged -- methods include solar chargers, wind powered chargers or battery chargers connected to some sort of shore power. It is good policy to make sure your batteries are fully charged on a monthly basis, this will minimise the likelihood of sulphating occurring.
  • Be aware of any gassing that may occur in flooded batteries and top up electrolyte levels as required.
  • Ensure that batteries are not drained below their safe operating level. For most lead acid deep discharge batteries (flooded, gel, Absorbed Glass Mat) this is around 50% but some battery types can be safely discharged further.

Batteries (Accumulators)

  • Lead-acid accumulators are made up of a number of cells which consist of two sets of lead plates separated by wooden or porous plastic separators, and filled with a dilute sulphuric acid solution called the electrolyte. This applies only to Wet Cell (flooded) Lead Acid Batteries.
  • One set of lead plates (alternate plates) are the positive plates which form lead-peroxide and a chocolate-brown colour when the accumulator is fully charged.
  • The plates and separators are usually placed into a vulcanite, hard rubber, glass or plastic case sealed with pitch, through which the terminals protrude.
  • When discharged both the positive and negative plates form lead sulphate. This is caused by the chemical action during discharge when sulphur and oxygen are transferred from the electrolyte to the plates, reducing the specific gravity of the electrolyte.

Plates in a discharged condition are easily recognised by the appearance of lead sulphate (a white deposit) on both positive and negative plates.

Upon re-charging the lead sulphate is transferred back to the electrolyte and the specific gravity increases, provided the accumulator has not been left standing in a discharged condition for a long time when a hard core of lead sulphate forms. When the accumulators have been left in a discharged condition for too long the lead sulphate hardens and causes the plates to thicken and buckle, damaging the separators and casing and no longer function efficiently, if at all, in which case the batteries should be reconditioned or replaced.

See also Boat Batteries.


The electrolyte consists of a solution of sulphuric acid diluted to correct specific gravity by distilled water.

The electrolyte should just cover the plates and maintained at that level by the additional of pure distilled water when the level drops below the top of the plates.

Testing Battery State

The voltage of a bank of batteries for the boat, normally 24V (or 12v), should be indicated by a voltmeter, preferably a digital one with two decimal place resolution, installed in an easy to read location so that the voltage can be ascertained at a glance. Many charge controllers display the current battery voltage via an LCD display, otherwise a multimeter can be used. The voltage of a 24V bank of batteries should be maintained at 24 – 26 volts off load and approximately 24V on load. The charging voltage should be more than 24V, usually 25 – 30 Volts, depending on the condition of the batteries which should be checked daily. The voltage of a 12V battery bank should ideally be between 12 - 13 volts.


A hydrometer can be used to test the state of a flooded lead acid battery. For sealed, Gel, AGM, or Lithium batteries it is of no benefit. A hydrometer which measures the specific gravity of the electrolyte, by taking a small amount of electrolyte into the tube and reading the specific gravity from a float inside the tube.

The specific gravity will vary between 1,270 when fully charged and 1,150 when discharged. It is advisable not to allow the accumulate to discharge below 1,200 to prevent the formation of hard lead sulphate on the plates. Ideally the specific gravity should be maintained at approximately 1,250 by regularly charging them or by keeping them on float-charge.

Battery Monitors

The essential function of a battery monitor is to calculate ampere-hours (aHr) consumed and the state of charge of a battery. Ampere-hours consumed are calculated by detecting the current flowing in or out of the battery.

Battery monitors need to be programmed for different types of battery, some of which will have different levels of charging efficiency to others. For example the charging efficiency of a flooded lead-acid battery is around 80% whereas that of a lithium battery is closer to 99%.

Battery monitors are available from companies such as Victron Energy and Nasa Marine.

It is debatable whether a battery monitor provides a significantly better view of battery condition than does a simple voltage meter.

Battery Care


  • Ensure sufficient initial charge.
  • Give batteries plenty of work and liberal charging.
  • Do not charge too much or too little.
  • Do not charge at too high a rate.
  • Do not run batteries too low (voltage and specific gravity).
  • Do not allow batteries to stand discharged too long.
  • Charge batteries daily.

Warning: The charging voltage of Gel and AGM batteries is lower then the charging voltage of Wet Cell (flooded) Lead Acid Batteries.These batteries can be damaged if overcharged. Most modern charge regulator have appropriate settings for the various battery types. Make sure that all regulators are at the proper setting.

Electrolyte (Wet Cell)

  • Keep plates covered with electrolyte, using distilled water for topping up.
  • Test specific gravity weekly.

Equalization (Wet Cell)

Sulfated plates on Wet Cell batteries can be cured by charging the battery at elevated voltage levels. This is called equalization. To maintaining your battery in good condition you should equalize 1 or 2 times every year. Many "smart" regulators have a special setting for equalization. Equalize the battery for several hours, while you are in the boat. This is a safety precaution.

Desulfation usually causes a bit of gassing, and so you should top up the fluid levels in wet cell batteries after running an equalization charge on them. Use distilled or de-mineralised water for this.

Equalization should not be performed on either Gel or AGM batteries.


  • Keep terminals clean and coated with petroleum jelly or similar.
  • Do not allow open lights including cigarettes near batteries.
  • Keep batteries and battery box dry.
  • Check casing for cracks.
  • Ensure terminals are clean and tight.
  • Earth connections should be properly connected.
  • Vents in filler caps should be clean and open.
  • The plates should not be buckled.
  • The electrolyte should just cover the plates e.g.5mm.
  • The batteries should not be over-filled with distilled water.
  • Lead sulphate should not be allowed to form on the terminals on plates.


  • Check voltages of batteries and cells on load.
  • Check voltage at least once daily.

Additional Information

  • If you have no hydrometer to test the specific gravity of the battery, shine a torch into the filler holes and note the colour of the plates.
    • Charged batteries: positive plate is light brown, negative plate is slate grey.
    • Discharged battery: positive is whitish, negative light grey.
  • While battery is being charged, the plates give off gasses causing bubbling, as it becomes fully charged, the amount of gas and bubbling increases.
  • Batteries to be inspected once every 24 hours and to be kept fully charged.
  • When making up the electrolyte always add acid to the water. (If water is added to the acid an explosion may occur.)
  • A 12V battery when fully charged, should register about 13.5 volts when there is no load on it, and this value should drop slightly when there is a load on it.
  • Check the battery for cracks or leaks when inspecting.




See Also


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