Boat Batteries

Boat batteries are one important, but neglected, piece of equipment. Many of the systems on your boat would not run without the batteries and yet, because of their size, weight and expense, they tend to be installed in a difficult to access place enduring heat, moisture and vibration for long periods of time without a second thought. Or, at least, without a thought until the motor won’t start; the refrigerator won’t keep up; the winch won’t grind; the radio won’t work; or any other of the boat functions that we don’t think about until we don’t have them… and they don’t last forever.

BatteriesThe most common battery type used in boating is a 12 volt lead acid battery similar to that used in your car. The battery will be rated by how much energy it can store; expressed as ‘Amp Hours’ and would typically be from 80 Ah up to 150 Ah. Multiple batteries may be connected in parallel when more capacity is required.

A battery is also rated on how much of a ‘punch’ it can deliver. The ‘cranking amps’ of a battery represent the short term high current output a battery can deliver when trying to do things like, surprisingly enough, cranking your motor to get it started.

The ‘marine’ variation of the classic lead-acid battery is constructed to withstand the physical and operational rigours of a marine environment. On a 12 volt battery there are 6 cells comprising lead plates (actually, these days most batteries have plastic plates with a lead coating) in a sulphuric acid electrolyte solution. All this lead and the need for a solid robust construction make the battery quite heavy. Anyone who has spent an afternoon in the bilge, sweating and swearing their way through a battery changeover will understand just how heavy marine batteries can be!

Each cell generates 2V and they are wired together in series to give you your 12V. Don’t get too hung up on the ’12V’ designation – the battery voltage will vary somewhat depending on its state of charge and its condition. In practice you would generally expect 13.8 volts out of a fully charged, healthy battery, 14.5V when charging, and down to 11.5V when discharging. Properly designed marine electronics should be able to handle this variation. Most marine equipment will continue to work once the battery voltage drops below 11V, but you are starting to do irreversible damage to the battery which will reduce its future capacity.

In an old style battery there will be openings in each cell to inspect the liquid levels in the cells and top it up if necessary. Water will slowly evaporate from the cells over successive charge-discharge temperature cycles and this should be checked and topped up at regular intervals. To take a technical diversion, you should really check the sulphuric acid concentration by measuring its density with a hydrometer, but in practice, distilled water is all that is needed. This liquid acid construction can be a bit risky in a boat where the boat is not always horizontal and so there has been an evolution of batteries over the years. A ‘Sealed Lead Acid’ battery or a ‘low maintenance’ battery is (mostly) sealed to prevent liquid escape. A ‘gel battery’ replaces the liquid electrolyte with a gel which allows the battery to operate in any orientation. Exotic battery constructions are becoming more common with Lithium batteries which are bigger versions of the batteries you have in your phone and they have better storage life and much lighter weight as the main advantages. As with most new technologies, Lithium batteries also make your wallet much lighter.

Each of the available battery technologies has its advantages and disadvantages which must be weighed up when choosing which type you will use.

Many thanks to Steve Cody for the above information; Steve is our resident electrics guru at RBYC and is happy to answer any questions at

– Jeremy Larkin

Jeremy heads up our office at Royal Brighton Yacht Club