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updated on 11-08-2008

Electric Points ‘Fully Charged’

In the second of a series of short articles on technical aspects of electric boating, EBA Technical Officer Paul Lynn discusses batteries and battery maintenance.

Generally hidden from view, often taken for granted, occasionally cursed — batteries are a vital and expensive ingredient of electric boating. Properly looked after they give us years of reliable service, hut abused or ignored they may let us down. So what do we need to know and do about batteries — especially as the laying-up season approaches and (sorry, Australia!) our thoughts turn to winter pursuits?

The so-called lead-acid battery is the workhorse of most electric boats. Made up of an assembly of cells, each filled with dilute sulphuric acid and generating about 2 volts, its basic operation a complex electrochemical reaction — is the same as in a car battery. However there are some important differences. Whereas a car battery’s most arduous duty is to supply a very large current (typically hundreds of amps) of short duration to start the engine, a boat’s batteries supply much smaller currents (typically tens of amps) to the propulsion motor for hours on end. And whereas the car battery is not designed to cope with ‘deep discharge’, we quite often need to discharge our boat batteries heavily (say down to 20% — try not to go further than this) before recharging them at a charging point. So they should ideally be capable of many ‘cycles’ of charge-discharge, and are commonly described by terms such as leisure, deep cycle, or semi-traction.

Whatever the precise description it is probably fair to say that you get what you pay for. Give high prices for high quality and be a proud owner, or opt for cheap leisure batteries (widely advertised for caravans and boats) and accept that you will probably have to replace them more often. Whatever the decision, there are certain do’s and don’ts which should help you get the best from them.

In Use

•  Keep your batteries topped up with distilled water, so that the liquid electrolyte comfortably covers the plates.

•  Always recharge your batteries as soon as they have been discharged. Never leave them discharged for long, as they may suffer irreversible damage.

•  Avoid heavy overcharging, which can also damage batteries and cause excessive ‘gassing’ and loss of liquid electrolyte (Note: a good battery charger automatically regulates its current to prevent this).

•  Keep the batteries clean and ensure that the terminals are connected tightly, a little Vaseline (or approved grease for battery terminals) on the terminals helps ensure good electrical contact.

In Winter

•  Do not simply ‘switch off and forget’. Ensure your batteries have sufficient electrolyte in each cell and are fully charged at the start of winter.

•  It is important to recharge the batteries every two months over winter and keep an eye on the electrolyte level, topping up with distilled water if necessary.

•  Remember that devices like a battery meter use a small amount of current, which is insignificant in the short term but can flatten a battery over several months. If the boat is in the water, small leaks can lead to the bilge pump switching on regularly, again using up battery power.

•  When charging is completed, the battery charger should be disconnected from the mains. Most battery chargers switch off and stay off after charging. They will not start up again automatically but have to be switched off and on again.

•  Give your batteries another full charge before using them again (Note: you may find that they need to go through a few charge-discharge cycles before getting fully back into their stride).

 And finally, ‘In Earnest’

•  Always remember that a charging battery tends to ‘gas’, the more so as it approaches full charge. The hydrogen gas given off forms a potentially explosive mixture with air, it is therefore essential to give batteries adequate ventilation — whether they are on the boat or elsewhere.

•  Never forget that a lead acid battery is a powerful beast which can cause damage or personal injury if mishandled (for example, by accidentally short-circuiting its terminals). Please don’t mistake apparent lethargy for impotence! If in doubt about any connections or procedures, always obtain expert advice.