by Kurt Küpper*
Flooded Lead Acid Batteries
Flooded lead acid (FLA) or wet cell batteries are the most commonly found lead acid batteries available. The name stems from the fact that the electrolyte (sulphuric acid solution) is in liquid form. Traditionally these have had removable caps that allow the level of the electrolyte to be checked and, if necessary, topped up.
This technology has been used for many years as automotive starting batteries, bringing an economy of scale and thus good value for money pricing. Deep cycle wet cell batteries require very strong plate grids, more lead, extra plate separators and stronger cases, and are therefore more expensive than starting batteries.
Well designed and maintained flooded lead acid batteries can last through thousands of charges – discharge cycles, making them the standout choice of all lead acid battery technologies. However, in practice this theoretical lifetime is hardly ever achieved, as it is conditional on an optimal usage and care regime that very few owners are able to maintain in practice.
During charging, charge will initially be deposited on the surfaces of the battery plates. The surface charge will dissipate deeper into the plates, allowing further charge acceptance on the surface. For this reason a battery that has just been charged will have a high surface voltage as measured at the terminals, which will reduce as the battery is rested until it stabilizes at a voltage that is equal throughout the lead plates.
Deep cavities are created in the lead plates during severe battery discharges. Extended recharge periods are required to allow lead to be deposited in these cavities. If such batteries are never charged long enough for this process to take place, a hard crust of sulphur crystals can form in these cavities that prevents the chemical reaction between lead and acid required for discharge and charge. Over time this leads to a reduction in the effective capacity of the battery.
Sulphation of plate surfaces also occurs if batteries are left in a semi discharged state for any length of time. Batteries should thus best be maintained in a fully charged state at all times when not in use.
Where sulphation has occurred, it may be possible to break down the sulphur crystals by periodically applying a charge of 15V to 16V for a few hours. This is referred to as equalization.
As the battery comes to the point of full charge, the electrolyte starts to bubble off hydrogen, oxygen and other gases. It is therefore essential that FLA batteries are only ever charged in well ventilated spaces.
FLA batteries therefore have to be charged for protracted periods at limited charge rates to ensure full charging while preventing excessive loss of electrolyte.
The electrolyte must be checked regularly and topped up with enough distilled water to ensure that the lead plates are covered to replace the liquid boiled away during charging and equalising.
Fully charged FLA batteries should have a charge of 12.8V once rested after charging. Deep cycle batteries should not be discharged to below 12V, at which point they will be about 50% charged. Ensure that batteries are fully charged when the boat is not in use by using a shore powered charger or wind or solar charging. The state of charge can also be determined by measuring the specific gravity of the electrolyte with a hydrometer.
FLA batteries sold nowadays are increasingly being labelled ‘Sealed’ or ‘Maintenance Free’. Such batteries are filled with an excess of electrolyte. The downside of such batteries is that they will become damaged due to insufficient electrolyte once the excess has been used up or if there is a rapid loss due to overcharging.
Calcium batteries are another variety of the species. Calcium or Calcium/Silver is added to the lead alloy to reduce the rate of water loss in sealed batteries. These batteries generally require a charging voltage of about 15V though and should therefore be avoided on boats unless the charging systems on the boat can be set to the specific parameters required.
FLA batteries therefore have significant inherent downsides, which more than negate the cost advantage the offer over GEL and AGM batteries. We will discuss those next month.
*Kurt Küpper is director of Aquavolt Electric Boat Parts. Tel: 02 9417 8455 www.aquavolt.com.au