Maintenance and Storage of Lead-Acid Batteries

Safty Considerations

Lead-acid batteries contain a sulphuric acid electrolyte, which is a highly corrosive poison. It can cause blindness and severe skin burns. Battery electrolyte mixed with salty sea water will produce chlorine gas that may KILL you - even in small quantities!

• an appropriate amount of ventilation must be provided,
• protective eye-wear (safety glasses) and clothing are required,
• the manufacturer's instructions for testing, installing, charging and equalizing batteries must be followed,
• smoking as well as all sources of electric spark or open flame in the vicinity of the battery must be avoided,
• the battery charger algorithm should match the specification of the battery,
• all electric power cords should be located so that they cannot be stepped on, tripped over, or otherwise subjected to damage or stress,
• the charging process should be monitored and checked for unusual heat development or boiling cells.

As a side reaction of lead-acid battery recharging operation, water electrolysis occurs. In this reaction water is decomposed in hydrogen (H2) and oxygen (O2) in a 2/1 volume ratio. This gas mixture is very explosive and has to be disposed of safely. Ventilation is important to reduce the hazard, and avoiding overcharging will prevent the generation of the gasses.

Sealed battery designs cause recombination of the oxygen at the same rate it is produced, therefore eliminating the explosive mixture. Hydrogen which is produced will partly permeate plastic containers and as long as the sealed battery is not in a sealed small area, the hydrogen will harmlessly dissipate into the atmosphere. It is always good practice to allow for ventilation even with sealed batteries because of the possibility of a charger failure causing abnormal charge rates. If this happens, the battery will vent to prevent internal pressure build up.
Another precaution is to prevent short circuits of the battery terminals. This can cause high heat and potential fire hazard. The same can happen if the polarity of the terminals is erroneously reversed. When designing battery compartments, measures to avoid the possibility of accidental terminal reversal must be taken.
Multiple batteries combined together to increase voltage or capacity, should be as similar as possible. If one battery is weaker than the others in a series connection, the voltage may reach 0V during the final stages of discharge and then reverse causing rupture or venting from overheating.

Maintenance

Most batteries today are "maintenance-free". Nevertheless, if long battery life is an issue, some operation guidelines should be taken into account.

• keeping batteries properly charged is the prior issue for long battery lifetime,
• operating temperature should always below 40°C since overheating will quickly degrade the battery performance,
• operating temperatures below 0°C should be avoided since battery capacitance will strongly decrease and frozen electrolyte may damage the cells,
• fast charging with currents excessively higher than recommended, will degrade lifetime considerably,
• deep discharge levels will also shorten battery lifetime, partial discharges will extend life (lead-acid batteries do not have the "memory effect", which is characteristic for Nickel-Cadmium batteries),
• the proper electrolyte level is important; service-able lead-acid batteries require periodic checks of the level and addition of water occasionally. Only distilled or de-mineralized water can be used to keep levels above the plates,
• overfilling the battery with electrolyte must be avoided as well as spills of fluid outside the container. Spills can be neutralized with a baking soda in a water solution.

Storing Lead-Acid Batteries

Batteries tend to self discharge even if they are disconnected from the power system (e.g. if the main power switch is off). If allowed to completely discharge, sulphation occurs, which will eventually destroy the battery electrodes. Long intervals of inactivity normally is extremely harmful to a battery. The self discharge rate depends on temperature. Higher temperatures increase the self-discharge process, cooler temperatures slow down self discharge. If batteries are stored over a longer time period, they should be kept in a dry and cool place and precaution to keep the battery loaded must be taken. The best option is to connect an automatic solar "trickle" charger to the battery system, which keeps the battery loaded. This will require a solar panel supplying about 13.5V at 0.5A, which will also compensate the parasitic loads.

Sulphation of Lead-Acid Batteries

Lead sulphation starts when the State of Charge (SoC) drops below 80%. The plates become coated with a hard and dense lead-sulphate layer that fills up the pores of the electrodes. The positive plates will be light brown (Pb02) and the negative plates will be white (Pb). Over time, the battery losses capacity and cannot be recharged.

• Batteries should be kept charged at 100% (also during storage) avoiding overcharging.
• Continuous undercharging of the battery (less than 90%) will also allow sulphation. Not reactivated Sulphur-Oxide from the electrolyte will coat the electrodes.
• Low electrolyte level. Electrodes exposed to air (oxygen) will immediately sulphate.
• Self-discharge will discharge the battery even without external load connected. This effect increases with increasing temperature. Batteries stored at high temperatures (40°C or higher) must be recharged in shorter intervals.

Using normal tap water to refill batteries can produce calcium sulphate that will also coat the plates and fill the pores, resulting in capacity reduction.

• apply a constant current of 1-2A for 48-120 hours at 14.4V, depending on the battery capacity;
• discharge (50%) and recharge the battery a couple of times and test its capacity (in most cases it will not be possible to obtain the full battery capacity any more);
• in order to break down the hard lead sulphate crystals, the charging voltage may have to be increased;
• If the battery temperature rises above 40°C stop the charging and allow the battery to cool down before continuing charging.