One of the most detrimental conditions for a battery is high temperature, particularly above 55C, because the rates of corrosion, solubility of metal components, and selfdischarge increase with increasing temperature. High operating temperature during cycle service requires higher charge input to restore discharge capacity and selfdischarge losses. More of the charge input is consumed by the electrolysis reaction because of the reduction in the gassing voltage at the higher temperature. While 10% overcharge per cycle maintains the state of charge at 25 to 35C, 35 to 40% overcharge may be required to maintain state of charge at the higher (60 to 70C) operating temperatures. On float service, float currents increase at the higher temperatures, resulting in reduced life. Eleven days float at 75C is equivalent in life to 365 days at 25C. Batteries intended for high-temperature applications should use a lower initial specific gravity electrolyte than those intended for use at normal temperatures. Manufacturers should be consulted on acceptable temperature ranges for operation of their batteries and on the associated effects of temperature. Nickel-cadmium batteries may be more suitable for higher-temperature applications.

Supplying an Equalizing Charge

Often a multi-cell battery will have one or more cells at a significantly lower voltage than other cells in the battery. When the battery is discharged the cells with lower voltage may become overdischarged. As noted earlier, overdischarge may cause enough damage to a cell that it needs to be replaced. When a difference exists between the potentials of cells in the same battery, an equalizing charge is applied to bring them up to an equal potential to the other cells. Criteria for applying an equalizing charge may be found in ANSI/IEEE 450, IEEE Recommended Practice for Maintenance, Testing, and Replacement of Large Lead Storage Batteries for Generating Stations and Substations.

Safety Precautions

Safety problems associated with lead-acid batteries include spills of sulfuric acid, potential explosions from the generation of hydrogen and oxygen, and the generation of toxic gases such as arsine and stibine. All these problems can be satisfactorily handled with proper precautions. NFPA 70, National Electrical Code, provides guidance on battery room ventilation. Wearing of face shields and plastic or rubber aprons and gloves when handling acid is recommended to avoid chemical burns from sulfuric acid. Flush immediately and thoroughly with clean water if acid gets into the eyes, skin, or clothing and obtain medical attention when eyes are affected. A bicarbonate of soda solution is commonly used to neutralize any acid accidentally spilled. After neutralization the area should be rinsed with clear water.

Precautions must be routinely practiced to prevent explosions from ignition of the flammable gas mixture of hydrogen and oxygen formed during overcharge of lead-acid cells. The maximum rate of formations is 0.42 L of hydrogen and 0.21 L of oxygen per ampere-hour overcharge at standard temperature and pressure. The gas mixture is explosive when hydrogen in air exceeds 4% by volume. A standard practice is to set warning devices to ring alarms at 20 to 25% of this lower explosive limit (LEL). Hydrogen detectors are available commercially for this purpose.

With good air circulation around a battery, hydrogen accumulation is normally not a problem. However, if relatively large batteries are confined in small rooms, exhaust fans should be installed to vent the room constantly or to be turned on automatically when hydrogen accumulation exceeds 20% of the lower explosive limit. Battery boxes should also be vented to the atmosphere. Sparks or flame can ignite these hydrogen mixtures above the LEL. To prevent ignition, electrical sources of arcs, sparks, or flame should be mounted in explosion-proof metal boxes. Flooded batteries can similarly be equipped with flame arrestors in the vents to prevent outside sparks from igniting explosive gases inside the cell cases. It is mandatory to refrain from smoking, using open flames, or creating sparks in the vicinity of the battery. A considerable number of the reported explosions of batteries come from uncontrolled charging in non-automotive applications. Often batteries will be charged, off the vehicle, for long periods of time with an unregulated charger. In spite of the fact that the charge currents can be low, considerable volumes of gas can accumulate. When the battery is then moved, this gas vents, and if a spark is present, explosions have been known to occur. The introduction of calcium alloy grids has minimized this problem, but the possibility of explosion is still present.

Some types of batteries can release small quantities of the toxic gases, stibine and arsine. These batteries have positive or negative plates that contain small quantities of the metals antimony and arsenic in the grid alloy to harden the grid and to reduce the rate of corrosion of the grid during cycling. Arsine (AsH₃) and stibine (SbH₃) are formed when the arsenic or antimony alloy material comes into contact with hydrogen, generated during overcharge of the battery. They are extremely dangerous and can cause serious illness and death. Ventilation of the battery area is very important. Indications are that ventilation designed to maintain hydrogen below 20% LEL (approximately 1 % hydrogen) will also maintain stibine and arsine below their toxic limits.

The following summary of safety precautions as found in DOE-STD-3003-94, Backup Power Sources for DOE Facilities, will aid in preventing personal injury and damage to facilities:

a. Rate of gas Formation is from Linden, 1984 (Handbook of Batteries and Fuel Cells).

1. Follow applicable site and area safety rules for work on batteries.

2. Obtain an approved Work Clearance permit per the Site Safety Manual before initiating any work on batteries.

3. The use of personal protection articles such as acid-resistant gloves, apron, face shield, and goggles is required.

4. Electrolyte is extremely corrosive and extreme care is required during handling.

5. Use only non-conductive/insulated/non-sparking tools in the battery room.

6. Do not smoke or use open flames, do not cause arcing in the vicinity of the battery.

7. All metal objects such as jewelry (rings, bracelets, necklaces) must be removed before working on batteries.

8. Neutralize static buildup just before working on batteries by having personnel contact the nearest effectively grounded surface.

9. Ensure the entrance and exit from the battery area is unobstructed.

10. Verify availability of currently inspected and operable (portable or stationary) water facilities for rinsing eyes and skin in case of an acid spill.

11. A Radiation Work Permit is required for all work in a radiologically controlled area.

12. For safety reasons, a person shall not work alone. At least two persons (Two Man Rule) must always be present when working on electrical power systems.