Different types of battery used for auxiliary power supply in substations and power plants

In industrial or substation applications mainly three types of batteries are used namely:

1. Vented / Flooded Lead Acid batteries
2. Sealed maintenance free batteries/Valve Regulated Lead Acid
3. Nickel Cadmium (Ni-cd) batteries

Vented / Flooded Lead Acid batteries

Vented / Flooded Lead Acid batteries

There are two types for vented or flooded lead acid batteries namely tubular and Plante. The difference between the two is the construction. For tubular battery normal life is 8-10 years. The Plante battery is both mechanically and electrically more durable. The normal life for Plante batteries is 15-20 years. Because this type of battery generates corrosive fumes when charging and because the sulfuric acid electrolyte does evaporate to some extent, these batteries must be used in a special room, which is well ventilated to the outside and kept away from delicate electronic equipments.

It needs separate room/racks with acid proof tiles for installation. Because of evaporation, it needs regular maintenance to check specific gravity, to add water and acid. These batteries can withstand high temperature, voltage, and deep discharge with minimum damage to itself. A notice should be exhibited in the battery room prohibiting smoking and use of naked flames. These batteries cannot be transported in charged condition and therefore need charging at site.

Typical initial charging of the battery will take about 55 to 90 hours. Nominal cell voltage is 2V/cell. The charger for this battery should be able to provide the first charge at 2.6 to 2.7 V/cell.

Tubular Type Lead Acid Battery

These types of batteries are typically used for UPS Systems of very high rated capacity, typically engaged for plant application, wherein maintenance and space is not really an issue.

Sealed Maintenance Free batteries (SMF)

VRLA /SMF Type Lead Acid battery

These are also known as Valve Regulated Lead Acid (VRLA) batteries. These batteries are the most popular for usage with UPS systems for computer or commercial application. Being sealed, these batteries do not emit any fumes and hence can be very well installed next to electronic equipment. These batteries also can be housed in a close enclosure if necessary. These batteries are also maintenance free and avoid any hassles of checking specific gravity, adding water or acid, etc.

These batteries have a relatively lesser life of approx. 3-5 years. The life expectancy typically depends on the number of charge/discharge cycle experienced by the batteries and the ambient temperature in which the batteries are used. These batteries are primarily the most popular for commercial applications due to “Install and forget” approach.

The Performance and service life of these batteries can be maximized by observing the following guidelines:

1. Permissible operating temperature range of SMF batteries is 15 deg C to 50 deg C, but using within an operating range 5 C to 35 C will extend service life. Below – 15 deg C, the battery changes its chemical composition and cannot hold a charge. You will enjoy longer service life, if batteries are operated in ambient temperature range of 20 deg C to 25 deg C (68F to 77F). At lower temperature they have longer life and lower capacity while at higher temperature they have higher capacity and lower life.
2. A good rule of thumb when determining battery service life in relation to temperature is that for every 8.3 deg C (15F) average annual temperature above 25 deg C (77F), the life of the battery is reduced by 50%. Therefore warranty of the battery should be ideally reduced to 50% for every 8.3 deg C (15F) increase in operating temperature above 25 deg C (77F).
3. SMF batteries are designed to have a float voltage of 2.3 V/cell. This means that a 12 V battery (with 6 internal cells) has a float voltage of 13.8 Volts. Most of the battery manufacturers recommend float voltage of 2.25 – 2.3 volts per cell. When there are more cells (generally >120) in series, to compensate for higher temperatures, float voltage should be decreased by approx. 3 mV per cell per deg C above 25 deg C. It should be increased by the same amount when operated at a temperature less than 25 deg C to avoid undercharge. The Cutoff voltage is 1.67 V/cell for high rate of discharge.
4. It is recommended that SMF batteries should not be left in totally discharged state more than 72 hrs. The batteries may get partially or fully damaged due to SULPHATION if charging does not start within 72 Hrs from totally discharged state. Sulphation is the formation of lead sulphate on negative plates which acts as an insulator and has a detrimental effect on charge acceptance.
5. In normal float / equalize use (2.25 to 2.35V/cell), gas generated inside battery is recombined into negative plates, and return to water content of the electrolyte. Thus electrical capacity is not lost because of this recombination. There is no need to add external water, but due the corrosion of the electrodes battery will eventually lose capacity.
6. At ambient temperature of 30 – 40 deg C, the shelf life of batteries is 5-6 months only. A freshening charge must be given to the batteries every 6 months, if needed to be stored for longer periods. Batteries should be kept in dry, cool place. At ambient temperature of 20 deg C (68F), the self-discharge rate is 3-4% (approx.) of rated capacity per month. The self-discharge rate varies with ambient temperature.
7. SMF batteries are equipped with a safe, low pressure venting system, which operates at 7 psi to 10 psi (can vary slightly from manufacturer to manufacturer), automatically releasing excess gas in the event that gas pressure rises to a level above the normal rate ensuring no excessive buildup of gas in the batteries. Resealing is automatic once the pressure is returned to normal.
8. Cyclic life of the battery depends on ambient operating temperature, the discharge rate, the depth of discharge, and the manner in which the battery is recharged. The most important factor is the depth of discharge. At a given discharge rate and time, the shallower the depth of discharge, the longer is the cyclic life.
9. Failure mode at the end of life includes:
   1. Capacity decrease
   2. Internal short circuit
   3. Damage to container/lid
   4. Terminal corrosion
   5. Reduced open circuit voltage.
10. The IEEE defines “B” (Bend of useful life) for a UPS battery as being the point when it can no longer supply 80 percent of its rated capacity in ampere-hours (AH). The relationship between AH capacity and runtime time is not linear, a 20% reduction in capacity results in a much greater reduction in runtime. When battery reaches 80% of its rated capacity, the aging process accelerates and the battery should be replaced. Some UPS/ Battery manufacturers define “B” (Bend of useful life) for a UPS battery when battery capacity reaches 50-60% of its rated capacity.
11. 
    
    Mixed use of batteries with different capacities, different makes should be avoided as it will cause accelerated aging of the whole string.
12. If two or more battery groups are to be used, connected in parallel, they must be connected to the UPS through lengths of wires, cables or busbars that have the same loop line resistance as each other. This makes sure that each parallel bank of batteries presents the same impedance to the UPS as any other of the parallel banks thereby ensuring correct equalization of the source to allow for maximum energy transfer to the UPS load.
13. The normal life SMF battery will support approx. 200 charge/discharge cycles at 25 deg C (77F) and 100% depth of discharge.
14. The term “B” (Bend of useful life) for a UPS battery refers to the fact that these batteries do not require fluid. But preventive maintenance like checking for cracks and deformation of the container & lid, electrolyte leakage/spills tightening of the connection etc, particularly for higher AH capacity batteries should be done to prevent any damage.

Nickel Cadmium Batteries (Ni-Cd)

Ni-cd batteries do emit hydrogen and oxygen gas, products of electrolysis, but there are no corrosive gases as lead acid batteries, so these can be installed near electronic equipment. Water consumption is relatively low and so therefore maintenance is low. Normal service life is 20-25 years. These are most expensive of the various types of batteries previously discussed. Initial cost may be approximately three times that of lead acid battery depending upon their AH capacity.

These batteries do not experience the severe shortening of life when operated at elevated temperatures and perform better at low temperatures than do the lead acid batteries. Nominal cell voltage is 1.2 V/cell. The battery chargers and inverters have to be designed to operate with low end cell cutoff voltages and higher recharging voltages needed for such batteries.

These batteries occasionally demand boost charging and typically find their applications wherein UPSs support critical equipment in hazardous environment such as chemical, fertilizer, cement industry.