In industrial or substation applications mainly three types of batteries are used namely:
- Vented / Flooded Lead Acid batteries
- Sealed maintenance free batteries/Valve Regulated Lead Acid
- Nickel Cadmium (Ni-cd) batteries
For UPS applications batteries are the most popular and hence are widely used. Hence, in this detailing, mainly emphasize has been put on these type of 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.
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)
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.
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:
- Capacity decrease
- Internal short circuit
- Damage to container/lid
- Terminal corrosion
- 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.
As spelled earlier, all the above discussed types of batteries have their own merits and demerits. Let us now look at them individually.
A) Vented / flooded Lead acid batteries
- Most economical among three types of batteries.
- Life is higher than SMF batteries.
- Robust- not much sensitive to temperature.
- Needs periodic maintenance- twice a month.
- Emits corrosive fumes.
- Needs special battery room with acid proof tilling.
- Cannot be transported in charged condition, initial charging takes 55 to 90 hours.
- Needs specially trained persons for handling due highly hazardous sulphuric acid.
B) Sealed maintenance free batteries / Valve Regulated Lead Acid
- No maintenance as far as water filling, specific gravity check etc is concerned.
- Can be shipped in charged conditions so ready to use.
- User friendly.
- Leaving batteries in discharged state for longer life will reduce life significantly or can damage them permanently.
- Very sensitive to temperature
- Service life lowest among the three types
- Costlier than flooded / vented lead acid battery
C) Nickel Cadmium (Ni-cd) batteries
- Moderate maintenance
- Higher service life
- less sensitive to temperature
- Fumes not corrosive so can be installed near electronic equipment
- Most expensive among three types
- Cannot be transported in charged conditions.
- Compatibility with respect to charger and inverter needed to be considered.
Summary of Techno-Commercial Analysis between Ni-Cd & Lead acid Batteries
|1||Reliability Criteria||Worst case failure is cell short circuit. Result is reduced performance. The battery will continue to support the system.||Worst case failure is cell open circuit. Result is complete loss of battery, known as ‘sudden death’, resulting in an unpredicted system failure.||This point makes NiCd to be superior in terms of purpose|
|2||Requirement of Air-Conditioning & additional cost of Electrical Energy||Nil||Yes||Additional Cost due to Air conditioning equipment and running cost-For VRLA|
|3||Battery Life as claimed by UK manufacturers (Industrial Batteries)||20 Years||5-7 Years||Replacing cost of VRLA after every 5-7 years will involve manufacturing cost escalation during that year. Eg.at 6th year, 12th year and 18th year.|
|4||Battery Cost||Approximately 3 times than VRLA||–||–|
|5||Relative Costs Initial & Life Cycle||Ni-Cd cells have a higher initial cost than lead acid but superior lifetime and characteristics, giving a lower life cycle cost in many applications.||Lead Acid has a low initial cost but a restricted lifetime. In many applications they can have a poor life cycle cost.||–|
|6||Physical Size||Ni-Cd cells are generally larger than VRLA cells.||In terms of Ah, Lead acid (VRLA) is the most compact battery.|
|7||Water Topping||Occasionally Required||Not Required|
Summary of Comparison between tubular flooded (ordinary Lead-acid) battery with SMF Lead Acid Battery
|No.||Tubular Lead Acid battery||VRLA Battery|
|1||Tubular positive Plates:|
The positive active material is held in a polyester tube. This does not allow the materials to shed during charge-discharge cycling resulting in long cyclic life- minimum 1200 cycles at 80 % depth of discharge.
|Flat pasted positive plates:|
The positive active material is a paste form pasted over a lead alloy grid. This results in a shorter cyclic life compared to tubular construction.- minimum 500 cycles at 80% depth of discharge
|2||Electrolyte: Stationary batteries of UPS and Power plant back up works on low specific gravity (1.200) electrolyte and larger in volume. This results in less corrosion of grids and longer life. The larger volume keeps the battery comparatively cooler which also adds to life.||Electrolyte: VRLA batteries work on high specific gravity electrolyte (1.280 to 1.300) and less volume. The cells get heated up during charging and high rate discharges which affects the service life.|
|3||Separators: Micro porous poly ethylene separators are used. This permits the ion movement and positive to negative plate separation.||Separators: Adsorptive glass mat separators are used in these batteries with a closer spacing between the positive and negative plates. Chances of cell shorting are therefore more.|
|4||Charging compatibility: Tubular stationary lead acid batteries can be charged with constant current and constant voltage mode. Flooded electrolyte batteries can withstand more abuses during charging. Water lost in electrolyte during such abusive overcharging could be easily made up manually by periodic top-up. Lagging cells could be brought to normal life by an extended equalizing charging without significantly affecting the other cells.||Charging compatibility: VRLA batteries require constant voltage charging with a specified limit only specifically to avoid overcharging. If by any chance, the charging conditions are altered, the battery will get heated up which will deteriorate the battery life. Though there is no need to top-up due to the recombination principle, it is not always 100% efficient, so some water loss during use is not avoidable. This results in a further increase in electrolyte specific gravity and life reduction. Equalizing (Extended) charging in VRLA batteries results in some cells getting heated up and life reduction.|
|5||Containers: We use transparent SAN containers. The electrolyte level is easily seen for topping up needs and ease of maintenance. Low maintenance.||Not applicable: Cannot monitor Electrolyte|
|6||Need for temperature control: Due to the large electrolyte volume the temperature of the cells generally do not rise abnormally during charging. No need for Air conditioning.||Need for temperature control: The close packing arrangement and compactness of the stack in the cell assembly rises the temperature sharply during charging. Due to this, VRLA battery manufacturers invariably recommend the need for air conditioned environment for the battery compartments. This adds to the cost of maintenance.|