A battery can be any device that stores energy for later use. The word battery is limited to an electrochemical device that converts chemical energy into electricity, by use of a galvanic cell. A galvanic cell is a simple device consisting of two electrodes – positive & negative and an electrolyte solution. Batteries consist of one or more galvanic cells with a potential difference between the electrodes.
Lead-acid is the oldest rechargeable battery in existence. Invented by the French physician Gaston Planté in 1859, lead-acid was the first rechargeable battery for commercial use.
Lead Acid batteries use chemical reaction between lead dioxide positive electrode, pure lead negative electrode and dilute sulphuric acid to store and regenerate electrical energy as per requirement.
Lead Acid Batteries can be classified by application or by construction or by design.
By Application
Automotive Battery : Used in automobiles for starting the engine, ignition, and lighting
Industrial Battery : Used for UPS, Inverter, Telecom, Solar
Traction Battery : Used for Forklifts, Golf Carts, Railway coaches
By Construction
Flooded Battery : Traditional battery design where the battery container has liquid electrolyte and typically the containers are large and bulky. These batteries require periodic maintenance by topping up distilled water as gases are vented during the operation of the battery.
Sealed Maintenance Free Battery : The battery container is sealed and there’s no free electrolyte that will spill. This type of battery does not require any topping up of distilled water.
By Design
Flat Plate Battery : The positive and negative plate inside the battery is of a flat construction. The plate is made by pasting a mixture of lead oxide on to a grid made from lead alloy. This is then cured and the paste hardens into a cement like mass.
Tubular Plate Battery : Here the positive plate is made of a set of parallel tubes filled with lead oxide. The negative plate remains as a flat plate.
Most people tend to associate a lead acid battery with an automotive battery and hence end up using automotive batteries for UPS & inverters leading to problems. The two applications are vastly different from each other.
Automotive Lead Acid batteries:
These batteries are used for starting an engine which requires a very high current for a few seconds. Once the engine starts a small current is used for ignition and lighting and the battery is already charging in parallel. Hence the automotive batteries are designed with thin flat plates that are designed to provide high current discharge for a very low duration. The plate thickness is typically below 2 mm. However the containers are very robust since they are mounted in moving vehicles and should withstand jolts and severe vibrations.
Industrial Lead Acid batteries:
These batteries are designed for variable and extendable loads and are hence more complex in design than automotive batteries. The different types of Industrial Batteries are
Flat Plate flooded batteries : These are typically used for inverters.
Tubular Plate flooded batteries : These are used for inverters, UPS and Solar applications
SMF VRLA Batteries : These are sealed maintenance free batteries used for UPS, Telecom & Solar applications. ‘VRLA’ stands for ‘Valve regulated lead acid’. In sealed batteries, due to chemical reactions inside the battery gases are formed. Sometimes due to over charging, more gases are formed that needs to be released from the battery container. This is done by using valves that release under a certain pressure. Otherwise the batteries will explode.
Flat plate batteries are cheaper & easier to manufacture but offer lower cyclic life. Cyclic life is the number of times the battery can be charged and discharged over its life span. A flat plate battery will typically offer between 200 – 400 cycles (depending on design, make etc) at 80% Depth of discharge. Depth of discharge is the extent to which the battery is discharged before being charged. 80% DOD means it’s discharged to 80% of it’s capacity before being charged again.
Tubular Battery Advantage :
Long life span – 3 times of a flat plate battery.
Lower cost of ownership.
Requires less frequency of topping up than flat plate batteries. Battery can be revived even if plates sulphate due to improper maintenance.
Tubular Battery Plate Disadvantage :
Initial higher cost.
Difficult manufacturing process.
Gives slightly lower backup time than flat plate battery of the same AH rating.
Tubular batteries use a special technology where the active material is encapsulated in tubes that prevent ‘shedding’. This prolongs the life of battery while undergoing long charges and discharges. Typically tubular batteries give more than 1000 cycles of life at 80% DOD which is 3-5 times that of a flat plate.
Can be used in extreme conditions like high temperature, since electrolyte loss due to high temperatures can be compensated by frequent topping up of distilled water.
Lower Cost per AH compared to SMF batteries.
Flooded Battery Disadvantage:
Regular topping up and maintenance required, otherwise battery will die.
Gases generate unpleasant odour, hence cannot be used in regular office environment.
Needs to be stored / used in vertical position.
Needs far more space than SMF batteries.
SMF Battery advantage:
No topping up required.
Environment friendly and can be used in office environment.
Can be stored / used in any position.
Requires very less space.
SMF Battery disadvantage:
Cannot be used in extreme conditions, since water loss cannot be compensated by topping up. Life of SMF battery comes down by half for every 10 degree rise in temperature. Typical life is 250-300 cycles at 100% DOD at 25 degree Celcius.
Can be used only with UPS or any device with clean DC charging. Use with inverters that has a high ripple content in the charger will lead to shortened life of the plates.
AH is a measure of how much current a battery can discharge over a certain period. It’s the ratio between current (A) and time – hour (H). For example a 100AH battery means the battery can discharge a current of 5 amps over a period of 20 hours. It can also mean the battery can discharge a current of 10 Amps over 10 hours. However the capacity of a battery that discharges 5 amps over 20 hours is less than that of a battery that discharges 10 amps over 10 hours, even though both are arithmetically 100AH. This is because higher the current discharge, the more the energy loss and hence lower the time it can discharge.
That’s why it’s important to understand the standard of discharge to ensure accurate understanding of the rating. The industry standard is ’20 hours’. Therefore the rating of any battery means the quantum of current the battery can discharge over a period of 20 hours. A few examples are given below for understanding.
A 7AH battery means it can discharge 0.35 amps for a period of 20 hours
A 26AH battery means it can discharge 1.3 amps for a period of 20 hours
It’s a measure of the discharge time over which the battery is rated. C20 is a 20 hour discharge, C10 is a 10 hour discharge, C5 is a 1 hour discharge and so on. For example a 7 AH battery should be able to discharge 0.35 amps over 20 hours. But the same battery will probably deliver only 9 hours of discharge if discharged at 0.7 amps i.e double the current. This ratio delivers only 6.3 AH (0.7 amps * 9 hours)
While purchasing a battery it’s important to check the discharge ratios over different time periods to understand the efficiency of the battery. Any good manufacturer will always provide this data in the battery specifications.
It’s important to note that in real life, most batteries used for UPS & Telecom backup applications are discharged over a period ranging from a few minutes to maybe 4 hours. So the real AH delivered is always less than half to 3/4th’s of the rated capacity at 20 hours. This is one of the reasons why the battery over sizing has to be considered while calculating backup times for a UPS.
Float life refers to the period the battery will last when it’s under ‘float use’ i.e the battery voltage is maintained at approximately 13.5V during usage. For a layman’s understanding, this is the condition the battery is in when connected to a UPS and there is no powercut or there’s generator backup i.e battery is never discharged.
Battery float life for a VRLA battery varies from 3 years to as high as 25 years depending on the design. Lower the design life, lower the cost of the battery and the quality.
Cycle life refers to the number of cycles of charge and discharge that the battery can withstand before it dies. Cycle life is referred to as ‘No. of cycles’ at a certain ‘DOD or Depth of discharge’ It can vary from as low as 300 cycles at 80% DOD for an SMF to as high as 5000 cycles for some specialised batteries like submarine batteries. Most good manufacturers will provide details of float life as well as cycle life at different DOD’s by means of a graph.
There’s no single formula for determining the life of a battery. The factors affecting life are many – Quality, cyclic capacity, temperature, the manner of charging, maintenance etc
All batteries have a finite shelf life. Most charged batteries will lose between 2% to 5% of capacity for every month of storage at 25 degree Celsius. Higher the temperature, higher the capacity loss. Therefore it’s important to ensure that batteries that are stored for long are given a regular maintenance charge at least once in 3 months to prolong shelf life.
Overcharging is the most destructive element in battery service. During overcharging, excessive current causes the oxides on the plates of the battery to ‘shed’ and precipitate to the bottom of the cell and also heat the battery, thus removing water from the electrolyte. Once removed, this material (which represents capacity) is no longer active in the battery. In addition, the loss of water from the electrolyte may expose portions of the plates and cause the exposed areas to oxidise and become inactive, thus reducing additional capacity. Sealed batteries are not immune from the same internal results when overcharged. In fact, sealed batteries are particularly sensitive to overcharging. Once moisture is removed from the battery, it cannot be replaced. Portions of the battery damaged due to overcharging are irretrievable. However, if detected early, corrective adjustments to the charging device will save the undamaged portion of the battery. Initial signs of overcharging are excessive usage of water in the battery, continuously warm batteries, or higher than normal battery voltages while under the influence of the charger.
Undercharging over a period of time also damages batteries. Insufficient charging leads to a problem called sulphation of the plates. This is a typical problem for long back duration UPS’s where the charger current capacity is less than 10% of the battery current rating. For example, while using a 100AH battery, the charger capacity should be ideally 10 amps.
Batteries go into a state of discharge when it reaches a voltage of 1.75 volts per cell or 10.5 volts in case of a 12 volt battery. Discharging the battery beyond this point may result in the battery reaching a point of no return. The lowest a 12V battery should be discharged to is 9.6V i.e 1.6V per cell. Continuous cycling to low voltage levels reduces the cycle life of the battery.
Most devices have a ‘low battery cut off’ voltage setting beyond which the battery is cut of from the circuit and stops discharging. If this is not present in a device using a battery, most certainly the life of the battery will be drastically reduced.
The health of an SMF battery can be determined by checking the OCV (Open circuit voltage) of the battery. A healthy battery will have an OCV above 12.8V. Any voltage below this means the battery is not sufficiently charged and needs a charge. The typical OCV readings should be as follows
Just after unpacking at site: Minimum 12.80 V
Just after connecting on float: Minimum 13.30 V
After 2 days of float charging: Minimum 12.95 V
The health of a flooded battery can be determined by checking the OCV and also by checking the specific gravity of the electrolyte. Specific gravity is a unit of measurement for determining the sulphuric acid content of the electrolyte. The recommended fully charged specific gravity of flooded batteries is 1.255 to 1.265 taken at 80°F. More than .025 spread in readings between fully charged cells indicates that the battery may need an equalization charge. If this condition persists, the cell is failing and the battery should be replaced. Since water has a value of 1.000, with electrolytes having a specific gravity of 1.260, it means that it is 1.260 times heavier than pure water even while pure concentrated sulphuric acid has a specific gravity of 1.835.
The following table illustrates typical specific gravity values for a cell in various stages of charge:
Battery OCV should be minimum 12.8V. If less than 12.8V, then the battery should be given a float charge till it reaches the desired voltage of 12.95 to 13V. In case the battery cannot take charge, the battery has to be replaced.
If connecting in a string, then the OCV of all the batteries in the string should be in the range of 300 millivolts of each other. For example if there are 16 batteries being connected, then the OCV ideally should be between 12.8 to 13.1V. Any higher or lower readings should be noted and the batteries given an ‘equalisation charge’. This is done by float charging the battery for a minimum period of 12 hours (13.5 – 13.8V) and then doing a C10 discharge for about 6 to 8 hours. Regular voltage readings of all batteries should be noted every 30 minutes. The cycle of charge and discharge is done till all batteries reach the desired voltage range within 300 mv. In case there’s a battery not within the range even after equalization, replace the battery.
Physically examine the battery for any sign of electrolyte leakage or visible damages to the battery container.
Check the condition of the positive and negative terminal connectors – they should not be corroded.
Site condition at time of installation:
The site should be well ventilated and airy to ensure proper dissipation of hydrogen gas that is emitted by batteries. Hydrogen is highly inflammable and therefore dangerous when accumulated in closed spaces.
Proper earth fault and insulation of the battery racks / stand to prevent current leakage to the ground.
UPS condition at time of installation:
Ensure charging current is proper.
Check float and boost voltages of the UPS. Should be 13.5 – 13.8V for float charge and 14.5 in boost charge mode with automatic return to float mode.
Other precautions:
Battery type, capacity, make and age should be similar in the string.
The correct length and cross section of interconnecting cable to be used.
Torque used will be same for type of bolt used. Torque recommendations are typically given in the battery specification sheets. It’s recommended that the technician doing installation use a torque wrench for tightening the bolts.
More than 3 strings of batteries should not be paralleled. Here, it’s recommended that each string has a separate charger to ensure proper charging.
