What is a VRLA Battery Bank?
The full form of VRLA is “Valve Regulated Lead Acid”. Therefore, a VRLA battery means a Valve-Regulated Lead-Acid battery. It is essentially a modified version of a conventional low-maintenance lead-acid battery.
Overchargings form oxygen and hydrogen gases due to electrolysis of water. These gases escape through vents provided on the top of the conventional battery cells. This hydrogen and oxygen loss leads to water loss. So we need to top up water periodically in conventional lead-acid battery cells. A VRLA battery cell does not allow gases to escape easily. The oxygen combines with the hydrogen to form water again. Because of this recombination cycle, we normally do not need to add water from outside.
However, a VRLA battery includes a safety relief valve on the container. This valve opens only when internal pressure rises due to abnormal charging conditions. It releases excess gas to prevent damage or explosion. Because of this valve, we call it a Valve-Regulated Lead-Acid (VRLA) battery.
Why is water reduced in a lead-acid battery?
Why do we need to top up the water in a lead-acid battery regularly?
The discharging reaction generates hydrogen ions. During charging, oxygen consumes those hydrogen ions and forms water molecules. So there will be no chance of generating extra hydrogen or oxygen during normal discharging and charging processes.
However, during overcharging, the extra energy injected starts the electrolysis of water molecules and forms oxygen and hydrogen gases. These oxygen and hydrogen gases are exhausted through the vent of the battery cell. Therefore, day after day, the amount of water in the cell reduces. That is why we have to top up the water in lead-acid battery cells often.
How to calculate the size of a stationary battery?
If we use a C10 battery, we must consider all DC loads during an emergency for 10 hours. The battery must supply protection, control, indication, alarm, and interlocking loads for 10 hours. We must also calculate all DC lighting loads for 10 hours. Then we must include intermittent DC loads. These include breaker closing and tripping, isolator operation, and other DC outlets. We must consider the simultaneous tripping of all incoming and outgoing breakers during busbar protection operation. Also, we assume this heavy load lasts for one minute. We must also include the DC power needed to restore the AC supply. We consider closing at least one incoming feeder and one station service transformer.
Then we draw the DC load characteristic curve. Then, we usually add 20% spare battery capacity. Additionally, we apply the required factors. Finally, we calculate the approximate battery size. For details, we can read the article on battery sizing calculation of a substation.
What are the maximum and minimum voltage ranges of substation batteries?
The battery voltage ranges from + 10% to – 10% of the rated battery voltage. For a 220V battery bank, it will be from 220 + 22 = 242V to 220 – 22 = 198V. The voltage per battery cell will be . As per the Standards, the lower range of cell voltage is 1.75V. That means, when the cell voltage comes down to 1.75V, we consider the battery bank to be fully discharged. Many utilities consider it to be 1.85V, too.
| Battery Voltage | Maximum Voltage | Minimum Voltage (discharged to 1.85 V/cell) |
|---|---|---|
| 220 V | 242 V | 198 V |
What is the voltage regulation of a battery charger?
Voltage regulation of a battery charger is the change in output voltage from no-load to full-load conditions. It shows how the output voltage changes when the load current varies. We vary the load current from 0% to 100% of full-load current and measure the output voltage. The output voltage variation should remain within 1% as recommended by many utilities.
What is the line regulation of a battery charger?
In a battery charger, line regulation is the change in output DC voltage with respect to changes in input AC voltage. We vary the input AC voltage from +10% to −10% of its rated value. We then measure the change in output voltage, usually at no-load and full-load conditions. The output voltage variation should remain within 1% as recommended by many utilities.
Why is it not allowed to connect battery cells in parallel in a battery bank?
It is nearly impossible to have identical internal characteristics for any two battery cells. In maximum times, their internal resistances differ slightly. As a result, if we connect two battery cells in parallel, a circulating current can flow between them. This circulating current creates localized heating. This heating can accelerate the aging of one or both cells. To avoid this, we can not connect two or more cells in parallel to increase the current rating of a battery bank.
What is the ampere-hour efficiency of a battery?
Ampere-hour (Ah) efficiency shows how much charge a battery gives back during discharge compared to the charge it receives during charging. It is the ratio of discharge Ah to charge Ah for the same voltage change.
What is the watt-hour efficiency of a battery?
Watt-hour (Wh) efficiency shows how much wattage a battery gives back during discharge compared to the wattage it receives during charging. It is the ratio of discharge wattage to charge wattage for the same voltage change.
When are Ni-Cd batteries preferred over lead-acid?
What are the differences between VRLA and flooded lead-acid batteries?
- Sealed Design and Venting System: A valve-regulated lead-acid (VRLA) battery is sealed. It has a pressure-regulated valve. The valve releases gas only during abnormal overcharging. A flooded lead-acid battery has open vents. These vents release hydrogen and oxygen gases during overcharging.
- Gas Recombination and Water Loss: In a VRLA battery, the hydrogen and oxygen formed during overcharging recombine inside the cell. They form water again. This water returns to the electrolyte. So, the water quantity stays almost constant. We do not need to top up water to a VRLA battery. In a flooded lead-acid battery, gases escape to the atmosphere through the vents. This causes water loss. The water level drops day by day. So, we must add distilled water as required.
- Electrolyte Form and Battery Orientation: A flooded battery uses liquid dilute sulfuric acid. The liquid can spill. So, we must always keep the battery cells upright. A VRLA battery uses AGM or silica gel paste. These hold the liquid dilute sulfuric acid electrolyte inside. The electrolyte does not spill. So, we can install VRLA battery cells in vertical or horizontal positions.
- Size and Space Requirement: A VRLA battery cell is more compact. It needs less space. Its footprint is smaller than a flooded battery.
- Temperature Effect and Pressure Control: In a VRLA battery, gases cannot escape easily. The battery relies on gas recombination. So, temperature control is important. High temperature increases the volume of the gases. Excess gas volume raises internal pressure. This can deform the container. Therefore, air-conditioning in the battery room is recommended. Since the gases are not sealed in a flooded lead-acid battery cell, it can tolerate temperature variations better. Normal ambient temperature variations do not affect them much. So, air-conditioning is not always necessary in the battery room.
- Service Life Comparison: VRLA batteries usually have a shorter life than flooded batteries.
- Tap Cell Provision and Boost Charging: We should avoid overcharging VRLA batteries. Therefore, we should also avoid boost charging. Tap cell charging is not gladly recommended for VRLA batteries. Flooded batteries can accept boost charging. Tap cell provision is useful in flooded battery banks.
These are the main differences between VRLA and flooded lead-acid batteries.
What specific gravity do we maintain of electrolyte in a battery, and why is it important?
The specific gravity of dilute sulfuric acid in a lead-acid battery cell is one of the most important parameters to maintain. If the specific gravity is high, corrosion increases. If the specific gravity is low, the battery capacity decreases. The rated battery capacity depends on the value of the specific gravity. A specific gravity of 1.280 is typically maintained in a lead-acid battery. When we measure the specific gravity, we must correct the reading to 27 °C room temperature.
What is the C-rating of a battery bank?
The C rating of a battery bank shows its capacity. It tells us how long the battery can supply rated current before it discharges. A C10 rating means the battery will discharge in 10 hours from its fully charged condition. A C5 rating means the battery will discharge in 5 hours from its fully charged condition.
For example, a 600 Ah battery with a C10 rating can supply rated current continuously for 10 hours before it reaches its minimum voltage threshold. If we divide 600 Ah by 10 hours, we get 60 A. So, the battery can continuously deliver 60 amperes for 10 hours.
What is float charging?
The charger is directly connected to the DC distribution bus. The battery is connected across the charger output terminals. So, the charger must supply current to the DC load of the substation and also provide charging current to the battery.
So, the battery does not supply current to the DC system. Instead, it remains connected across the charger output in a floating condition. However, due to the battery’s internal resistance, a small loss of charge always occurs inside the battery bank. To compensate for this loss, a small continuous current flows into the battery from the charger. This maintains the battery in the charged state. This condition is called float charging. This process is known as the float charging method.
Why is Wh efficiency lower than Ah efficiency?
Ampere-hour (Ah) efficiency tells us how much total charge the battery delivers compared to how much charge it receives from the source. It compares the output charge to the input charge.
Watt-hour (Wh) efficiency tells us how much energy the battery delivers compared to how much energy it receives. Here, both voltage and current are considered, so the result is in watt-hours. Because of the internal resistance of the battery, some voltage drops during discharge. This voltage drops reduces the energy output. Therefore, watt-hour efficiency is always lower than ampere-hour efficiency. As a general standard, the ampere-hour efficiency of a battery is about 90%. The typical watt-hour efficiency is about 80%, which is 10% lower than the ampere-hour efficiency.
Why is ventilation required in battery rooms?
During overcharging, batteries can emit hydrogen and oxygen. Hydrogen is highly flammable. If it accumulates, a small spark can cause an explosion. Ventilation dilutes and removes these gases.
What is a battery discharge test?
First, fully charge the battery. Then keep it open-circuited for 2 to 24 hours. After that, discharge the battery using a resistor bank. Discharge it at the rated current. For a C10-rated battery, the rated current is one-tenth of its Ah capacity, because it should fully discharge in 10 hours. Divide the Ah rating by 10 to get the discharge current. Adjust the load resistance so the battery can supply this current continuously. For example, a 600 Ah battery at C10 should discharge at 60 A. Set the resistance so it draws 60 A continuously.
At the start, connect a voltmeter and an ammeter. Check the load voltage and discharge current. Take readings every 5 minutes for the first 15 minutes, then continue at a specified interval. Keep the discharge current within ±1% of the rated value. Measure temperature at intervals. The terminal temperature is enough because it is nearly equal to the electrolyte temperature. Continue discharging until each cell reaches 1.75 V. Then stop the test and note the total time. Calculate the delivered capacity by multiplying the current by the time in hours.
Then correct the capacity to 27°C using the temperature correction formula.
During the test, make sure the voltage does not fall below 1.98 V after 6 minutes. It should not fall below 1.92 V after 6 hours. At 10 hours, it will reach about 1.75 V. In the first cycle, the battery should give at least 85% of the rated Ah capacity. You can repeat the test up to five times. Stop testing once the battery reaches 100% capacity. If it does not reach 100% after five tests, or if it exceeds 120%, reject the battery.
How do we conduct C1 capacity test of a battery cell?
- C1 capacity is not equal to C10 capacity. C1 capacity is usually about 50% of C10 capacity.
- First, keep the battery or sample cells fully charged for 2 to 24 hours.
- Then set the load resistance so the battery can deliver 50% of the C10 current.
- Connect an ammeter and a voltmeter. Measure load current and load voltage. Take readings every 5 minutes. Try to keep the current within ±1%.
- Measure the battery terminal temperature every hour. Terminal temperature is taken as equal to electrolyte temperature.
- Stop discharging when the cell voltage drops to 1.7 V. Record the total discharge time. Multiply the measured continuous current by time to get the discharge capacity.
- Correct the capacity to 27°C. Check if it is above 90% of C1 capacity.
- If it is below 90% in the first cycle, repeat the test for at most 3 cycles under the same conditions.
- When any cycle gives 100% capacity, stop further testing. If the battery does not reach 100% of C1 capacity in three consecutive test cycles, reject it.
Why is the C1 capacity of a battery lower than the C10 capacity?
The first reason is the discharge current. The discharge current of C1 is ten times higher than that of C10. When this ten times higher current flows through the battery, a large voltage drop occurs due to the internal resistance of the battery. Because of this voltage drop, the battery reaches the end cell voltage faster.
Second, this high current creates heat loss in the internal resistance of the battery. So the battery cannot deliver its full capacity to the load. Some capacity is lost as heat before it can be delivered.
Thirdly, the chemical reactions inside the battery need some time. At high discharge current, the reactions do not get enough time to complete. This is another reason why the capacity reduces at higher discharge current.
What is battery ripple current and its effect?
The substation battery receives the charge from a battery charger. The charger converts AC input to DC output. This DC supply feeds the DC load and also charges the battery connected in parallel. Some ripple is always present in the DC output voltage. When this DC is applied to the battery, the ripple also enters the battery. Ripple causes small charging and discharging cycles. These rapid cycles generate heat inside the battery. The extra heat reduces the battery’s life.
Therefore, a ripple in the charger’s DC output is not desirable. Normally, the charger keeps the ripple below 1%. A battery may not tolerate ripple above 1%.