Battery Sizing

The battery capacity shall be based on the design load. The design load equals 125% of the DC load demand, including 15% spare for ancillary loads and 10% considered for battery aging. If not determined in project design specifications, the selected battery should usually supply power for approximately 48 hours. Therefore, the calculation is given as:

$$ C_{50} = 125\% \times \text{DC demand load (A)} \times \text{Autonomous hours (typically 48 h for stand-alone systems)} $$

\( C_{50} \) is the 50-hour rate capacity of the battery, which can be found in documents provided by the battery manufacturer. Generally, \( C_x \) is the battery capacity in Ampere hours (Ah) specified for an appropriate discharge rate (A). Note that batteries have different capacities for different discharge rates.

Project Requirements

I addition to above, design must adhere with project requirements which may differ project to project.

• “Batteries supplied as part of the battery charger system shall be either sealed lead-acid battery or lithium-ion battery. Standard (non-sealed) lead-acid batteries generate hazardous substances (e.g., hydrogen) during operation and shall not be used.”
• “Batteries shall be sized to supply the base station load for a minimum period of 48 hours to an end voltage of 87.5% of the normal voltage.”

Charger Sizing

The battery charging rate and voltage should follow the manufacturer’s recommendations. Battery chargers are typically sized at the 10-hour rate (AS/NZS 4509.2). This means that the charger is sized so that the recharge time from maximum depth of discharge, with no load on the system, does not exceed 10 hours. Note that this guideline is for charger sizing purposes and does not imply that the battery can be fully charged in 10 hours. The following equation can be used to estimate the required size:

$$ I_{bc} = 0.1 \times C_{10} + I_s $$

Where:

• \( I_{bc}\) : Maximum charge current of the battery charger in Amps.
• \( C_{10}\) : 10-hour rate capacity of the battery.
• \( I_s\) : The current feeding field consumers by the battery charger while charging the battery bank (if required by project specifications; otherwise, it can be neglected).

Note that the battery bank selection is based on the \( C_{50}\) supply capacity. Consider that \( C_{10}\) does NOT always equal \( 5 \times C_{50} \). \( C_{10}\) should be obtained from the manufacturer’s documents. If unavailable, use the estimation \( C_{10} = 5 \times C_{50} \).

Project Requirements

I addition to above, design must adhere with project requirements which may differ project to project.

• “Battery chargers shall be capable of supplying the maximum station load as well as charging the attached batteries to 85% of capacity within 16 hours after a full battery discharge.”
• “Battery chargers shall supply an output voltage not exceeding 25.8 VDC in a 24 VDC system. This output shall have less than 2% ripples.”
• “Battery voltage sensing for all battery chargers shall be directly at the battery banks and not at a remote termination.”

Charger Selection

Battery chargers should be selected based on the charging method appropriate for the battery type. For lead-acid cells, constant voltage charging is used, while constant current charging is preferred for NiCd cells.

Definitions

• Equalizing charge (AS 2676.2): An extended charge to ensure complete charging of all the cells in a battery.
• Float charge (AS 2676.2): The permanent connection of a battery to a voltage-regulated DC system, ensuring the battery remains fully charged and ready to supply power to the system if normal charging fails.
• Boost Charge (AS 4044): A charge applied at a higher potential than a float or equalizing charge to restore the capacity of a discharged battery more quickly than a float charge.
• Float voltage: The minimum constant potential needed to offset the internal losses of a battery.

Charger Classification

• Type 1: Suitable for charging a battery with a load connected in parallel.
• Type 2: Suitable for providing supply to a load with or without a battery connected in parallel.
• Type 3: Suitable for providing supply to a load while sharing the load with another battery charger. Type 3 chargers can operate with or without a battery connected in parallel.
• Type 4: Suitable for supplying loads sensitive to electrical noise, such as communication equipment. Type 4 chargers can operate in parallel with another type 4 charger, with or without a battery connected in parallel. The user should specify the maximum level of electrical noise and the method of measurement.

Protection (AS4044)

A battery charger must include protection against damage in the event that one or more phases of the AC input are lost. If this protection shuts down the charger, it must restart automatically when the supply is restored.

A battery charger shall shut down if the output voltage exceeds the pre-set value.

Reduction in Battery’s Capacity or Battery Life

Low temperatures reduce capacity, and high temperatures shorten battery life. Additionally, improper usage patterns like high discharge rates and maintaining higher final cell voltages contribute to reduced actual capacity and efficiency. Proper thermal management and adherence to manufacturer specifications are key to optimizing battery performance and lifespan.

Low Temperatures and Battery Capacity

Batteries function less efficiently at low temperatures due to reduced chemical reaction rates inside the cells. The electrolytes inside the battery, which are responsible for conducting ions between the anode and cathode, become more viscous at lower temperatures, reducing ion mobility. This slows down the electrochemical processes, causing a drop in the available capacity compared to its rated capacity at standard conditions.

While low temperatures reduce battery capacity by slowing down electrochemical processes, they do not generally shorten the battery’s life unless prolonged or combined with improper charging/discharging cycles under such conditions.

Discharge Rate and Actual Capacity

Battery capacity is typically rated for a specific discharge rate, often given in hours (e.g., C-rate). The C-rate is the rate at which a battery is discharged relative to its capacity. A higher discharge rate (C-rate) causes a reduction in the actual capacity available due to internal resistive losses. This phenomenon occurs because faster discharge rates generate more heat within the battery and increase internal resistance, leading to lower energy efficiency and higher energy loss.

At high discharge rates, the chemical reactions within the battery do not have enough time to fully convert all active material into electrical energy, resulting in lower usable capacity.

Final Cell Voltage and Capacity

Manufacturers typically determine a battery’s rated capacity based on a specific cut-off voltage, which is the voltage at which the battery is considered fully discharged. If a higher final cell voltage is maintained (i.e., the battery is stopped from discharging earlier than specified by the manufacturer), the usable capacity decreases. This is because less of the battery’s stored energy is being used.

The capacity of the battery is proportional to the depth of discharge (DoD); limiting the depth of discharge (by maintaining a higher final voltage) reduces the amount of energy drawn from the battery.

High Temperatures and Battery Life

Operating a battery in temperatures above the manufacturer’s recommended ambient range can significantly reduce battery life. Higher temperatures accelerate chemical reactions within the battery, which can lead to:

• Increased degradation of electrodes: Elevated temperatures can cause the electrodes to degrade more rapidly, leading to reduced battery efficiency and capacity.
• Electrolyte decomposition: Higher temperatures can cause the electrolyte to break down or evaporate, reducing the battery’s ability to facilitate ion flow.
• Thermal Runaway: In extreme cases, overheating can lead to thermal runaway, a self-perpetuating cycle where the battery heats up uncontrollably, causing further degradation or even catastrophic failure.

Contribution of Battery Set in Short-Circuit Fault

The contribution of each cell to the short-circuit current is calculated from the following equation:

$$ I_{sc} = \frac{V_{oc}}{R_i} $$

where:

• \(V_{oc} \) : open circuit voltage
• \(R_i \) : cell internal resistance at full charge

If information regarding the short-circuit protection of a battery is not available from the manufacturer, the prospective fault level at the battery terminals should be considered to be 35 times the nominal battery capacity at the 3-hour rate (AS 3011.2).

Any cable, busway, or busbar forming the connection between a battery terminal and a DC switchboard should be rated to withstand the prospective short-circuit current which the battery is able to deliver for a period of at least 1 second (AS 2676.2).

Hydrogen Release of Sealed Cells

Under normal float charge conditions, sealed cells do not emit significant hydrogen due to the internal recombination of gases. However, in cases of overcharging, overvoltage, or overcurrent, the electrolysis of the electrolyte increases, releasing hydrogen gas. To mitigate this risk, charging systems must incorporate suitable protections such as voltage regulation, current limiting, temperature compensation, and proper ventilation to prevent hazardous hydrogen buildup.

Hydrogen Release in Sealed Cells

Sealed cells, commonly found in valve-regulated lead-acid (VRLA) batteries, nickel-metal hydride (NiMH), and lithium-ion (Li-ion) batteries, are designed to minimize gas emission during normal operation. However, under abnormal charging conditions, hydrogen gas can be released, posing safety risks such as fire or explosion.

In a float charge configuration, the battery is kept at a voltage slightly above its nominal voltage to compensate for self-discharge. This is typically done according to the manufacturer’s guidelines, maintaining the charge at a low level to ensure the battery remains fully charged without causing excessive overcharging.

During normal operation, sealed cells undergo controlled electrochemical reactions. For lead-acid batteries, the oxygen recombination cycle ensures that oxygen generated at the positive plate is absorbed by the negative plate, preventing hydrogen from being released. This is achieved through the internal pressure regulation mechanism.

In this controlled environment, no significant amounts of hydrogen are emitted, as the gases produced are recombined inside the cell. Manufacturers specify the float voltage to ensure that hydrogen evolution is kept within safe limits.

Hydrogen Evolution under Abnormal Charging Conditions

Overcharging: Overcharging occurs when the cell is subjected to a voltage higher than the manufacturer’s recommended float or charge voltage. This excess voltage increases the rate of electrolysis of the water in the electrolyte, causing more hydrogen and oxygen gases to be produced.

Overcurrent: Excessive current during charging also contributes to overcharging, causing increased internal pressure within the cell and more hydrogen production due to the faster breakdown of water in the electrolyte.

In sealed battery systems, pressure relief valves are designed to release gas when internal pressure exceeds safe levels. If abnormal charging conditions persist, hydrogen will escape, creating safety hazards.

Hydrogen is highly flammable and can form explosive mixtures with air at concentrations as low as 4% by volume. If hydrogen is allowed to accumulate in an enclosed space, it poses a significant risk of explosion or fire.

The average hydrogen concentration by volume in a battery room or enclosure shall be maintained below 2%. If mechanical ventilation is installed, an airflow sensor shall be incorporated to initiate an alarm should the ventilation fan be inoperative (AS 3011.2).

Preventive Measures for Hydrogen Release

• Overcurrent and Overvoltage Protection: Chargers should be equipped with control mechanisms to prevent excessive current and voltage. This includes:
• Voltage regulation: Ensuring that the charge voltage remains within the manufacturer’s specifications for the particular battery type.
• Current limiting: Restricting the charging current to avoid excessive gassing.
• Float charge control: A charger should be designed to shift to float mode after the battery reaches full charge to prevent overcharging.
• Temperature Compensation: As temperature influences the charging voltage, many charging systems include temperature compensation to automatically adjust the charging voltage. This ensures that the battery is charged safely in varying environmental conditions, preventing overcharging and the associated hydrogen release.
• Ventilation: For installations where large battery banks are used, adequate ventilation must be provided to disperse any hydrogen gas that might escape in the event of abnormal charging, avoiding the accumulation of explosive gas mixtures.

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