Aircraft batteries are critical components on the aircraft and are used to start engines and auxiliary power units (APUs). They also provide emergency backup power for essential avionics equipment and lighting systems, to assure no-break power for navigation units and also power the fly-by-wire computers. Batteries also provide ground power capability for maintenance and preflight checkouts. All of these functions are critical for safe operation of the aircraft, so the state of health of an aircraft battery is of utmost importance. Aircraft batteries are certified to have a certain minimum capacity for emergency operations in an electrical generator system failure. If the battery is used to satisfy essential or emergency power requirements, its capacity must be tested periodically to assure airworthiness.
Aircraft Batteries – Types and Selection:
From the 20th century, three battery types are used by OEMs as the industry standard for aircraft batteries. They are:
- Lead-Acid Battery: Lead-acid batteries are cheaper compared to NiCd and Li-ion batteries and hence were used as an early option for airline operators. However, due to lesser current capacities, efficiency, and life span, these were soon replaced by NiCd and Li-ion batteries.
- NiCd Battery: Nickel-cadmium batteries are widely used for their good low-temperature performance and long service life. Before the NiCd battery, Lithium based batteries were used, however, due to operational advantages, NiCd batteries have replaced large chuck of Lithium based batteries.
- Li-ion Battery: In the 21st century, Li-ion batteries have made massive strides in areas such as energy density, current delivering capability, and the associated protection and power electronics and hence, OEMs are switching to Li-ion based batteries.
Aircraft Battery Charging – Types and Operation:
Two common battery charging methods are followed across the industry as standard:
- Constant Current Charge Method (CI): In this technique, the current remains at a Pre-set level, while the voltage reaches a higher level. The advantage of this technique is that the ampere-hour input to the battery can be determined accurately by multiplying the charging current with the time taken for charging. However, overcharging the battery using CI Technique is not advisable because of overheating, excessive gassing and potential battery damage.
- Constant Potential Charge Method (CP): In this method, the voltage supplied to the battery remains constant, and the current supplied fluctuates with the battery voltage. Compared to the CI method, CP charging is preferred due to following reasons
- Less danger of gassing at excessive rate.
- Batteries of the same nominal voltage (with different capacities) can be connected in parallel to the charging source.
- The rate of charge is higher with the CP method.
- Pulse Charge Method: Often used for quick charging of aircraft batteries, especially when the battery voltage is below approximately 10 Volts, Battery chargers pulse-charge a series of high current pulses of approximately 10 amperes, and one millisecond in duration, at a repetition of about 2 pulses per second. With this pulse charge, the charger is protected, but the battery charges up efficiently with minimum heaing, and helps in conditioning (breaking chemical barriers) of the battery.
Procedure for Charging using Battery Charger:
Following is the standard procedure for charging batteries, as recommended by Charger unit OEMs:
Step 1: Preparation for charging – Ensure that the charge time and current or voltage from the battery OEM are read and followed
Step 2: Verify the End voltage – Turn the unit power off and disconnect the battery from the unit. Next, turn the current to a maximum and turn the unit power on. The end voltages are displayed on the units’ digital voltmeter.
Step 3: Charge time – Set the charge time based on the battery condition and the rate of battery charging
Step 4: Select and confirm the Mode of charging – Adjust between Constant Current (CI) and Constant Potential (CP) mode
Applicable Standards for Battery Charging:
Battery charging is dependent on the type of battery used and the respective OEM recommendations based on Discharge times. The following are standard procedures followed while charging different battery types:
- Nickel-Cadmium Batteries: These batteries can be initially charged with CP mode and then shifted to low CI Mode. Though they can be fully charged using CI mode, this is majorly used to eliminate fading, earlier known as the “memory effect” from the NiCd batteries.
- Lead-Acid Batteries: These batteries shall be charged using CP Mode at a steady voltage of 28.6+/-0.2 Volts until the charge current drops below one (1) Amp. If Lead-Acid batteries are charged using CI Mode, the voltage can increase to 34-37 volts, damaging the batteries.
Battery Charger Calibration and Maintenance – Why is it essential:
Every electrical equipment needs to be calibrated to ensure that the performance is restored to design standards. For Battery chargers, calibration is done every 12 months from the date of first use. This is further reduced based on the frequency of operations to ensure functions are error-free. The following components should be periodically calibrated:
- Voltmeter – Provides the voltage reading on the battery, used for testing discharge times and critical while charging the battery under CI mode.
- Ammeter – Provides the rate of flow of electrons to the battery, used for testing discharge current and considered a critical parameter while charging the battery in CP mode.
- Timer – Timer is a very accurate crystal-controlled device on a battery charger, and the last equipment is prone to error. Though timers on battery chargers cannot be re-calibrated, the accuracy of timers should be validated during predictive maintenance activity.
- Discharge current operating range: Every battery charger is preset to discharge a certain Amperage of current to the battery. The range can be verified and adjusted to meet operational requirements.
- Shunt verification: Although it is not mandatory to calibrate the shunt while performing PM activity on the battery charger, the shunt can be verified using an external calibrated shunt and a millivoltmeter.
- Maintenance activities: Standard electrical maintenance and cleaning procedure shall be followed while operating the battery charger. Battery cable and leads can be inspected periodically and if found damaged or worn, should be immediately replaced.
- Maintenance of vents and cooling fan: Inspect and clean the fan vents to avoid blockages, and ensure designed cooling to the equipment.
Though usage of Battery Charger for aircraft operations seems straightforward, the equipment needs to be calibrated and maintained following OEM SOPs. To comply with international standards, the repeatability and accuracy of battery chargers should be supported by following an annual calibration process.
Calibration of aircraft battery chargers also determines the calibration uncertainty value and the deviation from the actual reading. These readings are used as correction factors or to decide if the charger is fit for use.
e2b calibration offers industry-leading ISO-certified Battery charger calibration services. Our labs are ISO/IEC 17025 accredited and operated by a team of qualified calibration experts to test and calibrate your chargers. Our verifiable services are unmatched in the industry. We are registered with ANAB. We are also ANSI/NCSL Z540-1-1994 certified. We have the NIST Traceable Wide scope of ISO/IEC 17025 accreditation. Contact e2b calibration for all your equipment calibration needs.
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