Batteries, Generators, and DC Power

AC power supply XL1 = 10Ω XL2 = 15Ω XLT = 25Ω The total reactance of inductors connected in parallel is found the same way as the total resistance in a parallel circuit. [Figure 9-20] Thus, the total reactance of inductances connected in parallel, as shown, is expressed as: 1=XLT 1 + 1 + 1 XL1 XL2 XL3 1=XLT 1 + 1 + 1 15 15 15 XLT = 5Ω AC power supply XL1 = 15 Ω XL2 = 15 Ω XL3 = 15 Ω Capacitive Reactance Capacitance is the ability of a body to hold an electric charge. In general, a capacitor is constructed of two parallel plates separated by an insulator. The insulator is commonly called the dielectric. The capacitor’s plates have the ability to store electrons when charged by a voltage source. The capacitor discharges when the applied voltage is no longer present and the capacitor is connected to a current path. In an electrical circuit, a capacitor serves as a reservoir or storehouse for electricity.

The basic unit of capacitance is the farad and is given by the letter F. By definition, one farad is one coulomb of charge stored with one volt across the plates of the capacitor. In practical terms, one farad is a large amount of capacitance. Typically, in electronics, much smaller units are used. The two more common smaller units are the microfarad (μF), which is 10-6 farad and the picofarad (pF), which is 10-12 farad. Capacitance is a function of the physical properties of the capacitor: 1. The capacitance of parallel plates is directly proportional to their area. A larger plate area produces a larger capacitance, and a smaller area produces less capacitance. If we double the area of the plates, there is room for twice as much charge.

2. The capacitance of parallel plates is inversely proportional to the distance between the plates. 3. The dielectric material effects the capacitance of parallel plates. The dielectric constant of a vacuum is defined as 1, and that of air is very close to 1. These values are used as a reference, and all other materials have values relative to that of air (vacuum). When an AC is applied in the circuit, the charge on the plates constantly changes. [Figure 9-21] This means that electricity must flow first from Y clockwise around to X, then from X counterclockwise around to Y , then from Y clockwise around to X, and so on. Although no current flows through the insulator between the plates of the capacitor, it constantly flows in the remainder of the circuit between X and Y . As this current alternates to and from the capacitor, a certain time lag is created. When a capacitor charges or discharges through a resistance, a certain amount of time is required for a full Dielectric X Y 80 µF Capacitor AC generator 110V@400cps 9-13 charge or discharge. The voltage across the capacitor does not change instantaneously. The rate of charging or discharging is determined by the time constant of the circuit. This rate of charge and discharge creates an opposition to current flow in AC circuits known as capacitive reactance. Capacitive reactance is symbolized by X C and is measured in ohms.

This characteristic of a capacitor may also create a phase shift between voltage and current of the circuit. The phase shift created by capacitive reactance always causes current to lead voltage. That is, the current of a capacitive circuit reaches its peak values before the voltage reaches peak values. Capacitive reactance is a measure of how much the capacitive circuit opposes the applied current flow. Capacitive reactance is measured in ohms. The capacitive reactance of a circuit is indirectly proportional to the capacitance of the circuit and the applied frequency to the circuit. By increasing either the capacitance or applied frequency, the capacitive reactance decreases, and vice versa. This relationship is given as: 1XC = 2πfC Where: XC = capacitive reactance in ohms, C = capacitance in farads, f = frequency in cycles per second, and π = 3.1416.

In Figure 9-21, a series circuit is shown in which the applied voltage is 110 volts at 400 cps, and the capacitance of a condenser is 80 mf. Find the capacitive reactance and the current flow. To find the capacitive reactance, the following equation: 1XC = 2πfC First, the capacitance, 80 μf, is changed to farads by dividing 80 by 1,000,000, since 1 million microfarads is equal to 1 farad. This quotient equals 0.000080 farad. This is substituted in the equation: 1XC = 2πfC 1XC = 2π(400)(0.000080) XC = 4.97Ω Impedance The total opposition to current flow in an AC circuit is known as impedance and is represented by the letter Z. The combined effects of resistance, inductive reactance, and capacitive 110V AC R = 11Ω I = 10A consists of resistance only. Impedance (Z) = Resistance (R).

reactance make up impedance (the total opposition to current flow in an AC circuit). In order to accurately calculate voltage and current in AC circuits, the effect of inductance and capacitance along with resistance must be considered. Impedance is measured in ohms. The rules and equations for DC circuits apply to AC circuits only when that circuit contains resistance alone and no inductance or capacitance. In both series and parallel circuits, if an AC circuit consists of resistance only, the value of the impedance is the same as the resistance, and Ohm’s Law for an AC circuit, I = E/Z, is exactly the same as for a DC circuit. Figure 9-22 illustrates a series circuit containing a heater element with 11 ohms resistance connected across a 110-volt source. To find how much current flows if 110 volts AC is applied, the following example is solved: EI = Z I = 110V 11Ω I = 10 amps If there are two resistance values in parallel connected to an 50V AC Power supply R = 20Ω Ammeter I = 5A A R = 20Ω AC voltage. Impedance is equal to the total resistance of the circuit.

9-14 AC voltage, as seen in Figure 9-23, impedance is equal to the total resistance of the circuit. Once again, the calculations would be handled the same as if it were a DC circuit and the following would apply: 1RT = 1 + 1 R1 R2 1RT = 1 + 1 20 20 RT = 10Ω Since this is a pure resistive circuit R T = Z (Resistance = Impedance) ZT = RT ZT = 10Ω To determine the current flow in the circuit use the equation: EI = Z I = 50V 10Ω I = 5 amps Impedance is the total opposition to current flow in an AC circuit. If a circuit has inductance or capacitance, one must take into consideration resistance (R), inductive reactance (X L), and/or capacitive reactance (XC) to determine impedance (Z).

In this case, Z does not equal R T. Resistance and reactance (inductive or capacitive) cannot be added directly, but they can be considered as two forces acting at right angles to each other. Thus, the relation between resistance, reactance, and Reactance Resistance Z R Impedance XL − XC impedance may be illustrated by a right triangle. [Figure 9-24] Since these quantities may be related to the sides of a right triangle, the formula for finding the impedance can be found using the Pythagorean Theorem. It states that the square of the hypotenuse is equal to the sum of the squares of the other two sides. Thus, the value of any side of a right triangle can be found if the other two sides are known.

In practical terms, if a series AC circuit contains resistance and inductance, as shown in Figure 9-25, the relation between the sides can be stated as: Z2 = R2 + (XL – XC)2 The square root of both sides of the equation gives: Z = √ R2 + (XL – XC)2 This formula can be used to determine the impedance when the values of inductive reactance and resistance are known. It can be modified to solve for impedance in circuits containing capacitive reactance and resistance by substituting XC in the formula in place of XL. In circuits containing resistance with both inductive and capacitive reactance, the reactances can be combined; but because their effects in the circuit are exactly opposite, they are combined by subtraction (the smaller number is always subtracted from the larger): Z = XL – XC or X = XC – XL containing a resistor and an inductor are connected to a source of 110 volts at 60 cycles per second. The resistive element is a simple measuring 6 ohms, and the inductive element is a coil with an inductance of 0.021 henry. What is the value of the impedance and the current through the circuit?

Solution: First, the inductive reactance of the coil is computed: 110V AC 60 cycles R = 6Ω A XL = 0.021 H 9-15 XL = 2π × f × L XL = 6.28 × 60 × 0.021 XL = 8 ohms inductive reactance Next, the total impedance is computed: Z = √ R2 + XL2 Z = √ 62 + 82 Z = √ 36 + 64 Z = √ 100 Z = 10Ω Remember when making calculations for Z always use inductive reactance not inductance, and use capacitive reactance, not capacitance. Once impedance is found, the total current can be calculated. EI = Z I = 110V 10Ω I = 11 amps Since this circuit is resistive and inductive, there is a phase shift where voltage leads current.

Example 2 is a series circuit illustrated in which a capacitor of 200 μf is connected in series with a 10 ohm resistor. [Figure 9-26] What is the value of the impedance, the current flow, and the voltage drop across the resistor? R = 10Ω A C = 200 µF110V AC 60 cycles Solution: First, the capacitance is changed from microfarads to farads. Since 1 million microfarads equal 1 farad, then 200 μf = 0.000200 farads. Next solve for capacitive reactance: 1XC = 2πfC 1XC = 2π(60)(.00020) 1XC = 0.07536 XC = 13Ω To find the impedance, Z = √ R2 + XC2 Z = √ 102 + 132 Z = 16.4Ω Since this circuit is resistive and capacitive, there is a phase shift where current leads voltage: To find the current: EIT = Z 110V IT = 6.4Ω IT = 6.7 amps To find the voltage drop across the resistor (ER): ER = I × R ER = 6.7A × 10Ω ER = 67 volts To find the voltage drop over the capacitor (EC): EC = I × XC EC = 6.7A × 13Ω 9-16 EC = 86.1 volts The sum of these two voltages does not equal the applied voltage, since the current leads the voltage. Use the following formula to find the applied voltage: E = √ (ER)2 + (EC)2 E = √ 672 + 86.12 E = √ 4,489 + 7,413 E = √ 11,902 E = 110 volts When the circuit contains resistance, inductance, and capacitance, the following equation is used to find the impedance.

Z = √ R2 + (XL – XC)2 Example 3: What is the impedance of a series circuit consisting of a capacitor with a capacitive reactance of 7 ohms, an inductor with an inductive reactance of 10 ohms, and a resistor with a resistance of 4 ohms? [Figure 9-27] Solution: Z = √ R2 + (XL – XC)2 Z = √ 42 + (10 – 7)2 Z = √ 25 Z = 5Ω To find total current: E TIT = Z 110V IT = 5Ω IT = 22 amps Remember that inductive and capacitive reactances can cause a phase shift between voltage and current. In this example, inductive reactance is larger than capacitive reactance, so the voltage leads current. It should be noted that since inductive reactance, capacitive reactance, and resistance affect each other at right angles, the voltage drops of any series AC circuit should be added using vector addition. Figure 9-28 shows the voltage drops over the series AC circuit described in example 3 above.

To calculate the individual voltage drops, simply use the equations: ER = I × R EXL = I × XL EXC = I × XC To determine the total applied voltage for the circuit, each individual voltage drop must be added using vector addition. ET = √ ER2 + (EL – EC)2 4Ω 10 Ω 7 Ω110V AC 60 cycles capacitance. 110V AC 60 cycles ER = 88V EL = 220V EC = 154V 9-17 ET = √ 882+ (220 – 154)2 ET = √ 882+ 662 ET = √12,100 ET = 110 volts Parallel AC Circuits When solving parallel AC circuits, one must also use a derivative of the Pythagorean Theorem. The equation for finding impedance in an AC circuit is as follows: 2 2Z = 1 1 1+ – R X L XC To determine the total impedance of the parallel circuit shown in Figure 9-29, one would first determine the capacitive and inductive reactances. (Remember to convert microfarads to farads.) XL = 2πFL XL = 2π(400)(0.02) XL = 50Ω 1XC = 2πFC 100µf = 0.0001F 1XC = 2π(400)(0.0001) XC = 4Ω Next, the impedance can be found: 1Z = 2 21 1 1+ – R X L XC 1Z = 50 2 21 1 1+ – 50 4 1Z = √ ( .02 )2 + ( .02 – .25 )2 1Z = √.0004 + .0529 110V AC 400 Hz C = 100 µF R = 50Ω L = 0.02H Z = 1 .23 Z = 4.33Ω To determine the current flow in the circuit: ETIT = Z 100VIT = 4.33Ω IT = 23.09 amps To determine the current flow through each parallel path of the circuit, calculate IR, IL, and IC.

EIR = R 100VIR = 50Ω IR = 2 amps EIL = XL 100VIL = 50Ω IL = 2 amps EIC = XC 100VIC = 4Ω 9-18 IC = 25 amps It should be noted that the total current flow of parallel circuits is found by using vector addition of the individual current flows as follows: IT = √ IR2 + (IL – IC)2 IT = √ 22+ (2 – 25)2 IT = √ 22 + 232 IT = √ 4 + 529 IT = √ 533 IT = 23 amps Power in AC Circuits Since voltage and current determine power, there are similarities in the power consumed by both AC and DC circuits. In AC however, current is a function of both the resistance and the reactance of the circuit. The power consumed by any AC circuit is a function of the applied voltage and both circuit’s resistance and reactance. AC circuits have two distinct types of power, one created by the resistance of the circuit and one created by the reactance of the circuit.

True Power True power of any AC circuit is commonly referred to as the working power of the circuit. True power is the power consumed by the resistance portion of the circuit and is measured in watts (W). True power is symbolized by the letter P and is indicated by any wattmeter in the circuit. True power is calculated by the formula: P = I2 × Z Apparent Power Apparent power in an AC circuit is sometimes referred to as the reactive power of a circuit. Apparent power is the power consumed by the entire circuit, including both the resistance and the reactance. Apparent power is symbolized by the letter S and is measured in volt-amps (V A). Apparent power is a product of the effective voltage multiplied by the effective current. Apparent power is calculated by the formula: S = I2 × Z Power Factor As seen in Figure 9-30, the resistive power and the reactive power effect the circuit at right angles to each other. The power factor in an AC circuit is created by this right angle effect.

Power factor can be defined as the mathematical difference between true power and apparent power. Power factor (PF) is a ratio and always a measurement between 0 and 100. The power factor is directly related to the phase shift of a circuit. The greater the phase shift of a circuit the lower the power factor. For example, an AC circuit that is purely inductive (contains reactance only and no resistance) has a phase shift of 90° and a power factor of 0.0. An AC circuit that is purely resistive (has no reactance) has a phase shift of 0 and a power factor of 100. Power factor is calculated by using the following formula: True Power (Watts) PF = × 100Apparent Power (V A) Example of calculating PF: Figure 9-31 shows an AC load connected to a 50 volt power supply. The current draw of the circuit is 5 amps and the total resistance of the circuit is 8 ohms. Determine the true power, the apparent power, and the power factor for this circuit.

Solution: Reactive power Watts True power Apparent power volts x amperes A 50V AC power supply Capacitor Inductor Resistor Ammeter I = 5A 9-19 P = I2 × R P = 52 × 8 P = 200 Watts S = E × I S = 50 × 5 S = 250V A TP PF = × 100S 200PF = × 100250 PF = 80 Power factor can also be represented as a percentage. Using a percentage to show power factor, the circuit in the previous example would have a power factor of 80 percent. It should be noted that a low power factor is undesirable. Circuits with a lower power factor create excess load on the power supply and produce inefficiency in the system. Aircraft AC alternators must typically operate with a power factor between 90 percent and 100 percent. It is therefore very important to carefully consider power factor when designing the aircraft electrical system.

Aircraft Batteries Aircraft batteries are used for many functions (e.g., ground power, emergency power, improving DC bus stability, and fault clearing). Most small private aircraft use lead-acid batteries. Most commercial and corporate aircraft use nickel- cadmium (NiCd) batteries. However, other lead acid types of batteries are becoming available, such as the valve-regulated lead-acid (VRLA) batteries. The battery best suited for a particular application depends on the relative importance of several characteristics, such as weight, cost, volume, service or shelf life, discharge rate, maintenance, and charging rate.

Any change of battery type may be considered a major alteration. Types of Batteries Aircraft batteries are usually identified by the material used for the plates. The two most common types of battery used are lead-acid and NiCd batteries. Lead-Acid Batteries Dry Charged Cell Lead-Acid Batteries Dry charged cell lead-acid batteries, also known as flooded or wet batteries, are assembled with electrodes (plates) that have been fully charged and dried. The electrolyte is added to the battery when it is placed in service, and battery life begins when the electrolyte is added. An aircraft storage battery consists of 6 or 12 lead-acid cells connected in series.

The open circuit voltage of the 6 cell battery is approximately 12 volts, and the open circuit voltage of the 12-cell battery is approximately 24 volts. Open circuit voltage is the voltage of the battery when it is not connected to a load. When flooded (vented) batteries are on charge, the oxygen generated at the positive plates escapes from the cell. Concurrently, at the negative plates, hydrogen is generated from water and escapes from the cell. The overall result is the gassing of the cells and water loss. Therefore, flooded cells require periodic water replenishment. [Figure 9-32] Valve-Regulated Lead-Acid (VRLA) Batteries VRLA batteries contain all electrolyte absorbed in glass-mat 9-20 separators with no free electrolyte and are sometimes referred to as sealed batteries. [Figure 9-33] The electrochemical reactions for VRLA batteries are the same as flooded batteries, except for the gas recombination mechanism that is predominant in VRLA batteries. These types of battery are used in general aviation and turbine powered aircraft and are sometimes authorized replacements for NiCd batteries.

When VRLA batteries are on charge, oxygen combines chemically with the lead at the negative plates in the presence of H 2SO4 to form lead sulfate and water. This oxygen recombination suppresses the generation of hydrogen at the negative plates. Overall, there is no water loss during charging. A very small quantity of water may be lost as a result of self-discharge reactions; however, such loss is so small that no provisions are made for water replenishment. The battery cells have a pressure relief safety valve that may vent if the battery is overcharged. NiCd Batteries A NiCd battery consists of a metallic box, usually stainless steel, plastic-coated steel, painted steel, or titanium containing a number of individual cells. [Figure 9-34] These cells are connected in series to obtain 12 volts or 24 volts.

The cells are connected by highly conductive nickel copper links. Inside the battery box, the cells are held in place by partitions, liners, spacers, and a cover assembly. The battery has a ventilation system to allow the escape of the gases produced during an overcharge condition and provide cooling during normal operation. NiCd cells installed in an aircraft battery are typical of the vented cell type. The vented cells have a vent or low pressure release valve that releases any generated oxygen and hydrogen gases when overcharged or discharged rapidly. This also means the battery is not normally damaged by excessive rates of overcharge, discharge, or even negative charge. The cells are rechargeable and deliver a voltage of 1.2 volts during discharge.

Aircraft that are outfitted with NiCd batteries typically have a fault protection system that monitors the condition of the battery. The battery charger is the unit that monitors the condition of the battery and the following conditions are monitored: 1. Overheat condition, 2. Low temperature condition (below –40 °F), 3. Cell imbalance, 4. Open circuit, and 5. Shorted circuit. If the battery charger finds a fault, it turns off and sends a fault signal to the Electrical Load Management System (ELMS). NiCd batteries are capable of performing to their rated capacity when the ambient temperature of the battery is in the range of approximately 60–90 °F. An increase or decrease in temperature from this range results in reduced capacity. NiCd batteries have a ventilation system to control the temperature of the battery. A combination of high battery temperature (in excess of 160 °F) and overcharging can lead to a condition called thermal runaway. [Figure 9-35] The temperature of the battery has to be constantly monitored to ensure safe operation. Thermal runaway can result in a NiCd chemical fire and/or explosion of the NiCd battery under recharge by a constant-voltage source and is due to cyclical, ever-increasing temperature and charging current. One or more shorted cells or an existing high temperature and low charge can produce the following cyclical sequence of events: 1. Excessive current, 2. Increased temperature, 9-21 3. Decreased cell(s) resistance, 4. Further increased current, and 5. Further increased temperature.

This does not become a self-sustaining thermal-chemical action if the constant-voltage charging source is removed before the battery temperature is in excess of 160 °F. Capacity Capacity is measured quantitatively in ampere-hours delivered at a specified discharge rate to a specified cut-off voltage at room temperature. The cut-off voltage is 1.0 volt per cell. Battery available capacity depends upon several factors including such items as: 1. Cell design (cell geometry, plate thickness, hardware, and terminal design govern performance under specific usage conditions of temperature, discharge rate, etc.).

2. Discharge rate (high current rates yield less capacity than low rates). 3. Temperature (capacity and voltage levels decrease as battery temperature moves away from the 60 °F (16 °C) to 90 °F (32 °C) range toward the high and low extremes). 4. Charge rate (higher charge rates generally yield greater capacity). Aircraft Battery Ratings by Specification The one-hour rate is the rate of discharge a battery can endure for 1 hour with the battery voltage at or above 1.67 volts per cell, or 20 volts for a 24-volt lead-acid battery, or 10 volts for a 12-volt lead-acid battery. The one-hour capacity, measured in ampere hours (Ah), is the product of the discharge rate and time (in hours) to the specified end voltage.

The emergency rate is the total essential load, measured in amperes, required to support the essential bus for 30 minutes. This is the rate of discharge a battery can endure for 30 minutes with the battery voltage at or above 1.67 volts per cell, or 20 volts for a 24 volt lead-acid battery, or 10 volts for a 12 volt lead-acid battery. Storing & Servicing Facilities Separate facilities for storing and/or servicing flooded electrolyte lead-acid and NiCd batteries must be maintained. Introduction of acid electrolyte into alkaline electrolyte causes permanent damage to vented (flooded electrolyte) NiCd batteries and vice versa. However, batteries that are sealed can be charged and capacity checked in the same area.

Because the electrolyte in a valve-regulated lead-acid battery is absorbed in the separators and porous plates, it cannot contaminate a NiCd battery even when they are serviced in the same area. Warning: It is extremely dangerous to store or service lead- acid and NiCd batteries in the same area. Introduction of acid electrolytes into alkaline electrolyte destroys the NiCd, and vice versa. Battery Freezing Discharged lead-acid batteries exposed to cold temperatures are subject to plate damage due to freezing of the electrolyte. To prevent freezing damage, maintain each cell’s specific gravity at 1.275 or, for sealed lead-acid batteries, check open circuit voltage. [Figure 9-36] NiCd battery electrolyte is not as susceptible to freezing because no appreciable chemical change takes place between the charged and discharged states. However, the electrolyte freezes at approximately –75 °F.

Note: Only a load check determines overall battery condition. Temperature Correction U.S.-manufactured lead-acid batteries are considered fully charged when the specific gravity reading is between 1.275 and 1.300. A 1⁄3 discharged battery reads about 1.240 and a 2⁄3 discharged battery shows a specific gravity reading of about 1.200 when tested by a hydrometer at an electrolyte temperature of 80 °F. However, to determine precise specific gravity readings, a temperature correction should be applied to the hydrometer indication. [Figure 9-37] As an example, for a hydrometer reading of 1.260 and electrolyte temperature of 40 °F, the corrected specific gravity reading of the electrolyte is 1.244.

Battery Charging Operation of aircraft batteries beyond their ambient temperature or charging voltage limits can result in excessive cell temperatures leading to electrolyte boiling, rapid Freezing PointSpecific Gravity 1.300 1.275 1.250 1.225 1.200 1.175 1.150 1.125 1.100 °C –70 –62 –52 –37 –26 –20 –15 –10 –08 °F –95 –80 –62 –35 –16 –04 +05 +13 +19 State of Charge (SOC) for Sealed Lead-Acid Batteries at 70° SOC 100% 75% 50% 25% 12 volt 12.9 12.7 12.4 12.0 24 volt 25.8 25.4 24.8 24.0 9-22 Electrolyte Temperature °C +60 +55 +49 +43 +38 +33 +27 +23 +15 +10 +05 –02 –07 –13 –18 –23 –28 –35 °F +140 +130 +120 +110 +100 +90 +80 +70 +60 +50 +40 +30 +20 +10 0 –10 –20 –30 Points to Subtract From or Add to Specific Gravity Readings 12 volt +0.024 +0.020 +0.016 +0.012 +0.008 +0.004 0 –0.004 –0.008 –0.012 –0.016 –0.020 –0.024 –0.028 –0.032 –0.036 –0.040 –0.044 deterioration of the cells, and battery failure. The relationship between maximum charging voltage and the number of cells in the battery is also significant. This determines (for a given ambient temperature and state of charge) the rate at which energy is absorbed as heat within the battery. For lead-acid batteries, the voltage per cell must not exceed 2.35 volts.

In the case of NiCd batteries, the charging voltage limit varies with design and construction. Values of 1.4 and 1.5 volts per cell are generally used. In all cases, follow the recommendations of the battery manufacturer. Constant Voltage (CV) Charging The battery charging system in an airplane is of the constant voltage type. An engine-driven generator, capable of supplying the required voltage, is connected through the aircraft electrical system directly to the battery. A battery switch is incorporated in the system so that the battery may be disconnected when the airplane is not in operation.

The voltage of the generator is accurately controlled by means of a voltage regulator connected in the field circuit of the generator. For a 12-volt system, the voltage of the generator is adjusted to approximately 14.25. On 24-volt systems, the adjustment should be between 28 and 28.5 volts. When these conditions exist, the initial charging current through the battery is high. As the state of charge increases, the battery voltage also increases, causing the current to taper down. When the battery is fully charged, its voltage is almost equal to the generator voltage, and very little current flows into the battery. When the charging current is low, the battery may remain connected to the generator without damage.

When using a constant-voltage system in a battery shop, a voltage regulator that automatically maintains a constant voltage is incorporated in the system. A higher capacity battery (e.g., 42 Ah) has a lower resistance than a lower capacity battery (e.g., 33 Ah). Hence, a high-capacity battery draws a higher charging current than a low-capacity battery when both are in the same state of charge and when the charging voltages are equal. The constant voltage method is the preferred charging method for lead-acid batteries. Constant Current (CC) Charging Constant current charging is the most convenient for charging batteries outside the airplane because several batteries of varying voltages may be charged at once on the same system.

A constant current charging system usually consists of a rectifier to change the normal AC supply to DC. A transformer is used to reduce the available 110-volt or 220-volt AC supply to the desired level before it is passed through the rectifier. If a constant current charging system is used, multiple batteries may be connected in series, provided that the charging current is kept at such a level that the battery does not overheat or gas excessively. The constant current charging method is the preferred method for charging NiCd batteries. Typically, a NiCd battery is constant current charged at a rate of 1CA until all the cells have reached at least 1.55V . Another charge cycle follows at 0.1CA, again until all cells have reached 1.55V . The charge is finished with an overcharge or top-up charge, typically for not less than 4 hours at a rate of 0.1CA. The purpose of the overcharge is to expel as much, if not all the gases collected on the electrodes, hydrogen on the anode, and oxygen on the cathode; some of these gases recombine to form water that, in turn, raises the electrolyte level to its highest level after which it is safe to adjust the electrolyte levels. During the overcharge or top-up charge, the cell voltages go beyond 1.6V and then slowly start to drop. No cell should rise above 1.71V (dry cell) or drop below 1.55V (gas barrier broken).

Charging is done with vent caps loosened or open. A stuck vent might increase the pressure in the cell. It also allows for refilling of water to correct levels before the end of the top-up charge while the charge current is still on. However, cells should be closed again as soon as the vents have been cleaned and checked since carbon dioxide dissolved from outside air carbonates the cells and ages the battery. Battery Maintenance Battery inspection and maintenance procedures vary with the type of chemical technology and the type of physical construction. Always follow the battery manufacturer’s 9-23 approved procedures. Battery performance at any time in a given application depends upon the battery’s age, state of health, state of charge, and mechanical integrity, which you can determine according to the following: • To determine the life and age of the battery, record the install date of the battery on the battery. During normal battery maintenance, battery age must be documented either in the aircraft maintenance log or in the shop maintenance log.

• Lead-acid battery state of health may be determined by duration of service interval (in the case of vented batteries), by environmental factors (such as excessive heat or cold), and by observed electrolyte leakage (as evidenced by corrosion of wiring and connectors or accumulation of powdered salts). If the battery needs to be refilled often, with no evidence of external leakage, this may indicate a poor state of the battery, the battery charging system, or an overcharge condition. • Use a hydrometer to determine the specific gravity of the lead-acid battery electrolyte, which is the weight of the electrolyte compared to the weight of pure water.

Take care to ensure the electrolyte is returned to the cell from which it was extracted. When a specific gravity difference of 0.050 or more exists between cells of a battery, the battery is approaching the end of its useful life and replacement should be considered. Electrolyte level may be adjusted by the addition of distilled water. Do not add electrolyte. • Battery state of charge is determined by the cumulative effect of charging and discharging the battery. In a normal electrical charging system, the aircraft generator or alternator restores a battery to full charge during a flight of 1 hour to 90 minutes.

• Proper mechanical integrity involves the absence of any physical damage, as well as assurance that hardware is correctly installed and the battery is properly connected. Battery and battery compartment venting system tubes, nipples, and attachments, when required, provide a means of avoiding the potential buildup of explosive gases, and should be checked periodically to ensure that they are securely connected and oriented in accordance with the maintenance manual’s installation procedures. Always follow procedures approved for the specific aircraft and battery system to ensure that the battery system is capable of delivering specified performance.

Battery & Charger Characteristics The following information is provided to acquaint the user with characteristics of the more common aircraft battery and battery charger types. [Figure 9-38] Products may vary from these descriptions due to different applications of available technology. Consult the manufacturer for specific performance data. Note: Never connect a lead-acid battery to a charger, unless properly serviced. Lead-Acid Batteries Lead-acid vented batteries have a two volt nominal cell voltage. Batteries are constructed so that individual cells cannot be removed. Occasional addition of water is required to replace water loss due to overcharging in normal service.

Batteries that become fully discharged may not accept recharge. Lead-acid sealed batteries are similar in most respects to lead-acid vented batteries, but do not require the addition of water. The lead-acid battery is economical and has extensive application but is heavier than an equivalent performance battery of another type. The battery is capable of a high rate of discharge and low-temperature performance. However, maintaining a high rate of discharge for a period of time usually warps the cell plates, shorting out the battery. Its electrolyte has a moderate specific gravity, and state of charge can be checked with a hydrometer.

Lead-acid batteries are usually charged by regulated DC voltage sources. This allows maximum accumulation of charge in the early part of recharging. NiCd Batteries NiCd vented batteries have a 1.2-volt nominal cell voltage. Occasional addition of distilled water is required to replace water loss due to overcharging in normal service. Cause of failure is usually shorting or weakening of a cell. After replacing the bad cell with a good cell, the battery’s life can be extended for 5 or more years. Full discharge is not harmful to this type of battery. 9-24