There are a number of approaches to recharging lead-acid batteries. Many will return the battery to service, but fail to fully rejuvenate the battery. To keep a battery fully charged, and attain maximum battery life, proper charging techniques must be observed.
The status of a cell is determined by the specific gravity of the electrolyte solution. A specific gravity of 1.280 (obtained by hydrometer reading) indicates a fully charged cell. A reading of 1.250 or better is considered good. A fully discharged cell exhibits a specific gravity of 1.150 or less.
BATTERY CHARGER DESIGN
The battery charger design shown in Figure 6-48 is based on a charging voltage of 2.4 V per cell, in accordance with most manufacturer’s recommendations. The battery charger circuit pulses the battery under charge with 14.4 V (6 cells x 2.4 V per cell) at a rate of 120 Hz.
The design provides current limiting to protect the charger’s internal components while limiting the charging rate to prevent damaging severely discharged lead-acid batteries. The maximum recommended charging current is normally about one-fourth the ampere-hour rating of the battery. For example, the maximum charging current for an average 44 ampere-hour battery is 11 A.
If the impedance of the load requires a charging current greater than the 11 A current limit, the circuit will go into current limiting. The amplitude of the charging pulses is controlled to maintain a maximum peak charging current of 11 A (8 A average).
The charger circuit is composed of four basic sections:
- Voltage Regulator
- Current Limiting
Series Pass Element
A full-wave rectifier configuration with a center-tapped transformer (Figure 6-49) achieves maximum performance with minimum component count. The breakdown voltage requirement for the diode is:
This design is set to current limit at 11 A so a rectifier rating of 25 A is recommended to handle the maximum current drain plus any current surges. A pair of 1N183 diodes was chosen (35 A/50 V rectifiers).
Voltage Regulator Section
The components which make up the voltage regulator portion of the circuit are: Zl, Q1, Rl, R2 and RB as shown in Figure 6-50.
Z1 is a TL431 programmable shunt regulator which serves as the control element, Q1 is the pass transistor, and R1 – R2 sense the output voltage providing feedback to Zl. R1 and R2 are chosen so that their node voltage is 2.5 V at the desired output voltage. This node voltage is applied to the TL43l’s error amplifier which compares it to the internal 2.5 V reference.
When the feedback voltage is less than the internal 2.5 V reference, the series impedance (anode-to-cathode) of the TL431 increases, decreasing the shunt current through the TL431. This increases the current available to the base of pass transistor 01, increasing the output voltage.
When the feedback voltage is greater than the internal 2.5 V reference, the series impedance of the
TL431 decreases, increasing the shunt current through the TL431. This decreases the current available to the base of 01, decreasing the output voltage.
Because the feedback voltage is sensed at the output, the TL431 will compensate for any changes in the base-emitter drop of Q1 or the voltage dropped across RCL for various currents.
Current Limiter Section
The components which make up the current-limit portion of this circuit are: Z2, Q1, and RCL as shown in Figure 6-51.
The value of the current-limit setting resistor, RCL, is chosen so that 2.5 V will be developed across it at the desired limit current. The voltage across RCL is sensed by a TL431 programmable shunt regulator (Z2). When the output current is less than the current limit. Vref is less than 2.5 V and Z2 is a high impedance which does not affect the operation of Q1.
When the output current reaches maximum. Vref is 2.5 V and the impedance of Z2 decreases, decreasing the current available at the base of Q1 and controlling the maximum output current. Under this condition, shunt regulator Z2 takes control of pass transistor Q1 and maintains a constant current, even into a short circuit.
Series Pass Element
The series pass clement used in this configuration is a conventional Darlington power transistor, whose control is derived from either Z1 or Z2 depending on the state of the battery being charged. See Figure 6-52.
The performance characteristics of Q1 are important in determining the circuit design and in the choice of the transformer to be used. This relationship is shown in the following section on the design of the battery charger.
The values of R1 and R2 set the output voltage level at 2.4 V per cell or 14.4 V for 6 cells. For optimum performance of Z1, 1 rnA should flow through the R1 and R2 combination.
For ease of final adjustment, a 20 kΩ potentiometer may be used for R1.
Current limiting starts when 2.5 V is developed across RCL at the desired current limit. For a 44 A hour battery, the maximum charge rate is 11 A.
After the pass transistor has been selected, its base drive resistor, RB, may be calculated. A TlP642 meets the requirements. From the data sheet:
To calculate RB, assume a worst case or short-circuit condition where:
While RB must be small enough so it does not limit the base current of Q1 at the desired ICHG of 8 A, however, it must be large enough to limit the current during short circuit conditions. This value should be less than the sum of the base drive current required by Q1 and ISHUNT(max) Z2.
A value of RB within this range assures sufficient drive to Q1 for a charging rate of 8 A. yet allows total control of 01 by Z2 during short-circuit conditions. RB was selected to be 200Ω.
Power Dissipation and Heat Sinking
To determine the power dissipation in the 1N1183 rectifier and the TIP642 Darlington, the RMS currents and voltages must be calculated. The voltage and current paths are shown in Figure 6-53.
If the pass transistor and rectifiers are mounted on separate heat sinks, the sinks must be capable of dissipating the heat transferred by each device and maintain a surface temperature which satisfies the temperature requirement for each device. Mounted separately, the respective heat sink requirements are as follows:
Depending on the mass of the heat sink and the type of cabinet, forced air cooling may be required.