Constant voltage at a constant current results in a very large initial current in a ‘flat’ battery and a very low current in a partially charged battery. To overcome this problem, the charger should be made
to vary the charging current in accordance with the existing terminal voltage of the battery.
In the circuit presented here the charging current is adjusted against the
H
igh-power lead-acid battery char- gers usually employ constant voltage charging method. In such chargers the charging is monitored against the battery terminal voltage.Fig. 1: Schematic diagram of lead-acid battery charger
terminal voltage in such a way that any battery with any level of charge can be connected to the charger without requiring any manual adjustment. The charg-ing voltage is held constant, while an appropriate charging current range is automatically selected at successive different battery terminal voltages. And when the battery gets fully charged, the charger switches over to trickle-charge mode.
The circuit consists of the
Fig. 3: Actual-size, single-sided PCB for battery charger
Fig. 4: Component layout for the PCB
following sections:
1. The DC power supply section.
2. The series DC voltage regulation section.
3. The battery status indication-cum-charge current regulation section.
The DC power supply section. The 230V AC mains supply is connected to a step-down transformer with a second-ary rating of 24V AC, 5A through DPDT toggle switch S1. When switch S1 is in ‘off’ position, the availability of mains supply is indicated by green LED1. When switch S1 is toggled to ‘on’
position, red LED2 glows to indicate that the charger is ‘on’. The four 15-kilo-ohm resistors R1, R2 and R3, R4 in the path of LED1 and LED2, respectively, are rated at 1 watt each.
The output from the secondary of transformer X1 is rectified by the bridge rectifier comprising 1N5408 diodes D3 through D6, rated at 800V, 3A. The rectified output is smoothed by three capacitors C1, C2, and C3 before being applied to the rest of the circuit. The 4.7-kilo-ohm resistor R6 acts as a bleeder resistance. LED7 indicates that DC is available at the output of this section.
The series DC voltage regu-lation section. This section is configured around power Darlington transistor TIP142 (T1) that functions in conjunction with transistor T3 (BC549) and preset VR2 to regulate the output voltage from the DC volt-age regulator section.
Since zener diode ZD1 conducts only after the output voltage reaches 15 volts, the output voltage needs to be adjusted in the vicinity of 15 volts with the help of preset VR2. When transistor T3 conducts fully, the base of transistor T1 is pulled towards ground via resistor R8 and it stops conducting after the output voltage exceeds a specific value.
Transistor T2 (also a BC549) helps in current limit adjustments.
Low-value, high-wattage resistors R15 (shunted by R14) through R19 connected in series form a current-limiting resistor network at the output of transistor T1. This resistor network limits the charging cur-rent depending on the energisation/
de-energisation state of relays RL1 through RL4 that select the cur-rent range. The resistors are either Fig. 2: Charging current versus battery terminal voltage
shorted or added by respective relay contacts RY1 through RY4 depending on the charging current requirement from the regulator.
The battery status indication-cum-charge current regulation section. In this circuit, a quad op-amp LM324 (IC1) is wired as a four-stage comparator to indicate the battery voltage with the help of four LEDs (LED3 through LED6), while at the same time selecting and driving corresponding relays to set the charging current range.
The 6.8V reference voltage developed across zener diode ZD2 is proportion-ately applied to the inverting terminals of comparators A1 through A4, while the sampled battery voltage is proportionately applied to the non-inverting terminals of all the comparators.
Preset VR3 may be adjusted to obtain the reference voltages as shown in Fig. 1.
Preset VR4 may be adjusted by applying an external fixed voltage of 10.5V, 11.5V, 12.5V, or 13.5V across the battery’s screw terminals, ensuring that the correspond-ing LEDs (and relays) light up (energise) in accordance with the table.
In the charge characteristic curve of Fig. 2, it can be seen that the terminal voltage is compared by the comparators against the preset values and the charg-ing current is selected accordcharg-ingly. Thus a battery of any charge level can be connect-Parts List T4-T7 - 2N2222A npn transistor D1, D2,
D7-D11 - 1N4007 rectifier diodes D3-D6, D12 - 1N5408 rectifier diodes LED1 - Green LED
C1, C2 - 2200µF, 40V electrolytic C3 - 1000µF, 40V electrolytic C4 - 470µF, 25V electrolytic C5 - 100nF ceramic VR1-VR2 - 2.2-kilo-ohm preset VR3 - 10-kilo-ohm preset VR4 - 15-kilo-ohm preset Miscellaneous: F1 - 750mA cartridge glass fuse
tabLe
LeD/relay Operation and Charging resistance
Battery LED/Relay status Charging Preset
voltage LED3 LED4 LED5 LED resistance current
/RL1 /RL2 /RL3 /RL4
* 0.5A is taken as the trickle charging current.
edand left unattended under the control of this charger circuit.
When the battery is flat with termi-nal voltage below 10.5 volts, the initial charging current is selected at just one ampere because a higher initial charging current may cripple both the battery and the charger. A higher charging current is selected only when the battery has reached a safe level of terminal voltage.
Later, as the battery starts charging and its terminal voltage starts rising, the charging current is decreased in proper steps. Upon reaching the full voltage of 13.5 volts, the charger switches to the trickle charge mode with resistor R19 coming into the charging path. Optionally, one can switch off the charger on energisation of re-lay RL4 by just removing resistor R19 from the circuit. Whenever the terminal voltage level of the battery goes low, the charger automatically resumes charging.
Figs 3 and 4 show the actual-size, sin-gle-sided PCB and the component layout, respectively, of the charger circuit.
Note. To ensure proper functioning of the circuit, use good-quality relays and precise-value resistors (R14 through R24) with tolerance as mentioned in the Parts List. Connect the metal housing of the charger circuit to the earth line of the AC mains supply for personal safety. ❏