1. circuit
ideas
Constant-Current s.c. dwiv
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Battery Charger
 Monoj Das 13.5-14.2V in the case of a 12V battery), 14+5=19V.
give indication and the charger will For the sake of simplicity, this con-
T
here are many ways of battery switch off automatically. You need not stant-current battery charger circuit is
charging but constant-current remove the battery from the circuit. divided into three sections: constant-
charging, in particular, is a 4. If the battery is discharged be- current source, overcharge protection
popular method for lead-acid and Ni- low a limit, it will give deep-discharge and deep-discharge protection sec-
Cd batteries. In this circuit, the battery indication. tions.
is charged with a constant current that 5. Quiescent current is less than 5 The constant-current source is
is generally one-tenth of the battery mA and mostly due to zeners. built around MOSFET T5, transistor
capacity in ampere-hours. So for a 6. DC source voltage (VCC) ranges T1, diodes D1 and D2, resistors R1, R2,
4.5Ah battery, constant charging cur- from 9V to 24V. R10 and R11, and potmeter VR1. Diode
rent would be 450 mA. 7. The charger is short-circuit pro- D2 is a low-temperature-coefficient,
This battery charger has the follow- tected. highly stable reference diode LM236-5.
ing features: D1 is a low-forward-drop schottky LM336-5 can also be used with reduced
1. It can charge 6V, 9V and 12V bat- diode SB560 having peak reverse volt- operating temperature range of 0 to
teries. Batteries rated at other voltages age (PRV) of 60V at 5A or a 1N5822 +70°C. Gate-source voltage (VGS) of T5
can be charged by changing the values diode having 40V PRV at 3A. Nor- is set by adjusting VR1 slightly above
of zener diodes ZD1 and ZD2. mally, the minimum DC source volt- 4V. By setting VGS, charging current
2. Constant current can be set as age should be ‘D1 drop+Full charged can be fixed depending on the battery
per the battery capacity by using a battery voltage+VDSS+ R2 drop,’ which capacity. First, decide the charging
potmeter and multimeter in series with is approximately ‘Full charged battery current (one-tenth of the battery’s Ah
the battery. voltage+5V.’ For example, if we take capacity) and then calculate the nearest
3. Once the battery is fully charged, full-charge voltage as 14V for a 12V standard value of R2 as follows:
it will attain certain voltage level (e.g. battery, the source voltage should be R2 = 0.7/Safe fault current
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2. circuit
ideas
R2 and T1 limit the charging cur- diode ZD1 starts conducting after LED2 will glow to indicate that the bat-
rent if something fails or battery termi- its breakdown voltage is reached, tery voltage is low.
nals get short-circuited accidentally. i.e., it conducts when the battery Values of zener diodes ZD1 and
To set a charging current, while voltage goes beyond a prefixed high ZD2 will be the same for 6V, 9V and
a multimeter is connected in series level. Adjust VR2 when the battery 12V batteries. For other voltages, you
with the battery and source supply is is fully charged (say, 13.5V in case of need to suitably change the values of
present, adjust potmeter VR1 slowly a 12V battery) so that VGS of T5 is set ZD1 and ZD2. Charging current pro-
until the charging current reaches its to zero and hence charging current vided by this circuit is 1 mA to 1 A, and
required value. stops flowing to the battery. LED1 no heat-sink is required for T5. If the
Overcharge and deep-discharge glows to indicate that the battery is maximum charging current required is
protection have been shown in dotted fully charged. When LED1 glows, the 5A, put another LM236-5 in series with
areas of the circuit diagram. All com- internal LED of the optocoupler also diode D2, change the value of R11 to 1
ponents in these areas are subjected to glows and the internal transistor con- kilo-ohm, replace D1 with two SB560
a maximum of the battery voltage and ducts. As a result, gate-source voltage devices in parallel and provide a good
not the DC source voltage. This makes (VGS) of MOSFET T5 becomes zero and heat-sink for MOSFET T1. TO-220 pack-
the circuit work under a wide range of charging stops. age of IRF540 can handle up to 50W.
source voltages and without any influ- Normally, zener diode ZD2 con- Assemble the circuit on a gen-
ence from the charging current value. ducts to drive transistor T3 into con- eral-purpose PCB and enclose in a
Set overcharge and deep-discharge duction and thus make transistor T4 box after setting the charging current,
voltage of the battery using potmeters cut-off. If the battery terminal voltage overcharge voltage and deep-discharge
VR1 and VR2 before charging the bat- drops to, say, 11V in case of a 12V bat- voltage. Mount potmeters VR1, VR2
tery. tery, adjust potmeter VR3 such that and VR3 on the front panel of the
In overcharge protection, zener transistor T3 is cut-off and T4 conducts. box. 
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