How this battery charger circuit works:
To make the simple explanation, lets divide this battery charger circuit into three sections: constant current source, overcharge protection and deep-discharge protection sections.
Constant Current Source
The constant-current source is built around MOSFET T5, transistor T1, diodes D1 and D2, resistors R1, R2, R10 and R11, and potmeter VR1. Diode D2 is a low-temperature-coefficient, highly stable reference diode LM236-5. LM336-5 can also be used with reduced operating temperature range of 0 to +70?C. Gate-source voltage (VGS) of T5 is set by adjusting VR1 slightly above 4V. By setting VGS, charging current can be fixed depending on the battery capacity. First, decide the charging current (one-tenth of the battery”s Ah capacity) and then calculate the nearest standard value of R2 as follows:
R2 = 0.7/Safe fault current.
R2 and T1 limit the charging current if something fails or battery terminals get short-circuited accidentally. To set a charging current, while a multimeter is connected in series with the battery and source supply is present, adjust potmeter VR1 slowly until the charging current reaches its needed value.
Overcharge and deep-discharge protection have been shown in dotted areas of the circuit diagram. All parts in these areas are subjected to a maximum of the battery voltage and not the DC source voltage. This makes the circuit work under a wide range of source voltages and without any influence from the charging current value. Set overcharge and deep-discharge voltage of the battery using potensiometers VR1 and VR2 before charging the battery.
In overcharge protection, zener diode ZD1 starts conducting after its breakdown voltage is reached, for example, it conducts when the battery voltage goes beyond a prefixed high level. Adjust the variable resistor VR2 when the battery is fully charged (say, 13.5V in case of a 12V battery) so that VGS of T5 is set to zero and hence charging current stops flowing to the battery. LED1 glows to indicate that the battery is fully charged. When LED1 glows, the internal LED of the optocoupler also glows and the internal transistor conducts. As a result, gate-source voltage (VGS) of MOSFET T5 becomes zero and charging stops.
Normally, zener diode ZD2 conducts to drive transistor T3 into conduction and thus make transistor T4 cut-off. If the battery terminal voltage drops to, say, 11V in case of a 12V battery, adjust the potensiometer VR3 such that transistor T3 is cut-off and T4 conducts. LED2 will glow to show you that the battery voltage is in low condition.
Values of zener diodes ZD1 and ZD2 will be the same for 6V, 9V and 12V batteries. For other voltages, you need to suitably change the values of ZD1 and ZD2. Charging current provided by this circuit is 1 mA to 1 A, and no heat-sink is required for T5. If the maximum charging current required is 5A, put another LM236-5 in series with diode D2, change the value of R11 to 1 kilo-ohm, replace D1 with two SB560 devices in parallel and provide a good heat-sink for MOSFET T1. TO-220 package of IRF540 can handle up to 50W.