Description.
This lead acid battery charger circuit is designed in response to a request from Mr.Devdas .C. His requirement was a circuit to charge two 12V/7AH lead acid batteries in series.Anyway he did not mentioned the no of cells per  each 12V battery. The no of cells/battery is also an important parameter and here I designed the circuit assuming  each 12V battery containing 6 cells. When two batteries are connected in series, the voltage will add up and the current capacity remains same. So two 12V/7AH batteries connected in series can be considered as a 24V/7AH battery.

The circuit given here is a current limited lead acid battery charger built around the famous variable voltage regulator IC LM 317. The charging current depends on the value of resistor R2 and here it is set to be 700mA. Resistor R3 and POT R4 determines the charging voltage. Transformer T1 steps down the mains voltage and bridge D1 does the job of rectification. C1 is the filter capacitor. Diode D1 prevents the reverse flow of current from the battery when charger is switched OFF or when mains power is not available.

Circuit diagram.

Notes.

• Assemble the circuit on a good quality PCB.
• T1 can be a 230V primary, 35V/3A secondary step down transformer.
• If 3A Bridge is not available, make one using four 1N5003 diodes.
• LM317 must be fitted with a heat sink.
• R2 = 0.85 ohm  is not a standard value. You can obtain it by combining a 6.2 ohm and 1 ohm resistors in parallel.
• F1 can be a 2A fuse.
• To setup the charging voltage, power ON  the charger and hook up a voltmeter across the output terminals and adjust R4 to make the voltmeter read 28V. Now the charger is ready and you can connect the batteries.
• This charger is specifically designed for two 12V/7AH/6 cell lead acid batteries  in series OR a  24V/7AH/12 cell lead acid battery.

Description.
The circuit diagram shown here is of a automatic changeover switch using IC LTC4412 from Linear Technologies. This circuit can be used for the automatic switchover of a load between a battery and a wall adapter.LTC4412 controls an external P-channel MOSFET to create a near ideal diode function for power switch over and load sharing. This makes the LT4412 an ideal replacement for power supply ORing diodes. A wide range of MOSFETs can be driven using the IC and this gives much flexibility in terms of load current. The LT4412 also has a bunch of good features like reverse battery protection, manual control input, MOSFET gate protection clamp etc.

The diode D1 prevents the reverse flow of current to the wall adapter when there is no mains supply. Capacitor C1 is the output filter capacitor. Pin 4 of the IC is called the status output. When wall adapter input is present the status output pin will be high and this can be used to enable another auxiliary P-channel MOSFET (not shown in the circuit diagram).

Circuit diagram.

Notes.

• Assemble the circuit on a good quality PCB.
• The wall adapter input can be anything between 3 to 28V DC.
• The battery voltage can be anything between 2.5V to 28V.
• Do not connect loads that consume more than 2A.
• Maximum continuous drain current of Q1 (FDN306P) is 2.5A.
• D1 (1N5819 is a 1A Schottky diode.
• Q1 (FDN306P) is a P-channel MOSFET.

Description.

Here is the circuit diagram of a battery eliminator circuit that can be used as a replacement for 9V PP3 batteries. The circuit given here can be used to power any device that operates from a 9V battery. The transformer T1 steps down the mains voltage and bridge D1 performs the job of rectification. Capacitor C1 is a filter. IC LM317T is the regulator here. The value of R1, R2 and R3 are so selected that the output voltage of IC1 will be steady 9 volts.

Circuit diagram.

Notes.

• Assemble the circuit on a good quality PCB.
• Transformer T1 can be a 230V primary, 9V secondary, 1.5A step down transformer.
• If 1A Bridge is not available, then make one using four 1N 4007 diodes.
• Do not connect loads that consume more than 1.5A to this circuit.
• A heat sink is recommended for IC1.

Description.

This is the circuit diagram of a very powerful and efficient Ni-MH battery charger using IC LT4060 from Linear Technologies. In addition to Ni-MH batteries Ni-Cd batteries can be also charged by slightly modifying the circuit.For charging Ni-Cd batteries connect the CHEM pin (pin12) of the IC to +Vcc.Here the circuit is configured to charge 2 cells connected in series. By altering the connections of SEL0 and SEL1 pins, upto 4 batteries in series can be charged using this circuit. For charging a  single cell, connect SEL0 and SEL1 pins to GND. For charging two cells, connect SEL1 to GND and SEL0 to Vcc. For charging  three cells,  connect SEL1 to VCC and SEL0 to GND. For charging 4 cells, connect SEL0 and SEL1 pins to VCC. The circuit also features temperature based charge qualification.An NTC thermistor connected at the NTC pin (pin11) of the IC serves as the temperature sensor.

Circuit diagram.

Notes.

• The circuit can be assembled on a Vero board.
• Mount the IC on a holder.
• Preset R2 must be set around 4K.
• Type no of Q1 is not very critical. Any 3 to 5A PNP transistor will do the purpose.
• LED D1 serves as  a charge indicator.

Description.
This circuit can be primarily used for charging 12V Ni-Cd battery packs. Any way 6V and 9V battery packs can be also charged by using this circuit a little compromise on power efficiency. The built in automatic current regulator regulates the charging current to somewhat 4 amperes. When the charging current reaches 4A, the voltage across resistor R1 becomes 0.7V and switches the transistor Q1 ON. The transistor Q2 which is now in ON state will shorts the base of Q3 to ground and inhibits the biasing of Q4 through which the charging is done. That is how the current regulation is achieved. When charging low voltage battery packs, the excess voltage will be dropped across Q4.It is advised to use a heat sink with transistor Q4.

Circuit diagram with Parts list.

Notes.

• The circuit can be assembled on a Vero board.
• T1 can be a 230V primary, 12V secondary,4A step down transformer.
• Bridge D1 can be made by using 1N5400 diodes.
• Fuse F1 can be a 500mA type.