battery charger construction


This simple and low cost battery charger uses a single transistor as a constant current source. The voltage across the pair of 1N4148 diodes biases the base of the BD140 medium power transistor. The base-emitter voltage of the transistor and the forward voltage drop across the diodes are relatively stable. The charging current is approximately 15mA or 45mA with the switch closed. This suits most 1.5V and 9V rechargeable batteries.The transformer should have a secondary rating of 12V ac at 0.5amp

 1

USB Battery Charger for Lithium Ion Battery

 This schematic diagram is used for charging lithium ion battery. The power source is from a computer‘s USB port. With this circuit, you do not need to build power supply circuits for charging your battery.
A USB port is a great power source for charging a single cell li-on battery. It is capable of supplying maximum 5.25V and 500 mA. The circuit above is a USB powered single cell li-on battery charger. LM3622 is used as the controller. This special purpose IC has a precise end-of-charge control and low battery leakage current about 200nA.

2

Simple Battery Charger using LM350

The schematic diagram can be used for charging the 12V lead acid batteries.
The circuit is designed as a constant voltage source with a negative temperature coefficient. The transistor Q1 (BD 140) is used as the temperature sensor. The transistor Q2 is used to prevent the battery from discharging through R1 when the mains power is not available. The circuit is designed based on the voltage regulator IC LM350. The output voltage of the charger can be adjusted between 13-15 V by varying the POT R6.

The LM350 will try to keep the voltage drop between its input pin and the output pin at a constant value of 1.25V. So there will be a constant current flow through the resistor R1. Q1 act here as a temperature sensor with the help of components R6/R3/R4 which more or less control the base current of Q1. As the emitter/base connection of transitor Q1, just like any other semiconductor, contains a temperature coefficient of -2mV/°C, the output voltage will also show a negative temperature coefficient. That one is only a factor of 4 larger, because of the variation of the emitter/basis of Q1 multiplied by the division factor of P1/R3/R4. This results in approximately -8mV/°C. The LED will glow whenever the mains power is available.

The transistor Q1 must be placed as close as possible to the battery.
Use a 20 to 30 V / 3A DC power supply for powering the circuit.
This circuit is not possible for charging GEL type batteries as it draw large amounts of current.

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