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In today’s world, there are many applications for portable devices. These devices must have extremely low or no battery drain when turned off and need to charge their on-board batteries when connected to an external power supply, whether the device is turned on or not. This circuit provides seamless switching from batteries to external power and provides a simple charging solution. In addition, there are several output signals for a microcontroller (MCU). This application uses a +15-V external dc supply to charge a +12-V lead-acid battery.

When power switch S1 is closed and the charger isn’t plugged in, the 12-V battery powers the circuit. If the user plugs in the charger to recharge the battery (with S1 still closed), U1 cuts off current flow from the battery and provides power to the circuit from the external source only. The +15-V source can provide up to 5 A of current to the load. R1 and R4 are used to signal to the processor when the device is plugged in.

Q1 and Q3 make up the heart of the battery-charging circuit. This has to be driven by the MCU. When the Charge Driver line is high, the battery will receive current from the +15-V source. D2 prevents current from flowing from the battery back into the charging circuit.

R3 and R6 provide a signal for the MCU, indicating when switch S1 is turned on. When the switch is turned off, no current will flow, preventing the battery from draining during storage. In some applications where temperature varies widely, these values may need to be dropped by an order of magnitude or two. R2, R5, and Q4 provide a 0- to 5-V signal related to the voltage level of the +12-V battery. This voltage helps ensure proper charging and gives the user an indication of how much battery life is left.

Because the user may want to charge the battery without turning the device on, the voltage divider needs to connect to the battery directly, without the power switch in its path. As a result, the processor can monitor battery voltage when turned off. Q4 prevents current from flowing when S1 is off and the circuit isn’t connected to the external power supply, again preventing the battery from draining when not in use.

All of these components are readily available and not very expensive. The circuit here is designed for a 12-V battery and a 15-V power supply. For different voltages, some minor tweaks to the monitoring resistor values may be required. In addition, adjustments to the U1 and Q1 circuits may be needed. The good folks at IR are very competent and would be happy to help get your design up and running quickly.

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