Atmel solar panel battery charger

Atmel solar panel charger

Batwatch is a simple monitor for solar panel chargers, using an Atmel ATtiny13V microcontroller. It periodically measures the charging current and battery voltage and displays them by flashing two LEDs. I built this circuit into the plug of a Volkswagen solar charger panel to prevent the battery from discharging when the car is not in use for extended periods. Modern cars contain a lot of electronics, and a quiescent current of 40-50mA (about 1Ah per day!) is considered "normal."

Every few seconds, the software samples the two analog inputs, calculates the charging current and battery voltage, and displays the voltage on a red LED and the charging current on a green LED. Voltage is displayed in 0.1V units using three decimal digits, and current is displayed in 10mA units using two decimal digits. Each number is represented by the number of short flashes corresponding to its value. Zero is represented by a single, longer flash. The green LED remains completely off when the charging current is below 10mA (rounded) or negative. I used 4x oversampling to obtain 11-bit results, following the technique described in the Atmel application note AVR121. I'm not sure if there's enough noise on the inputs to actually improve resolution, but it doesn't hurt. The raw A/D readings are converted to voltage using separate offset/gain constants for each input. These constants come from calibration data stored in the EEPROM. To conserve power, all latency is achieved by executing one or more sleep instructions that idle the CPU until the next timer interrupt. The interrupt frequency is 100 Hz, so each sleep pause lasts a maximum of 10 milliseconds.