Batwatch is a simple monitor for a solar panel battery charger, using an Atmel ATtiny13V. It periodically measures the charge current and battery voltage, and shows them by blinking two LEDs. I built this circuit into the plug of a VW solar charger panel that is used to prevent a discharge of the battery when a car is not used for some time
The device must be calibrated before it can show reliable figures. For each channel, samples of exactly 8V and 12V are taken and stored in EEPROM. At program startup, these values are transformed to offset/gain pairs that are used to convert a sample to a voltage.
To calibrate the device, disconnect the battery and make sure that the solar panel is covered. Now the following steps have to be followed:
Apply exactly 8V to the 10uF capacitor.
Pull the calibration input low, by shorting pins 6 and 7.
The green LED should now turn on for 2 seconds..
Apply exactly 12V to the 10uF capacitor..
Pull the calibration input low, by shorting pins 6 and 7..
The red LED should now turn on for 2 seconds.
To save power, all delays are implemented by executing one or more sleep instructions which cause the CPU to idle until the next timer interrupt. The interrupt frequency is 100 Hz, so each sleep pauses up to 10 mS
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