Here’s an improved Atmel ATtiny13V solar charger, source and schematic included, Dick writes – “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. A modern car contains a large amount of electronics, and a quiescent current of 40-50mA (about 1Ah per day!) is considered “normal”.
Because I wanted to keep an eye on the battery condition and charge current without opening the car, I devised a circuit that shows the voltage and current visually, by blinking a red and green LED. The charge current is determined by measuring the voltage drop over a 10 ohm series resistor. Two A/D inputs are used to measure the voltage before and after the resistor, which is the battery voltage.” [via] – Link.