@richard published a HaD project with an interesting new use case for the notorious W1209 thermostat: measure the internal resistance of lead acid batteries in a battery reconditioning set-up. All the hardware features of the W1209 are re-used: relay, LED display, keys, sensor input, and even EEPROM storage!

On Richard's project page there is also a video clip of the device!

The resistance measurement uses the relay and a 1A constant current circuit (a 7805 5V regulator and a 5R resistor), and calculates the resistance by comparing the open-circuit battery voltage with the voltage under load.

Check out Richard's blog - there is a ton of great hacks!