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The Battery Guardian

Don't get caught with a flat battery; this easy-to-build circuit can cut off the power to a 12V fridge or car stereo system if the battery voltages drops below critical level.

By John Clarke

Click for larger image
All the parts fit on a single PC board, so the circuit is easy to build. Note that the corners of the PC board must be removed to clear the corner mounting pillars inside the case.
Electric fridges in vans and 4WDs are a great idea but if you are not careful, they can severely discharge the battery and leave you stranded. Maybe the battery will end up with severe damage as well. The same problem applies if you have a big stereo system and you like to play it without the motor running.

Operation on 12V is fine when the motor is running and battery charge is maintained but if the fridge is allowed to run for too long when the motor is stopped, it can flatten the battery in a relatively short time. This is where the Battery Guardian comes into play. It monitors the battery voltage and disconnects power to the fridge before the battery becomes too flat to allow the engine to be started again.

  • Cuts power to load (eg, fridge) when battery voltage drops below a preset level.
  • 10A rating.
  • Low power drain.
  • Chirping sound during cut-out.
  • Flashing LED indication during cut-out.
  • Automatically reconnects power when battery recharged.

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