Arduino Mains Failure Battery Backup Circuit

The article explains a simple mains failure backup circuit for providing Arduino boards an uninterruptible supply during such situations. The idea was requested by Mr. Fredrik.

Technical Specifications

This blog gave me a lot of interesting information. Especially the power supply circuit with battery backup part.

The reason for this is that I am working on a Arduino based system for monitoring and controlling heating cables at my summer place.

This system will eventually be gsm controlled so I quickly can get an update on for example the temperature in the bathroom.

The part that I am stuck on is that I would like to have the Arduino to have a battery backup of some sort so it can still monitor the temperature around vulnerable waterpipes, and possible notify me if mains power goes out. I'm thinking of using a car battery so it can last for ages if power goes out.

What changes will I have to make to the "Power Supply Circuit with Emergency Backup" circuit to make it work with a 12V car battery and still have it trickle charge slowly?

Thank you in advance for any advice.

Sincerely
- Fredrik

Circuit Diagram

The Design

The simplest way to implement the proposed application is by using two diodes as shown in the above diagram.

The design shows two diodes with their cathodes connected together and anodes terminated to a 14 V source and anodes to the positive of a 12 V battery source respectively.

The common cathodes of the diodes are further connected to a IC 7805 IC whose output is finally applied to the Arduino board.

When mains is present the 14 V supply ensures s constant trickle charge supply to the attached battery via R1 and also feeds the Arduino borad through D1 and the 7805 IC.

In this situation D1 cathode experiences a much higher potential than the cathode of  D2 due a relatively lower battery potential at D2 cathode.

The above situation keeps D2 reverse biased allowing the battery charge to stay blocked and pass only the adapter voltage to the Arduino board.

But as soon as the mains supply fails, D1 instantly stops conducting and enables D2 to get forward biased so that now the battery instantly takes over and begins supplying the Arduino via the 7805 IC.