External Battery for Macbook Air 2011

This idea came with the need to extend the working time on my daughter's Macbook air when she is far from any electrical power plug ...
This type of external battery for MB air can be found online but it is pretty expensive, so I decided to assemble my own version.
I found a car battery booster ( 50€ at LIDL in France ) with a suitable 12vdc jack output.
I have then connected on the 12v jack a DC to DC converter ( 9€ on amazon ) to elevate the 12vdc to 14.5vdc.
Then connect a magsafe 1 cable ( > 10€ ) to the DC to DC converter output and plug it to the MB air power connector.
And there you go, my Macbook air power up on battery is extended by 3 times ;-)
Enjoy ...