I am trying to create a backup dc power supply that is charged by a battery which ranges between 10 and 21Vdc. The backup supply needs to output 20W (10V, 2A) of power for 5 seconds when the battery is disconnected. I have performed some research into how long the capacitor in the backup supply will take to charge and has been determined to be negligible. I was thinking of using a simple RC circuit if possible. Additionally, the backup circuit should only be active when the battery is disconnected. Any suggestions on how to tackle this is appreciated. I was thinking of using a sensor to determine when the battery voltage is zero, meaning the battery is disconnected and then "turn on" the backup circuit. Mainly, I am looking for any suggestions towards the circuit design of the backup circuitry and any suggestions about how to turn the backup circuit on only when the battery is disconnected.