So i need a power supply, with a battery backup, at 9Vdc @ 2amps. It wouldnt be that hard except id like it to have a self charging battery backup. and the self charging battery back up need to protect itself from over charge and Zero Point loss. This is for a micro controller project (Arduino) the project is an RFID deadbolt that will still let me in when the power is out. My electronics knowledge has degraded over the years of non use to the point of nothing. im not exactly sure how to go about this. i was going to just use a wall wart at 9V with the correct amperage and then run it through a relay. the relay would be powered by the wall wart to close the circuit allowing the wall wart to power the load. should the wal wart fail the relay would open and thus close the circuit allowing the batter pack to power the load. all of this with a cap at the end to hold a charge long enough to power the load during the brief time that the relay is in transition and niether the wall wart, nor the battery pack are powering the load. but then i got to thinking, what if i forgot to change the batteries, it wouldnt matter if there was a battery backup or not. thus the self charging battery backup idea came about. suggestions on what type of cells (NiCad, MiHM, LiOn, Li-Po) voltages, mA/h, voltage regulation. iv looked around and i have found similar ideas but none seems to contain the self switching mechanism. I could use all the help you all are willing to offer. Thanks
