I am looking to have a automatic switch that will switch to a small 12 volt back up battery when the car 12 volt power fails. It would help if it has a charger capability while the car system is working.this will prevent me from having to charge battery by itself on and off. Need it to have a full 12 volt out on the emergency out. Very small switch or circuit preferred , to fit existing gps storage area. 3 inches x 3 inches would be the max, preferred smaller. Can you guys help find such a switch or circuit?
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What about a pico ups, should be exactly what you need.
http://www.mini-box.com/picoUPS-120-12V-DC-micro-UPS-battery-backup
adi
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You could probably build something, but it is a really bad idea.
Running a car battery down is a bad idea, both from the life of the battery and the impact it has on the car electronics.
If you need to run the Pi in the car, you would be better to buy a battery (which you may already have) an an isolator, similar to those used to isolate fridges etc. in a 4WD. These are designed to protect the car battery from discharge by connected appliances, but keep an auxiliary battery charged. Run the Pi off the auxiliary battery.
Only you can decide if this is needed. At 700mA the Pi should run off a car battery for more than 24 hours - longer if you use a 12v => 5v switch mode power supply.
If you need longer, then you are going to need a pretty big battery.
Milliways
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