Hi I'm creating a case which will include a raspberry pi. I have never used a capacitor, so don't understand fully how the charge/circuit would work.

I have a 12V(wallwart) or 7.4V(battery) source. When the wallwart is plugged in the battery is disconnected and when the wallwart is plugged in the battery is disconnected, this is all done automatically because of the dc plug.

So what I want to do is when I unplug my wallwart the battery takes over, but the problem is that millisecond of interruption of power reboots my pi. I want there to be some temporary power so it doesn't turn off for those milliseconds. so thats why I'm thinking a capacitor.

First is what uf/voltage capacitor do I need.. Does it matter? I'm using a ubec to convert the 12v/7.4v power to 5v for the pi.

The total draw of everything is about 0.5A or lets say 0.6A for safety.
I may upgrade the monitor to something a little bigger (haven't received it yet so don't know the amps for that)

current circuit is

[Power] -> 12V -> Monitor
-> UBEC -- 5V --> Raspberry pi

I was thinking either putting the capacitor with my pi.

[Power] -> 7.4V/12V -> Monitor
-> UBEC -- 5V --> [C1] --> Raspberry pi

 

update: I tried with the battery and it runs at 0.8A. so perhaps a 1A safety net.

These measurements are from the power not from the [C1] point

[Power] -> [Amp meter] -> 7.4V/12V -> Monitor
|-> UBEC -- 5V --> [C1] --> Raspberry pi

 

How does the battery ever get charged?

Ignore the disconnect switch on the power plug. Wire the battery to the rPi through a 1N5817 rectifier in series. This way there is no switch-over time and the rPi is always powered.

 

I disconnect the battery and charge it. And I do have a power switch to turn off the whole system. Isn't meant to be on all the time. But want to make it so I can make it portable when I want during the day when in use.

Hmm that might be a problem I didn't quite describe my set up perfectly

Here is a small sketch.

The box to share power is a modular type thing so I can just use a single plug to share 12V or 7.4V power or 5V power to my devices (like a monitor, led, pi, other...)