As drawn, though, you can't control that, unless what you mean is that you only plan on connecting mains power to the charger when the Pi is off. In that case, it might work, but I wouldn't trust a cheap charger conencted across the battery without powering the charger. There should be something to stop the battery discharging back through the charger, but lithium batteries being as picky as they are, I wouldn't do it. You could wire in a changeover switch so that the battery is connected to either the charger or the Pi & screen, but I'm not sure why - it would be just as easy, and more certain, to wire up your batteries to quick-release connectors and just unplug and replug.mud2005 wrote: I plan on only charging the battery when the pi is off
That might be a good clue, check RC UBEC's, some might have a low voltage shutoff.Given your apparent familiarity with RC gear, beware that RC lipos generally have less in the way of built-in protection circuitry than the lipos in laptops and other consumer electronics - you are at risk of damaging cells if you over-drain them, and damaged cells can reputedly do bad things when you next charge them.
They note that it is NOT a good "UPS" due to the way the power behaves when charging during use.mud2005 wrote:thanks all, this gives me a lot to think about.
there is also this option: https://www.adafruit.com/product/1566
I could plug the PI into the 2A and the screen into the 1A and it has a charging circuit built in all for $40 and has a very high capacity, almost seems to good to be true.
Your initial idea lacks an under voltage protection. So your Li cells may have a short life. In addition, depending on the charger's regulating strategy, charging won't work at all, having a second power sink (Pi, display) connected. The power bank approach, using a power bank capable of being charged and provide power in parallel is the most "simple" solution from a user's perspective.
The USB battery will already have a 5v regulator. If you put in another voltage convertor, you will still need better power connections from your convertor to the Pi. Just get a better USB lead between your battery and Pi (eg 18awg, very short cord).
18650 Lithium Li-ion Batteries coupled with below should protect your cells from over charging/discharging and allow you to achieve what you have drawn.mud2005 wrote: ↑Sun Jun 04, 2017 1:26 amI'm new to the pi world and electronics in general, I bought a 7" display and a raspberry pi 3.
They both run great off AC adapters, but I'm looking to run off battery power.
my display can handle 5V-12V I'm wondering if this simple circuit would work to run the pi and display and also be able to charge the batteries using an external 2S battery charger.
I realize I may be in over my head and this circuit is way too simple, if so feel free to say so, I'm here to learn
I plan on only charging the battery when the pi is off