Hi,

I am in need of a circuit to power a raspberry pi, that should include a battery backup so it can run for a few hours if mains power fails.
I have come up with the attached circuit, which includes:
- My testing shows the raspberry pi uses on average around 800ma, peaks to around 950ma.
- 6v sealed lead acid battery (4ah or 7ah) (selected for cost, power requirements).
- charger limited to around 600ma.
- battery monitor to provide a level indicator, stop the charging when sufficient level reached, and cut-off the battery if low voltage reached.
- buck-boost to provide the final 5v to the raspberry pi (buck-boost due to the various possible supply voltages dipping below or close to 5v).

I would appreciate any pointers if I am on the right track and if this will work as expected?
I will probably add a trigger to the raspberry pi (via transistor or optocoupler) to let it know when a low battery voltage is reached so it can shut itself down.

I am concerned around the different possible voltages coming from the power supply and battery, where they meet up. Is this a correct way of doing it (don't want to use a relay to switch between them if I can help it).

Lastly, I am struggling with the buck-boost circuit. I have done some testing with a cheap Chinese board, but would prefer to end up with my own circuit eventually. Any suggestions or pointers to a proper sample circuit I can start with?

Thanks!
Alan

 

Welcome to AAC!

I am concerned around the different possible voltages coming from the power supply and battery, where they meet up. Is this a correct way of doing it?

A diode-OR arrangement, such as you show, is a standard way of connectng two supplies.

 

Looks good, diodes will separate the 5v and 12v supplies,, better to use a ready made buck booster, Sla batteries charge at a Constant Voltage, so the current will drop as the battery is full, Shotkley diodes are better for low voltage drops,.

 

Welcome to AAC!

A diode-OR arrangement, such as you show, is a standard way of connectng two supplies.

Thank you!

Looks good, diodes will separate the 5v and 12v supplies,, better to use a ready made buck booster, Sla batteries charge at a Constant Voltage, so the current will drop as the battery is full, Shotkley diodes are better for low voltage drops,.

Thanks, I shall build it and see how it all works out