Raspberry Pi UPS: Trickle Charger

LK1 is an option in case you want to change the design. Closing that circuit would close the relay as soon as 5V is provided, and would hold the relay closed until the batteries are depleted. In that case, the Run pin should be disconnected, as it could cause harm to your Raspberry Pi (the GPIO pins use 3.3v, so sending 5V to it could damage the pin). The schematic is drawn with LK1 normally open, and it’s designed to sense a power failure and shut down the Raspberry Pi when that happens, ... Read More

LK1 is an option in case you want to change the design. Closing that circuit would close the relay as soon as 5V is provided, and would hold the relay closed until the batteries are depleted. In that case, the Run pin should be disconnected, as it could cause harm to your Raspberry Pi (the GPIO pins use 3.3v, so sending 5V to it could damage the pin).
The schematic is drawn with LK1 normally open, and it’s designed to sense a power failure and shut down the Raspberry Pi when that happens, then disconnect the batteries so they don’t get depleted. If you want the Raspberry Pi to just run until the batteries are depleted, you probably don’t even need the power failure sensing circuitry from this design.
You can also do something like what I did in my design, which is to build a time delay so that the relay is held closed for a few seconds after the Raspberry Pi is shut down to ensure all the writes to the SD Card have finished.
The chemistry of the battery pack is critical to your decision to use this design. Lithium batteries should NOT be used with this design. If the RC batteries are NIMH cells, you would still want to trickle charge them. This would help prevent the batteries from overheating, while still keeping them fully charged. The alternative is to use NIMH batteries that you charge in a separate charging unit, then remove the charging circuit from your UPS design. The NIMH batteries would need to be charged every few days to ensure your Raspberry Pi doesn’t die on you when the power goes out.
I hope this helps answer your questions.

So I posted lots of photos of my UPS on Imgur. The link is here. In use Finished with the soldering Modified schematic I really invested a lot of time on this and learned a lot about electronics along the way. This has been an immensely enjoyable process, and I want to say thank you to Steve for his original design and software. I modified the software in some places. I used GPIO 14 and 15 instead of GPIO 17 and 19. So my /etc/rc.local file reads like this: # Pin 14 sends a Hi signal until ... Read More

So I posted lots of photos of my UPS on Imgur. The link is here.

In use

Finished with the soldering

Modified schematic

I really invested a lot of time on this and learned a lot about electronics along the way. This has been an immensely enjoyable process, and I want to say thank you to Steve for his original design and software.

I modified the software in some places. I used GPIO 14 and 15 instead of GPIO 17 and 19. So my /etc/rc.local file reads like this:

While testing Steve's Python program, I noticed a small issue with the syntax of the script. I am using Raspbian Stretch and Python 3, so I'm not sure if this applies to Python 2.

This should read:

Also, if you uncomment any of the other print commands, you'll need to add parentheses around the quotes.

I also made some modifications to Steve's original hardware design to include a time delay to hold the relay closed after the power has finished shutting down. Steve suggested this in one of his replies and I referenced his other Raspberry Pi UPS design when I modified this. The delay was only supposed to last for about 30 seconds (according to the simulator I used) to ensure a complete shutdown, but in practice it seems to last much longer (about 2 1/2 minutes.) This is likely due to not knowing the minimum amperage at which the relay will remain closed.

I also added a fast charging circuit (C/40) with 4 more transistors (one PNP and three NPN). I felt this was necessary because I am using a 19-24v power supply and the AA NiMH batteries I ordered claim to be 2700mAh. If I had used Steve's original simple design, T4 would have been shunting nearly 1/4 watt.

Also, I don’t run my RPi all the time, so I felt some extra charging at boot would be useful. The RPi runs a simple script on boot which turns a GPIO pin on for 2 hours (easily adjustable). Once 2 hours has elapsed, the script turns the pin off. The charging circuit is working perfectly so far, and amperages are within 10% of estimates.

I tried to over-engineer some things. I used 22ga solid core wire for most everything, and in some places I doubled that up. I doubled up the 2A RL207 diodes (in parallel) to replace the 1N5404 diodes in the original design because I didn't trust my logic. Everything is soldered onto an AdaFruit Altoids perma-proto board, and I wasn't sure how much current the traces could handle, so I added some extra wire to bridge long distances. In hindsight, this was probably unnecessary, but I would rather be safe than sorry.

Initially, I tested the UPS on a 12v 3A power supply I salvaged from an old NICAD flashlight battery charger. It was putting out about 12.5v under load, and everything worked great except the charging circuit was only putting out 9mA.

I ordered an 8A buck converter (sometimes listed as 12A "with additional cooling") instead of the cheap 3A ones. All told, I probably have about $70 invested in all the components because I ordered many things in bulk. I bought the NiMH AA batteries separately, and bought an 8-cell holder from AdaFruit. I didn't trust the low gauge wires on the AdaFruit holder, so I soldered on a fuse holder to the positive side and 22ga wire to the negative side. If I had to do it over again, I'd probably just buy an RC-style 9.6v NiMH battery pack with a Hitec connector.

If anyone wants to play with the simulated circuit, here is the link I used. Sometimes the links get wonky, so if it doesn't work, compare it with the Imgur pictures.

I'm running out of characters, so I'll post the scripts in my next post.