Hacking the Raspberry Pi Model B to use with Geekworm UPS Hat

Raspberry Pi Model B (not Plus) with Composite Video socket dismounted

I recently bought a Geekworm UPS Hat for the Raspberry Pi Models B Plus and later. It has, shall we say a few rough edges. Especially running loads of 500mA and above. It often functions as a UPS at loads of say 480mA. If I paired it with a say, Raspberry Pi Model B, it works quite well.

Now I have always wanted a solar-powered wifi repeater. In the day, a solar panel provides DC power and also charges up an NS60 (nominal 60Ah at 12v) car battery. At night it runs off the battery. It seems reasonable enough: the Raspberry Pi 3 Model B Plus wifi repeater took up more than 500mA at 5V 24 hours a day, whilst the solar panel might supply 2A at 19V for maybe 6 hours a day.

But before I could buy the NS60, the Covid-19 pandemic of 2019 intervened. Rather than wait for lockdown to pass, why not reconfigure it to run from solar power in the day, and seamlessly switch over to mains power at night. I would need a couple of relays: one each for mains and solar power 5V DC-DC buck converters. And to ensure a trouble-free switchover, a UPS Hat for the Raspberry Pi would be nice ...

The Geekworm UPS Hat worked well at 500mA, and misbehaved over 600mA: the WiFi Repeater would reset on switchover. Or it would not charge the lithium battery on switching back to mains power. Now this is actually self-recovering: on the battery running down it would reset the load and the battery would charge again. There might be a minute of WiFi repeater service interruption. TM Net my service provider certainly does that a few times a day. But this is humiliating; not tolerable for anyone other than TM Net.

My Raspberry Pi Model B Plus drew 510mA clean and 600mA once the WiFi dongle started firing up. On the other hand a Raspberry Pi Model B drew only  440mA and might just work. The trouble is, the Geekworm UPS Hat has a 40-pin socket and the Model B only has a 26-pin header.

Raspberry Pi Model B's 26-pin Header

Many hardware designers seek backward-compatibility when upgrading their designs. Often old hats will work on new models, but new hats will not. But this means the headers will have a lot of similarity. Sometimes enough to work. A quick comparison shows that the first 26 pins of the 50-pin header is pretty much identical, except for GPIO 19-21. The I2C pins are the same, and crucially, so are the power and ground pins.

Raspberry Pi Model B Plus' 40-pin Header

And as long as the 3 contentious GPIO pins are not used, they will default to GPIO input, and inputs when mis-wired and unused are harmless.

But there is another problem: the Model B's composite video output, an RCA socket is too tall and gets in the way of the Geekworm 40-pin socket. This is easily de-soldered.

Remember, an electronic engineer's favorite programming language is solder!

Once the RCA socket is removed, the Geekworm UPS Hat mounts nicely onto a Model B.

Geekworm UPS Hat on Raspberry Pi Model B

The Model B powers on nicely from battery. But the proof of the pudding is in the eating. Besides the Geekworm UPS I2C device at address 0x48 I also had an ADS1115 4-channel analog input card at address 0x36.

# i2cdetect -y 1

     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f

00:          -- -- -- -- -- -- -- -- -- -- -- -- -- 

10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 

20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 

30: -- -- -- -- -- -- 36 -- -- -- -- -- -- -- -- -- 

40: -- -- -- -- -- -- -- -- 48 -- -- -- -- -- -- -- 

50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 

60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 

70: -- -- -- -- -- -- -- --             

And from my last post, 

# ./ups_read

97

# ./ups_read -vc
4.161250V 96.550781%

Blinkenlights galore: Raspberry Pi Model B booting from battery power

To probe it a little further, I printed out a few more registers of the MAX17048:
# ./ups_read2 -a
MODE 10 00
CFG 97 1c
CRATE ff f9
VER 00 12
STAT 00 ff
VRST 00 0c
VCELL cf f0
CREG 60 65
4.158750V 96.394531%

The code is in my github repository.

Note that the 5V input into the Geekworm UPS Hat greatly affects its operation. In the picture of the WiFi extender below, a smaller 5V DC-DC buck converter (red PCB) was used, and this resulted in the Geekworm Hat not charging. Swapping it out for a 5V 3A unit (blue PCB) did the trick. Note that when the battery is at full charge, it takes a little while, maybe a few minutes, for the Hat to start charging.

Raspberry Pi Model B with Geekworm UPS Hat, installed as daytime solar powered WiFi extender . From top: ADS1115 I2C analog converter, WiFi dongle, Pi Model B with Geekworm UPS Hat, 2A buck converter(disconnected), and 3A CC CV buck converter

And yes, the Raspberry Pi daytime solar WiFi Repeater works for now. It's early days yet and there are many switchovers and switchbacks yet to come.

And there you have it: how to hack a 26-pin Raspberry Pi Model B for the Geekworm UPS Hat.

Happy Trails