Like many people I had a traditional AC-DC UPS supplying power to my home network.  One day I awoke it it beeping away and running on battery power even though the mains power was present.  And then it died.  So I did what any maker might do, I optimized the repair by replacing it entirely with my own DC-DC UPS powering an Arris Cable Modem, Routerboard GigE Router/Firewall, Ethernet Hub and a Wifi Access Point.

(Don't worry - this ugly mess is in my lab where normal people never venture)

This project - now over a year old - is what led me to contemplate the "Use It UP!"  campaign.

I was able to supply 100% of the parts from my own stash.

1. Custom Solar MPPT Charger
2. Dell 19V 3A Laptop power supply + 1000 uF electrolytic capacitor
3. 12V 18 Ah Lead Acid battery
4. PJRC Teensy-LC controller
5. Adafruit 96x96 pixel Sharp Memory LCD breakout board
6. MPT3055 N-Channel MOSFET
7. Meanwell 15W Boost-Buck DC-DC converter with 12 VDC output
8. Linear Tech LT3053 Buck DC-DC converter eval board with 3.3 VDC output
9. Plastic Project Box, various wire and cables, protoboards, jacks, connectors and hardware

The Solar MPPT charger is a device that I designed for a client to power wildlife cameras in Asia and Africa.  It has a Silicon Labs micro to implement the charge controller and a Laird smart LoRaWAN to remote control and status access.

Normally the LoRaWAN module accesses the charge controller via an I2C interface but I had a board with no LoRAWAN module.  This lead me to the idea of adding a status display using the Teensy-LC and Sharp Memory LCD.  A quick Arduino sketch let the Teensy read out various parameters and format them for the display.

(In reality I rarely look at it, but it has been useful - and shows the efficiency of a DC-DC UPS)

The laptop supply output voltage is just about the perfect solar panel voltage for a 12V system and the charger simply locks on to its output voltage as the MPPT setpoint.  I did have to add some bulk capacitance (1000 uF) at the charger input in order for it not to crowbar when the charger starts up and has a big inrush.

A total of four DC voltages are provided to the networking devices.

1. Regulated 12 VDC for the Arris Cable Modem using a Meanwell DC-DC converter.
2. Unregulated battery voltage for the Routerboard Router (it has a very wide input range).
3. Regulated 3.3 VDC for the Ethernet Hub using an LTC Eval board (replacing it's original AD-DC supply).
4. Regulated 5 VDC for my lab's local Wifi Access Point using the charger's built in DC-DC covnerter.

The charger is capable of low-battery shut down so I kludged a wire from DC-DC enable signal coming off of its charge controller to the gate input of a beefy N-Channel MOSFET.  This allows a low-side power switch for the remaining DC supplies.

(Guts of the UPS)

(Teensy-LCD and Sharp Breakout)

(LTC Eval board powering Ethernet switch)

Coda

I initially worried about the LTC board powering the Ethernet Switch since it can take a bit of power but the whole setup has been very reliable.  It's capable of keeping my network alive for most of a day. 

Sadly the same can't be said for the internet service itself, provided by Comcast (Xfinity).  Since I live outside of my city there is at least one battery-backed up box between me and their main plant.  It seems to have only enough power for about an hour of runtime when the power goes out.

Shameless Plug

In 2019 I made another smart Solar MPPT charger designed for the maker market.  You can buy it on tindie.  It can be the basis of a DC-DC UPS as well.  Which is where the idea for this UPS came from.