STFN

Powering an LTE modem from a 12V battery

off-grid
diy
15 minutes

Background

This project came from a simple need: I have a piece of land without grid electricity, and I want to have Wi-Fi there. I could just use my phone as a hotspot for a laptop, but I wanted something better, and not using the phone’s battery. And building something for living off-grid is always fun.

In one of the previous blog posts I wrote how I built a 12V powerbank, so the energy source is covered. I also have an LTE modem, a TP-Link TL-MR100 which I bought some time ago to be used on holidays in remote places. Those two sounded like a perfect match, the only issue was how to connect them.

Looking at the sticker on the modem, it needs voltage of 9 volts, and uses 0.85A (I assume that second value is at maximum gain and throughput). My powerbank provides from ~13.5V fully charged to 11.5V at shutdown threshold. That sounds like a job for a step-down converter!

If you want to build something similar for yourself, check the sticker under your LTE modem for power specs, if your modem uses 12 volts, you might not even need to a converter, just connect it straight to the battery. Or if you want to be on the safe side, get a step-down/step-up converter that will output 12V no matter the voltage of the battery. In every other case you’ll need a converter, maybe even a step-up one if your modem runs on, for example, 15 volts.

I am not responsible for any damage you cause to your equipment or yourself.

Parts List

Step-down converter

A step-down converter is a device that takes an input voltage and outputs an output voltage. I know, shocking. In my case I needed something that will reduce the voltage of the battery, fluctuating between ~13 to ~11 volts, and provide a stable 9 volts to the modem. After some searching and reading, I found that a perfect solution for me would be something based on the LM2596 chip. This chip can receive voltages from 40 to 4.5 volts, and output from 37 volts to 3.3 volts and handle a maximum current of 3 amps, so my use case would be well within its capabilities.

There is a lot of different converters using this chip, just go to your favourite online marketplace and search for the chip name, you’ll get pages and pages of devices. They come in fixed and variable output voltages. All of them are dirt cheap, for 2-4 euros. Just select one that you like. I chose a fancier product, with a display showing the input and output voltages.

And all the other stuff

Apart from the converter, you’ll need:

And typical electrical DIY tools: multimeter, side cutters, a cable stripper, soldering iron, hot glue gun, and screwdrivers.

Build Process

For the wires I went with speaker cables, as they are perfect for DIY direct current projects: cheap, quite weather resistant, come in pairs of wires, and are marked for positive and negative. And I already have 30m of them lying around for a project that never came to fruition. Mine are, I think, 2 x 1.5mm and they were a bit too fat, I had to trim the ends to make them fit into the converter connector holes.

When soldering the cables, pay special attention to polarity and at every step make sure you don’t mix it. Speaker cables have one of the wires marked in red, the convention is that it is the positive one. XT60 and barrel plug also have + and - signs on them, if you always go plus to plus, you should be fine. Just test often.

I cut two lengths of the speaker cables, one for the powerbank-converter part and second for the powerbank-modem part. I carefully split the positive and negative wires and stripped the ends of their isolation. At this time I added short lengths of heat shrink to give the cable an additional layer of protection at the ends. For the first part I soldered an XT60 connector to one end. I chose that connector type because my powerbank already has these. The second end of that cable I soldered to the input side of the step-down converter. Part one done.

For the second length I did the same, splitting, stripping, and soldering. One end went to the output side of the step-down converter, and at the second end I screwed a 2.1x5.5mm barrel plug. Check what plug your modem has, it is very possible that it will too have that one, it’s a very popular standard.

The final step was to add a case to the converter. Again I used something I already had lying around the workshop (aka my bedroom), a weatherproof electric box. The converter fit nicely inside. In accordance to ancient Chinese traditions, I hot glued the it to the inside of the box, and added additional hot glue on the soldered connectors to protect them from moisture. And some more hot glue to close the cable gaps and fix everything securely. Now just a sign explaining what’s inside the box (what’s in the box?!) and the project is completed.

With the project is basically done, now it’s time to configure and test it.

Testing

Setup and start

My LM2596 is a variable voltage one, and has a display to show input and output voltages. If you have a constant output voltage one, skip the setup part. If you have a variable output one, but without a display, you’ll need a multimeter to check the output voltage.

I plugged the input cable to my powerbank and powered it on. The display started showing the output voltage. Using a small screwdriver, I carefully turned the adjust screw until the display showed 9 volts. This is all that is needed to setup the converted, but I wanted to do some testing, so I connected a PC case fan to the output side. As I was changing the voltage, the fan was visibly spinning slower and faster, so that confirmed that the converter works. Finally I checked the output plug with a multimeter to be triple sure that I am getting exactly 9 volts on it. I also made sure the polarity is correct, for barrel jacks the inside should be positive, and the outer shell should be negative.

Finally it was the time for the great finale, I connected the LTE modem to the converter, and the converted to the powerbank. I turned on the powerbank… and it worked! The modem started as usual and quickly caught the mobile network. In moments, I was able to connect to the Wi-Fi and browse the intertubes on battery power.

Now I can browse Mastodon while sitting in a field, the future is incredible. Thank you for reading, I hope you enjoyed it!

I would love to get some feedback from you, dear reader, you can drop me an email, or contact me on Mastodon.

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