A DIY off-grid 12V powerbank
EDIT 9.10.2023: There is now a continuation of this post, in which I made an LTE modem to work with this powerbank. Off-grid LTE modem.
One of my hobbies is astrophotography, and that is something that is best done off-grid, far away from light sources such as cities. Astrophotography also requires a steady source of electricity to power the equipment, cameras, computerized tripods, a laptop etc. These two aspects combined suggest using a portable power source. Typical powerbanks won’t do, as they give out 5 volts to charge a phone or a tablet, and I need 12V of voltage, with currents of several amps for hours at at time. There are three basic solutions how to get a portable 12V power source:
- buy a cheap 12V powerbank dedicated to astronomers
- buy a very expensive, powerful portable power source like a Jackery
- build your own power source
At first I went with option 1, I bought a Bresser Power Station which was ok at the beginning, but once I move to a cooled astrocamera with a large power requirement, it just would not provide the needed power level (it was far below 9000).
As time passed and I got better with electronics and soldering, I decided I now know enough to build my own power supply.
I admit with shame that I do not have any photos from the time I was building the powerbank. I built it some time ago, and it was an iterative process, and I was not sure it will end up here on the blog. I hope the photos and diagrams I am showing here are understandable enough to show how its construction.
The first thing to decide was the battery. I needed a steady supply of 12V with enough capacity to work through a long night. I also wanted the battery to be safe and keep the factory capacity for a long period of time. On the other hand, I did not require it to be lightweight, as I would only carry it to and from my car. Budget was also an important factor.
I dismissed Li-ion batteries right away because of their flammability and cost. And they would require a BMS (Battery Management System) into which I do not want to delve yet. LiFePo4 would be awesome, but again the cost is for the time being prohibitive for me. So I settled for the good ol’, tested and tried AGM lead acid batteries. They weight a lot but that’s not really an issue for me, they can survive a lot of cycles without losing too much capacity, and they have a reasonably low self-discharge. The price per watt hour is not great, but also not terrible.
I would love to have an RTG like the Vikings, but hey, we don’t live in the future yet.
Going back to Earth, I searched for a good model of a battery, and I’ve settled with a YUASA REC22-12 deep-cycle battery. It has a capacity of 22Ah, which suits me fine, and is designed to survive a lot of cycles from full to almost depletion. I could have gone for a GreenCell with half the price, but I read mixed opinions about their longevity.
For the powerbank case I went with the simplest and sturdiest option, I bought a plastic tool box with internal dimensions enough to store the battery and the cables. Plastic is also easy to cut, and I had to do quite a few holes for the connectors. Speaking of connectors…
For those, I went with XT60 sockets. XT60 is a safe overkill for the current I will be using, and it’s cheap and easy to solder. They are also, at least in my opinion, sturdier than the 2.1 x 5.5mm barrel jacks.
The remaining parts are the cables, fuse, voltage meter, internal connectors for the cables, and finally a USB charger for phones. As for the USB charger, it took me quite some time to find, but eventually the “motorcycle or car usb chargers” is the category I found the one that I needed.
Overdischarge failsafe protector
One additional part that I added to my build is the overdischarge failsafe. It turns off everything taking power from it when it senses that the battery voltage fell below a set threshold. Batteries, especially lead acid ones should not be discharged too much, as then they can lose a lot of their capacity. 12V lead acids should not be discharged below 10.5-11V. You can go without this failsafe by just being careful and regularly monitoring the battery voltage while using it, I wanted to be double sure and have an automatic cutoff in case I forget to turn the equipment off or something breaks.
YUASA REC22-12 battery — 370PLN — 83EUR
Qbrick System Pro 500 Basic tool box — 65PLN — 15EUR
16AWG silicone cable (3 meters should be enough with a safe margin) — 7PLN — 2EUR
XT60 plug and socket (x2) — 10PLN — 3EUR
5 pin electric connectors (x2) — 6PLN — 2EUR
10A fuse with mount and cable (pack of ten) — 14PLN — 5EUR
3A USB 12V charger — 40PLN — 10EUR
15V analogue voltmeter — 10 PLN — 3EUR
assorted car cable connectors — depends on the pack, but around 15PLN — 5EUR
overdischarge protection — 20 PLN - 6 EUR
total: 557PLN — 134EUR
I am not providing any links as such small electronic components (with the exception of the battery) usually are not of a specific brand, so the best way to buy them is just to search for their name on any electronic components store and buy the that looks right.
For this project it is required that you have a soldering iron and know how to solder, as the connectors will be soldered.
Additionally you need a 12V lead acid battery charger, any car battery charger will do just fine.
For the tools, a drill and a hole saw are required. A glue gun and, a stripping tool and a connector crimper will be useful.
The overall design of the electrical system is simple: a power source and four receivers: a voltage meter, one USB charger and two XT60 sockets. From the battery power goes to a cable box that splits the power to all receivers. The receivers are connected in parallel. This way they all receive the same voltage, 12V, and the current taken from the battery is the sum of currents used by the individual receivers.
I have chosen 16AWG silicone cables as they should handle the currents I am forecasting to be used in the setup, and are resistant to heat and cold. One thing I could have done better was to use different colours for positive and negative, but I handled it with some red markers.
Battery and power switch
I started with fitting the battery. When looking for both battery and the case, I matched the external size of the battery with the internal volume of the case, so the battery fit snugly inside. I put some cardboard to fix it in place. If you plan to carry the powerbank a lot over rough terrain, consider fixing the battery in a more robust way, maybe with some angle brackets.
With the battery done, I drilled a hole for the switch. I used a small diameter hole saw, placed the switch and fixed it in place with hot glue.
The next part was the connection between the battery and the switch. It does not really matter from which terminal of the battery you start, I went with the positive side as that was closer to the side of the case I decided to be the control panel. For the connection. I used one of the fuses. The fuse casings I bought already had cables attached to both sides, so I just crimped the right connectors to both ends. For the battery end I used an M5 open connector, but the switch end I used the connector that came with the switch itself.
The switch has three connectors, not exactly sure why, so by testing I found out which two make a connection when the switch is ON, and I connected the lead from battery to one of those. One connection done.
Optional overdischarge protector
I went with adding an overdischarge protector but you can skip it. In my setup, I first two the PCB board and screwed it to a piece of plastic. Between the board and the plastic I placed a thermal pad I had lying around from some M.2 hard drive. I added it to have a soft base for the PCB and for some thermal conductivity. Then I glued the plastic piece to the case with hot glue. I connected the negative lead from the battery and positive lead from the switch to the IN port of the protector, and placed leads from the OUT port to the cable boxes. Finally, I configured the protector to turn off the battery at 11.5V. The battery manual says it can be discharged down to 10.5V, but I went with a higher value to increase its longevity.
Cable boxes and voltage meter
Next part was to prepare the two cable boxes which will act as manifolds to distribute the energy to the devices. I went with five element connectors to have four devices available (the fifth cable goes to the battery). I glued them to the bottom of the case. To one of them went the cable from the switch, to the second one I connected the negative terminal of the battery.
From now on, make sure that the power switch is in the OFF position, or disconnect the battery from the cable boxes!
Now came the time for the first device that will receive the electricity coming from the battery, and that is the voltage meter. The one I have has a round casing and for screws to secure it in place. I drilled the whole for the casing with a large hole saw, and eyed four holes for the bolts. It got in nicely, and I put nuts on the bolt to make sure it does not fall out. I placed one cable from the positive cable box and one from the negative cable box and connected to the meter by round cable connectors. Easy stuff.
Now, when you turn on the switch, the voltage meter should show the current voltage. If the meter needle goes the other way to negative it means you connected the cables the wrong way round. Ask me how I know this.
Now the hardest part. I had to make square holes in the box to fit two XT60 connectors. I tried using a hairsaw, I think that’s the correct name? but it did not work out. I eventually drilled some holes with a wood drill, and connected them with a knife. Once I had large enough holes, I soldered the cables to the XT60 sockets and fastened them to the plastic case with small screws and more hot glue. The XT60 sockets have small + and - signs on them, make sure you connect them the right way to the cable boxes.
The last step was very simple, I had to drill a round hole like for the power switch, I placed the USB charger in the hole, secured it with the provided plastic nut, and finally I took two lengths of cable, crimped car connectors at the ends, and connected the USB charge to the cables boxes.
Before plugging anything of value, I tested the powerbank with a multimeter. I turned the switch on, and checked the sockets whether they had the proper polarity and voltage. I then tested the USB charger with a cheap USB light. Everything was working fine, so I plugged in my phone, and it started charging! Finally I plugged my whole astrophotography setup to one of the XT60 sockets and it booted and operating like normal. What a relief.
12V light source
One of the use cases I have for this powerbank is to power the lights for my garden shed. I did not want to use LED strips or anything similar, as the lighting was meant to be warm and house-like to make the whole place cosy. After a lot of searching I finally found a lightbulb with a warm tone and a standard E14 thread that runs on 12V voltage. For the mount, I bought a simple IKEA lamp, cut off the 220V plug and soldered an XT60 connector. Works like a charm and gives really nice light.
I don’t want to talk too much about my astrophotography setup here as there will be a separate post about it, but to give a tl;dr version, I am using an Astrojolo Astrolink 4 Pi that takes 12V from the powerbank and uses it to power a Raspberry Pi together with all the other astro components that need the electrons to operate.
Real life usage
Because of bad weather so far I had one chance to test my project in full, and it worked perfectly, provided stable power for three hours and still had charge for a few more. And I could charge my phone at the same time.
To charge the powerbank, I am using a standard car battery charger. For now I am putting the charger clips right on the battery terminals, but one of my improvement plans is adding a separate connector only for connecting the charger. It will be a nicer solution and won’t require opening the case to charge the powerbank.
In the future I am considering adding another battery to connect them in parallel, doubling the overall powerbank capacity. Maybe I will also add a 220V converter, but for now I don’t really have a use case for it. An USB-C power delivery charger would be awesome, but I have yet to find one that does not look super cheap and suspicious. And I don’t want to plug my laptop into something looking like a 5$ e-waste.
So, this is it, I hope you like it. If you have any questions, comments, suggestions, or want to show your power solutions, hit me up on Mastodon or send me an email. I would love to get any feedback from you, even a short email saying “I read it until the end”, or “this post sucks!!!!11one” will be appreciated. Links at the bottom of the page.
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