DIY Solar Power Station

I found some small lead-acid batteries at my local surplus store and felt inspired to create my own solar power station.

My goals were to build something useful, recycle these batteries, and learn something along the way. I’ve seen YouTubers like Julian Ilett, Great Scott, and Big Clive tackle related projects and I felt that I had enough relevant tools and information to take on a project like this.

The Plan

My plan was to put two of these batteries into some kind of plastic container, connect a charge controller, and make it easy to connect and disconnect a solar panel. I wanted 12-volt output and 5-volt USB output for charging my phone and USB battery packs. I also wanted some kind of visual indicator of how full the battery is. This would give me a portable, 120 watt-hour, off-grid power pack that could operate anywhere the sun shines.

After measuring the size of the batteries, I determined that I could safely fit two of them into a plastic ammo box, the kind frequently found at surplus stores. The ammo box has a handle on its lid, so moving the batteries around would be convenient. This is important because lead-acid batteries are surprisingly heavy for their size. Being plastic meant that I could cut holes as needed with my rotary tool. After getting the box home, I found that I could fit three of the batteries in the box with just a smidgeon of room left over. Being able to use three batteries increased my target capacity to 180 watt-hours!

Charge Controller

Initially, I thought I could connect a solar panel directly to the batteries, but I learned that this is incorrect. The high voltage from the panels can cause a chemical reaction in the batteries that reduces their capacity and output. A charge controller modulates the power from a solar panel to a level that is safe for the battery. It prevents over-charging by disconnecting the solar panel from the battery after the battery reaches a target voltage.

I selected the Renogy Wanderer 10A charge controller because it was somewhat cheap and Renogy provides documentation on how to use it. The Wanderer has an LCD display that shows the voltage and current coming from the solar panel, the battery voltage, and the current on the output when the load is enabled. It prevents over-discharge by disconnecting a load when the battery voltage falls below a given limit, also called a low-voltage disconnect. It also includes two USB outputs! At rest, the Wanderer consumes only 10 milliamps of current, so it won’t drain the batteries all by itself.

Assembly

Using a solar panel and charge controller, I charged each battery on its own in order to bring all three batteries to the same voltage. I wanted to connect them in parallel and this step was necessary to prevent a large flow of current between batteries. Once they were at the same voltage, I connected all three, in parallel, to the charge controller using 18-gauge wire and Wago lever terminal blocks. This all fit neatly into the plastic ammo box.

I wanted to connect the solar panels to the charge controller inside the box without having to leave the lid open. Ideally, I would have used some kind of panel-mount socket, but panel-mount sockets for the XT30 connectors I’d settled on were impossible to find. I didn’t want to get sidetracked making my own. I fit the solar panels with an XT30 connector and cut a small slot in the box to run wires through. Because the handle is attached to the box’s lid, this also let me use the handle to move the box while the solar panels are connected.

Testing Capacity

Based on the labels, three 5 amp-hour, 12-volt batteries should provide 180 watt-hours of energy. After charging the battery pack to 13 volts, I ran a capacity test by attaching two fans to the output and letting it run until the charge controller turned off the output. I measured about 700 milliamps of current and it ran for fourteen hours. This means the battery pack gave me 117 watt-hours, 65% of nominal capacity. To give you a relatable example, that’s equivalent to 23.5 amp-hours at 5 volts, meaning I could charge about two of those 10,000 mAh lithium battery packs, probably less due to conversion losses, usually around 15%. This is somewhat disappointing given the price I paid for the batteries relative to what they cost new. But, hey, can you put a price on the feeling you get by keeping batteries out of the landfill?

My build cost was $130, not including the cost of wires and connectors. That cost also does not include all the parts I bought that didn’t make it into the final build. I’ll chalk that up to R&D costs. That works out to $1.11 per watt-hour. A new, 12-volt, 15Ah lithium battery can be had for about $60. Without discounting that capacity rating, using such a battery would have brought the per-watt-hour cost down to $0.81. More importantly, doing so would nearly double my battery pack’s capacity for a paltry $15 more.

Was it Worth it?

I believe I met all the goals I had for this project, but would I do it again? Certainly not.

As of today, I can buy a new, 256Wh power station with solar panel for $210. That’s $0.82 per watt-hour and it includes an inverter that can supply standard 120-volt A/C power, USB sockets, USB-PD sockets, 12V barrel sockets, and a 12V vehicle dash socket. It also weighs 8 pounds while mine weighs nearly twenty pounds. Larger power stations can get you down to around sixty cents per watt-hour, though they cost a lot more. (I only considered power stations that include solar panels in my comparison.)

My battery pack, then, is the most expensive per-watt-hour option and has only USB and bare terminals for 12-volt output on the Renogy Wanderer. The only feature it has that the others might not have is the load-switching feature of the Wanderer. It can be configured to turn the output on when the output from the solar panel drops off. This could be used to, for example, turn a light on automatically when it gets dark outside and turn it off when the sun comes back out.

Things I learned along the way: