Hello! My name is Nathan, and I am an amateur tinkerer. I needed a power bank for camping, bonfires, emergencies, and general tom foolery. Given my tinkery nature, buying one seemed some what sacrilegious. My build CAN be cheaper than your average power bank and will certainly have more functionality than most. My goal was to build a 12v system that had the following features:

Anyone can put it together
Has the capacity to last 10+ hours under moderate use.
Has some modularity to design
Doesn't modify basic components too much
Has solar charging capability.
Is splash resistant

After doing my design work, my power bank has the following features:

Flexible/detachable 18v solar charger
20AH 12v SLA battery
2 AC socket/2 USB socket inverter
Solar Controller with 2 USB sockets
2, 12v car sockets (1 connecting to the inverter)
SPST Toggle Switch for Power Cut-Offf
Sturdy/Splash Resistant Dewalt Tool Box Case

***DISCLAIMER***

I am NOT a certified electrician. Any instruction I give to you is second hand and reinterpreted from actual professionals; meaning what I give you may be the Google Translate equivalent of what you need to know. If you do not use proper safety precautions you WILL get hurt and maybe die. Don't do that.

Step 1: Still Here? Great! Here's What You Need.

I sourced almost all of my parts from Amazon due to the fact I get rewards for doing so (and 2 day shipping on most items). Those that didn't come from Amazon came from Lowes (most of the wiring equipment) though so that you have a picture of what I'm looking at, all things link to Amazon. Lowes was INCREDIBLY better priced for anything to do with actual wires though not necessarily the tools to manipulate them. You may be able to make this build a lot cheaper sourcing from electronics websites or hardware stores, though expect to wait a bit longer for parts and make sure you're not getting over charged for shipping. You can change pretty much all of them out with other brands/shapes/sizes but be aware of power values and functionality as that is something you cannot change safely. If you don't count tools (and I never do because I always reuse tools) the build is a little less than $200. Places you can definitely cut corners would include: battery capacity (NOT VOLTAGE), connectors for the wires, the container, the inverter (go for fewer ports, not less protection), the toggle switches (go less fancy than I did). My guess is if you do some of that and are a little bit better about sourcing materials you can make this build for around $150.

Materials

12v 20AH battery
18v Solar Panel - I liked this one because it already had built-in protection.
Sturdy Container - I found a toolbox works really well.
12v DC to AC Inverter
Solar Controler
12v Car Socket Splitter
SPST Toggle Switches - One costs half of what five cost for whatever crazy reason.
10 Gauge Red and Black Wire - You can add Yellow and White if you want. The solar panel and inverter that I picked come with their own cabling (a set of alligator clips and a 12 volt plug set). Most of those parts will usually mention if they have cabling, and you can factor that into how much you'll need.
10 gauge terminal connectors - Worth buying after you see the terminals on your switches
Solder - If you don't use rosin core, you need flux. Might wanna get flux anyway.
Battery Charger/Maintainer - For maintaining the battery when you get home/after not using for a while. You will need this to charge the battery after using the power bank for an extended period as the solar panel can only keep up with the load so much.

Tools

Wire Cutters/Strippers - This was cheaper on Amazon oddly enough.
Standard Soldering Iron - This is actually a REALLY good Amazon deal.
Handy Dandy Leatherman - If you don't have one, you can probably get away with needle nose and a good knife, but multi-tools are always useful. There's no particular reason I picked the Surge other than that's what I happen to have. Any mid to full-size multi-tool is gunna work fine.
Drill with Bits - If you don't have a bit set, I'm going to recommend you get a basic one. If you buy all of the bits you need for this project individually, you would have already bought a bit set. This link is another good Amazon kit deal if you don't already have a drill.
Rotary Tool - You'll need one with a cutting wheel, a stone grinder, and maybe an etcher depending on how you want to label things
Electrical Tape
A Sharpie

Step 2: Ready the Brain!

Take all the things out of all the packages. We're going to start by finding a spot for your solar controller. The solar controller (SC) is going to be the brain of the operation. It will control everything from the solar panel (SP), the battery and the different devices you're connecting to the battery. You can see here that I placed it in a central location. You want to make sure the device has enough clearance for the wires to bend at basically a 90 degree angle. Your design may not bend like mine, but that's generally a good idea to do to assure you're not damaging the controller or the wire.

Once I had the SC in the spot I wanted, I drilled holes for the panel mounts and made sure it fit in place with its' nuts and bolts. I marked about 3/4 of an inch out from each socket and then drilled holes there for the wires. Those holes need to be about the size of 10 gauge wire, maybe a little bigger. You can see that I marked the polarity of each wire. That's going to be redundant in most cases as the SC, your components, and the wires themselves will be marked, but that was helpful to me. Then I undid all of it. Why? Because it is a pain to try and bend wires into those ports.

I then took one of the alligator clip to terminal wire sets (from my inverter) and cut off the alligator clips, though you can use your own wire if you want to. Save those alligator clips, they're pretty useful and you can reattach them to other wires. Now, before I tell you want to do with them, let me tell you what I didn't do them: I DID NOT CONNECT THEM TO THE BATTERY!Connecting the wires to the battery will be my last step aside from beautification and christening.

Once you have those wires, push them through the appropriate holes (the middle two in most cases) and attach them properly. You may need to open the sockets they port into before putting them in. They operate on a righty-tighty lefty-loosey kinda deal. Remember, your red is your positive and your black is the negative. While you may have no charge right now, it's a good idea to get into the habit of connecting your positive wire first.

Step 3: The Brain Needs Limbs!

Now that you have the battery connector wires connected to the SC, it's time to give the SC something to do. We're going to take one of the solar panel's (SP) wire sets and disconnect either the alligator clips or the 12 volt car plug (CP). There are advantages to both, but what I chose to do was cut the alligator clip set. This was almost entirely by accident, you should probably cut the CP. You'll still be able to use the SP for your car in case of emergencies but aside from the inconvenience of having to lift the hood up on your car with the alligator clips, you'll be able to place the SP in more direct sunlight than you would the CP.

To disconnect the CP, you basically want to dismantle the plug to reveal its' wires/PCB. You can just cut it off near the edge, but I try and use as much of the wire in each device as possible. Most come apart by unscrewing a screw and some gentle encouragement. Try and cut or de-solder the wires as close to their connections as possible. The polarity of a CP is pretty simple: the tip is the positive and the side leaf springs are the negative. You may want to label the wire polarity as there's no guarantee that they will be marked.

You're going to do the same thing you did with the battery connections, only to the left side of the SC. Because our SP is going to rest on top of the build, I drilled a hole in the top and put the wires through. I also drilled into the main console to put the SP wires into the bottom of the system. I *tried* to A) have good cable management and B) have as few things for others to break as possible as I will not be the only one using this. Do NOT connect those wires to the SP. You can, it probably won't hurt anything, but don't. My thought during this process was to have as little interaction with a power source as possible until I was sure all connections were safe.

Now you'll move on to the 12v car socket splitter (SS). Here's where it gets a little tricky. Whatever SS you ended up buying needs to consolidate down to one positive and one negative wire. Mine just so happens to already be that way (like I planned it or something), but some will consolidate inside the CP itself. There's a number of ways to do this that I will cover in the next step. Again, like the SP and battery connections, you're going to connect the SS to the right side of the SC which is usually marked with a light bulb (the only non-obvious marking). At this point, you can remount your SC.

You may be wondering, "Why use the SS? You already have an inverter, why not just use more inverters?" I'll tell you why. One of the conveniences of the CP is that the - majority of the time - whatever has it is going to be 12v, likely be built with safeguards (as many DC devices are) and shouldn't be over taxing to the system as they are intended to run on a battery that's main purpose is powering your car. This can be anything from more inverters, phone chargers, hot plates, coffee makers, fans, and blow dryers. I wanted one socket to be free specifically so if I needed any of these wonderful devices, I could use them as need be.

Step 4: Here's Where It Gets Tricky..

You could stop here, cut the holes where you want to mount your inverter and extra socket and call it a day. The system (once connected to the battery) would work from here. The SC has safeguards that will keep all three components connected to it relatively safe, but I couldn't help myself. I wanted some extra safeguards - extra controls. I'm doing that, by adding an SPST toggle switch (SPST). You can see by my little markup of the build that I intended to put SPSTs in between the components and their sockets as well, but for the sake of getting the build running, I haven't done that yet. It would be the same process either way.

The SPST
Because I intend to connect the battery to a battery maintainer when I get to civilization, I wanted to put a switch between the SC and the battery. This is mostly so that the battery maintainer doesn't screw with the SC when charging the battery, and also to make it easier to charge down the road. Realistically, you could just unscrew the terminals from the battery and then connect the maintainer, but I don't want to have to keep insulated pliers around, nor try and keep track of bolts all the time. I realize if I'm at home, this isn't really a big deal, but if I'm at say Miguel's Pizza and they're kind enough to let me charge up there, I don't want anyone to feel like I'm wiring up an IED or turn into that guy from Home Alone.

So, here's how we do that. Place the battery in the build. DO NOT CONNECT IT TO ANYTHING. You're going to first place your switch where you want it, making sure that it does not get stuck on the battery, then measure how much of the positive battery connector wire you need to make it there from the SC. Give yourself at least a centimeter more for wire stripping so you have exposed wire soldering or adding a crimp connector. That wire will connect to the accessory terminal. They're either marked ACC or have an odd symbol. If you're unsure, google it, but the ACC is usually the middle one or the right one if you have a ground that's not on the bottom with the other terminals. Take the positive wire that will go to the battery and wire it to the positive terminal.

If your SPST has a ground, you now have a decision to make. Your switch as it is right now will function just like an on /off switch, all you have to do is connect the negative wire from the SC to the battery. If you choose not to ignore the ground, you'll need to connect the battery to the ground and then the negative wire from the SC either to the wire connected to the ground or directly to the negative terminal on the battery. Either way, if you choose to do the ground you're going to be doing some extra splicing and dicing.

There are a good amount of different ways to do the ground connection. You can get a 3-way connector box or you can suck it up and solder what's called a 3-way splice or "T-splice". Here's a video demonstrating a good way to do a T-splice. If you're not familiar with soldering, I recommend watching a tutorial on that first. The diagram above is a good example of how the wiring should look with the fan representing your SC. A lot of times the term "ground" is pseudo-synonymous with negative as it is in that diagram.

Step 5: It Lives!

At this point, your battery bank should be fully functional - at least once you connect it to the battery. To give power to the SC and allow the SC to charge your battery, you just turn the on switch on. As you can see in the pictures, I mounted the inverter and the spare CP into the main console. I basically just traced around both with a pencil and then cut out their silhouettes with my rotary tool. One thing I will say about doing this is that less is definitely more. I checked the fit frequently to make sure I wasn't going to have *too much* hot glue holding everything in place. This was another one of those steps I did to keep people's hands out of the wires on the build.

You will need to do some cable management to make the build a little bit easier to manage when doing maintenance/modifications. You will also need to fix the battery into the build so it's not rustling around in the box. The battery weighs a little more than 12 pounds so if you can manage to put it in a central spot, I would do so. I used aluminium strips to build a frame around the battery and then used screws to fix it into it's location in the build. There's a lot of ways you could fix it, but DO NOT forget to do so.

That's about it. At this capacity, you can charge your phone about 20 times or so even without the solar capabilities of the battery bank. I even had enough room that I could have added a second 20ah battery. With the AC plugs and multiple USB outlets, you have more than enough space for you and your friends to charge up overnight. Hope this was helpful!