What do an off-grid camping trip and a power outage have in common?
Well, in both cases it is good to have a battery backup at hand.
Fortunately for you, I am going to show you how to build one yourself. I know that there is already a wide variety of these kind of projects out there which is a good indicator to show you how easy it is. However, I'm going to show you how to build my particular version, which is a plug and play solution that is super easy to use and nice to look at.
I also created a project video if you prefer hearing my voice while watching me at work.
Without further ado, let's get straight into it.
Here is what you are going to need for this project:
• toolbox that fits your components
• 12V sealed lead acid battery (used ones are super cheap on ebay)
• solar charging controller
• dc jacks and sockets
• fuse holders and fuses
• wire (I used 2,5mm^2)*
• crimp connectors
• a laptop power supply with an output voltage above 16V (for the grid charging option)
*Choosing the correct wire size is very important . If the wires are to thin they overheat and can cause a fire. A simple formula to determine the size you need is this one:
( I x L x 0,018 ) / U = A
I : the maximum current that will flow through your cables
L : the total length of the wires in meters ( in this case from the fuse box to your grounding)
U : The maximum Voltage loss you want to have in your wiring ( I recommend 0,5 V)
A : the right cross section in mm^2 (for americans there are tables to find the AWG equivalent)
Step 1: Case Modifications
After you decided where you want to place the switches, the meter and the in- and output sockets, you mark the spots directly on the box or create a few templates and glue them on. I decided to mount most of the components on the right side of the box and went with the templates which you can download here.
The heaviest components should be placed in the center of the box in order to keep the weight balanced.
To keep everything in the right place you need to add a few deviders to the box.
Use a measuring tape to determine the right side and cut 2 fitting pieces out of plexiglas or a different kind of material and hot glue them in place. It might be a good idea to fix them with a few extra screws because hot glue does not really hold up to sudden shocks, wich can occur if you drop the box (I learned that when I accidentally dropped it while making the final photos;) ).
For easier access to the controller you can build a little flap for the battery compartment on which you can mount it. However that is only possible if your box is high enough, otherwise you can fix it to one of the deviders.
Now you drill all the holes and use a file to get them into the right shape. Test-fit the components multiple times and keep on filing until they fit in tightly.
Don't make the holes too small because it might have a negative effect on components like switches.
Get a rainproof toolbox. However if you don't have one that is already waterproofed from the top and bottom you can go for a combination of plastic profiles and oceans of hot glue to waterproof each individual hole. This does not look very nice, but it works.
One more outer case modification is to add a few custom stickers to the outside of the box. I bought myself a pack of glossy label paper for laser printers on amazon and went to a nearby copy shop. This way one Printed page costs me around 1€. I'm pretty sure that those stickers will hold up against a few rain drops just fine and they make the box look much better.
Step 2: Hooking All the Components Up
I'm not going to show you how I soldered everything together instead I'm going to explain how everything is connected.
The wiring diagramm can be split into three sections:
1.The input section
This section is pretty straight forward, an input socket, which can be a 12V car socket, a solar panel socket or a usual dc socket is connected to the input pins of the solar charging controller.
2.The battery section
The battery is fused with a 15A fuse and hooked up to the battery pins of the controller.
3.The output section
This is where most of the action takes place. The output line is hooked up to the output of the controller and fused with another 15A fuse to protect the controller if there is something wrong with the outputs. All of your output sockets and integrated devices ,such as an RGB-LED controller in my case, are simply connected in parallel. You can add individual switches to the different outputs or place a single switch on the line coming from the controller.
You can see that there are two more things in the circuit diagram that are additional. The first one, which can be seen in the center, is a Volt and Ammeter. It has 2 connectors on it's back. The first one features three thicker wires, those are the measuring wires. The Current is always measured by the voltage drop on a resistor which is connected in a row with the rest of the circuit. The black and the blue or green wire lead to this sensing resistor inside the meter, therefore the black one is connected to the negative outpin pin of the controller and the blue one is connected to the rest of the output section.
The other two thinner wires power the meter. You can simply connect both red wires to the positive output line. However the black power wire is not connected to the negative output pin of the controller, instead it is hooked up to the negative pole of the battery. The reason for this is simple: The charging controller features an over-discharge protection. If the battery Voltage is lower than 10,5V it breaks the connection between the negative battery pin and the negative output pin in order to shut off the output section. We still want to be able to check the exact battery voltage after that point so the shut off circuitry is simply bridged. It is still a good idea to add a little switch to the power wire of the meter to turn it off when it is not needed.
The interior light
The second extra is an LED light that is supposed to come on when you open the box. It can be seen on the bottom right of the diagram. On the plus line you can see a momentary switch which is normally closed (NC) and breaks the circuit if it is pushed down by the lid of the box. This thing works like the switch that turns off your refrigerator light.
On the negative side of the lamp, which by the way simply consists of two 40 cm pieces of LED stripe, you can see a three position switch. The top position of this switch connects it to the negative output line. This is the normal operating mode. The middle position turns the light off and the bottom position is the emergency position. In case the battery is already empty and the controller shut the output section off you can use this switch position to connect it directly to the battery, just like the Meter. As the name already says this should only be used if you really need light after the battery is empty and leaving the light on for longer periods of time can damage the battery.
In the pictures next you can see how the connections look inside the box. I also hot glued some cable canal into the box to hide the wires inside.
Step 3: The Finishing Touch
When everything is hooked up the box is basically ready to use. However, I felt like there was something missing, a feature that would make the box unique and even more easy to use. My idea was to have a panel on the front that displays all the information that the controller gives me, such as the charging status of the battery, rather or not the output section is turned on and if there is an input source connected.
In my first attempt I simply opened up the solar charging controller and measured the voltage of the on board LED's, which was just 1.8-1.9V. The five type 5050 SMD-RGB LED's that i planned to use for the panel need between 2V and 3V. I tried to find a few resistors on the controllers PCB that I could exchange in order to get a higher voltage but wasn't successful. As the circuit confused me more and more the longer I looked at it I decided to take a different approach.
If I can't hack into the electrical signals of the controller I can use the light signals of the original red and green LED's and feed them into a circuit that determines rather or not they are on and switches the LED's behind my panel accordingly.
The simple circuit that can be seen in the first picture has a voltage divider consisting of a light sensor (LDR) that controls the base of a transistor that switches an LED.
The LDR has a high resistance when it is dark so the base voltage is below 0,7 V , therefore the transistor doesn't turn on. If light hits the LDR it's resistance lowers which leads to the base voltage increases to more than 0,7 V and the LED turning on.
This circuit is simply multiplied for 5 LED's.
I furthermore added a small timer circuit that turns the panel on for 15 seconds. I did this with another transistor and a relay. If i push the button the condenser is charged up when the button is released the condenser begins to discharge through the resistors R1 and R6. As soon as the Voltage is below 0,7 V the transistor turns off the relay, which turns the panel off.
I figured all of the resistor and condenser values out on the breadboard and they might not fit for your circuit, However here is a list of my components:
- R1 6x 10KOhm
- R2 5X4,7KOhm
- R3 3K
- R4 500Ohm
- R5 4X450Ohm
- R6 100KOhm
- C1 100 microF
- push button
- 5x LDR
- 12V Relais OMROM G5V-1
The sensor board has to be placed directly on top of the charging controller's LEDs and shielded from other light sources by applying some tape. The LEDs are hot glued into suitable holes behind the panel and the power wires are hooked up to the battery terminals of the controller. Fix all the components inside the box and you are good to go.
Step 4: Charging From the Grid
The solar charging controller is build to work with a solar panel or a wind generator. Those are self limiting current sources. They can't deliver more current then they produce. A laptop power supply or any other normal power brick doesn't have such a limitation. If you simply plug in a 19V 3,5A laptop power adapter, it will be totally overloaded and supply more current than it is capable of. You might hear a noise coming from it because it steps down it's switching frequency into the audible range. It can also get pretty hot and you will probably destroy it over time.
There are two things that limit the current flow through a circuit: The voltage that pushes the electrons and the resistance of the circuit they are pushed through. Usual lead acid chargers limit the current with a regulator like the LM317t until the battery reaches a certain voltage, then they switch from the constant current into a constant voltage mode to top the battery charge of.
A simpler solution is to add a Resistor to limit the current .
The calculation for this is simple:
The charging controller needs a maximum voltage of 14,4V and I want a charging current of 2,5amp ,which is below the maximum stated in the batterys dataheet. My power supply delivers 19V so I need to get rid of 4,6V.
4,6V / 2,5A = 1,84Ohm ~ 2Ohm
But of you can't just use usual resistors as they are only capable of radiating between 1/4W and 1/2W of heat. Our resistance will generate 2Ohm *4,6V= 9,2W of heat so we are going to need a high power resistor.
Untill this point this is just a concept. I still have to get the right resistors and build it in order to verify that it actually works. I will update you as soon as possible. The solution will also be presented in the second part of the project video.
Step 5: Take It on a Camping Trip
You can use it with any kind of solar panel with a voltage between 14,4 and 20V as long as it's current doesn't exceed the maximum charging current stated in your batterys datasheet.
I hope you enjoyed this write up as well as the video and I inspired you to build your own power backup box.
If there are any questions left or if you have any critisism feel free to write a comment. Of course I'm happy about some positive feedback as well.
Don't forget to give this project a vote in the solar contest to help me win a professional power backup for my more extensive future camping trips.
As always I'd like to thank you for taking your time and reading this and which you a great day.