Introduction: Portable Power Box

About: I have a lifetime of working on things. Jack of all trades, master of none.

I had some extra parts laying around that needed a purpose and fortunately they fit together as well as if I had bought them for this purpose. That purpose is to provide a useful amount of power to an inverter in a compact portable package.

Just so happened that I had an extra Pelican 1460 case that I ordered without trays in error. I also had four batteries on hand which were previously purchased to be used to increase the run time of my MX650 build (, but then I decided that they weighed more than I wished to put into the bike. The inverter was purchased to install into my old RV, but I had not got around to installing it yet. Extra wire and electrical fittings are always at hand in my garage, although not always in the color I want.

I tried the batteries and inverter for fit in the Pelican case and an idea was born. I was going to build a portable battery box to power things around camp and to run lights and toys in my Brother’s backyard play house for his kids. It turned out that the Pelican case would perfectly fit two more batteries, so I asked my Brother pitch in too. He bought an additional two batteries making the count 6 each 22 Amp Hour Sealed Lead Acid Batteries for a combined 132 Amp Hours.

Step 1: Parts List, Tools, and Safety Warnings

SAFETY FIRST – Electricity can kill. So please have a basic understanding of the risks involved before proceeding with this or any other build. Solder in a well-ventilated area. Use caution when working with power tools and always wear proper PPE (personal protective equipment).

Tools used in this build include: Drill, Soldering iron, Pliers, Wire Stripper / Crimper, Jig Saw, Tape Measure, Vise, Marker, Knife, Screw Driver

Parts List:

6 Each 22Ah SLA batteries – Mine came from Monster Scooters $350

6 or 8 gauge wire and ring connectors – I had on hand but these can be purchased at any automotive or hardware store.

Metal Strapping Tape – Also known as hanging tape I believe and is available at hardware stores or lumber stores.

Inverter – I started with one that I had on hand 750W/1500W and ended with another due to power requirements 1500W/3000W – From Harbor Freight roughly $140

Screws & Bolts – few are required for this project and I had them on hand.

Miscellaneous foam to support batteries – I have some on hand. Wood scraps or other material could be used instead.

12 Volt battery Charger – I have a couple of different car battery chargers on hand. Any 12 volt charger will do the trick.

Pelican Case – I used a Pelican 1460 case available from roughly $175

Step 2: The Build

The build is straight forward.

I installed the six batteries into the lower tray of the Pelican case and supported them with foam to hold them in place. Because the box will never need to tip on its side this will offer plenty of support, even without securing them to the box. I then cut the lid to hold the inverter and fastened the inverter to the lid with metal strapping tape and screws. The batteries are wired in parallel and then connected to the inverter. I soldered each of the ring connectors to the wires and I used 6 or 8 gauge wire that I had on hand. Don't go too thin on the wire as this system is under a large load.

Note: Inverters can generate a fair amount of heat and should be mounted in areas of good air flow. Although I did not worry too much about this as the inverter I used has two built in cooling fans. It is always a good idea to let them breathe though so I cut the hole a little larger in the front of the lid to allow some air in and around the inverter. Never wrap an inverter in rags or place onto combustible material.

I added a 3D printed plate to dress up the rough hole cut in the front of the lid with a jig saw. I painted it with paint left over from a recent project that came close to the color of the case.

That is all that is to this build. Pictures should help illustrate how simple this build is.

My battery box is very well packed with dense lead acid 12-volt power. it weighs in at 95 pounds and therefore is best moved by two people even though the box has handles.

It would cost somewhere around $700 to assemble all the parts

Step 3: The Principles Behind the Build

I now have six 12-volt batteries supplying a useful amount of 120-volt AC power. Why does it all work though?

I may get myself deeper into this question than I want to, but I am going to try to clear up (define) some basic electrical terms and principles. Sometimes I need to look up some of the following terms and principles to get the desired results from projects that I build. So I thought that I would share some useful terms and concepts. I should state that I am not an electrical engineer, nor an electrician, so feel free to correct me if I get anything wrong and I will correct it. You may ask any questions you wish, but be aware that I may not know the answer.

What are parallel circuits and how to they compare to wiring in series? In my projects I am often referring to Parallel or in Series wiring. Batteries in parallel are wired plus terminal to plus terminal(s) and negative terminal to negative terminal(s). This does not change the total output voltage of the batteries. An example is 6 each 12 volt batteries in parallel yields 12 volts of power. That is how this battery project is wired.

Batteries in series are wired Plus terminal to Negative terminal and so one. With each battery adding its voltage to the last. An example is three AA batteries at 1.5 Volts each yields voltage of 4.5 volts when wired in series and only 1.5 volts when wired in parallel.

This can also refer to say LED bulbs as well. Let’s say we are using bulbs requiring power of 3 volts. These bulbs when wired in parallel will only need 3 volts supplied to them. Whereas the same bulbs requiring 3 volts when wired in series will require 6 volts for two and 9 volts for three.

Another note when combining batteries in either parallel or series they should be the same type of battery with the same ampere hour (Ah or mAh) values. These principles are noted in photos above with inserted notes calling out details. The screen shots are taken from " Tinkercad Circuits", which is a very nice new tool in Tinkercad.

What is an inverter and how does it work? An inverter converts electricity from DC current to AC. I am not going to get into much detail as to how it does this, but it increases the DC voltage and then changes it to alternating current before sending it off to the device. You need to know what the power requirements are of the device you are going to power with your inverter and what is the power of the source feeding the inverter. Most of the time the source will be 12 volts DC with 120 Volt AC output. You may be able to get by with a small 400 W inverter or you may need a 3000 W inverter depending on what you are powering: a light bulb or a circular saw. So, determine that the inverter you use is greater than the start-up (surge) power needed by the device(s) you plan to power with it. Also, be aware that the sine wave output of an inverter is often a “square” wave (modified sine) rather than the nice even rounded sine wave of AC power from the wall. This may not matter if you are powering an electric motor, but it might matter when powering communication, medical, or navigation equipment for example. (sine wave and power requirement chart in photos above)

AC vs DC power – AC, alternating current, is what you have at the outlets in your house. DC, direct current, is what you find in batteries of all kinds; such as the battery that powers your car or the AA batteries that you put into your remote control.

In DC power the direction of the electrons flows from the negative terminal to the positive terminal in one direction like the movement of water through a hose. DC power is generally used in lower voltages than AC power.

In AC current the direction of the electrons is constantly switching directions. Up to 60 times per second in most US power systems. AC power is easier to operate at higher voltages than DC power.

What happens to amp hours when you wire batteries in series vs in parallel? Amp hours when batteries are wired in series equals what the batteries read. If in this build I had wired all 6 batteries in series instead of parallel they would yield 72 volts, but only 22 Ah. Whereas the 6 batteries of this project are wired in parallel at 12 volts and together they yield 132 Amp hours. Yeah!!!!

What is an SLA battery? SLA = Sealed Lead Acid. Meaning a DC battery that will not leak if tipped over or mounted on its side.

What are sine waves and how does it affect power? With AC power from "The Grid" the sine wave is very smooth like that waves in the ocean without wind yielding smooth well rounded peaks and valleys. With AC power generated from a DC source with an inverter you can have fairly “square” sine waves. This is generally not a big problem if you are powering a motor, lights or other nontechnical items. However, if you are powering navigation, medical, or communication equipment this can create interference. Invertors can be built to provide clean sine wave power, but the cleaner the wave generated the greater the cost of the inverter.

What is Amperage (A)? The number of electrons is measured in Ampere (Amps) and is known as current.

What is Amp Hours (Ah)? Think of Ah as the fuel tank. Where Ah is a unit of electric charge multiplied by time. This is equal to the charge transferred by a steady current of one ampere flowing for one hour. You will often see this expressed as milliampere, mAh, which is one-thousandth of an ampere hour.

What is Wattage (W)? Wattage is the measurement of power required to run a device and is measured in Watts. Another way of putting this is: Electric power is the rate, per unit time, at which electrical energy is transferred by an electric circuit. Where one watt equals one joule per second. This is the gauge for the size of inverter needed to power your device or devices.

What are Ohms? Ohms are a unit of measurement for electrical resistance. Some materials flow power easily while other materials create resistance thus impeding the movement of electrons. This is also why you need to use 8 gauge wire at a minimum (6 gauge is better) so that we can get the flow needed between batteries and to the inverter without melting wires.

What is Voltage? Electric pressure (potential energy) between two points measured as a volt.

Step 4: First Use in the Real World

The first task we used the portable power box for was to power a baseball pitching machine. Unfortunately the first inverter I installed in the box was a 750 Watt / 1500-Watt peak inverter and it was not enough power to run the pitching machine for my Son’s little league team. Batting practice is important of course so I bought a 1500 Watt / 3000-Watt Peak inverter and installed it into the box in place of the smaller inverter. The larger inverter works, but it too goes into alarm/cut off and it blows the fuse in the pitching machine if I do not start the fly wheel spinning by hand before turning on the pitching machine’s power switch. I really can not say why this happens as the pitching machine will start fine when plugged into an AC wall socket without blowing the fuse. I think that it may be how the power is delivered from the inverter or perhaps it may be the sine wave of the power delivered as discussed earlier. The pitching machine takes the 120 V power supplied by the inverter and converts it back to 90 V DC power to run the motor.

There has been a couple of solid feedbacks from others on Instructables regarding why it blows the fuse on the pitching machine without a rolling start. Here is one of the comments that I feel explains well the situation: In relation to your comment on why the fuse blows using the power box and not when using a AC supply. Most inverter's put out a square wave, some produce what is called a pseudo sine wave which is a stepped square wave that grows and decreases in height (voltage) by steps following approximately the curve of the 180 degrees of the sine wave cycle in the positive and negative directions, this type of inverter behaves nearly identically as the AC power from the grid, but the inverter's that do no do this, that are just square wave generators as you can see with an oscilloscope you have basically two types, one that generates a square wave that lasts the 180 degrees of the positive and negative cycle, the other type generates a square wave that lasts less than the 180 degrees of the positive and negative cycle. The inverter's of the first type, the output voltage I believe must equal the RMS voltage of the sine wave, if it does not and the output is higher between the RMS and peak value it will probably blow the fuse depending on the time lag that the fuse has and the starting current that the motor draws when starting from a dead start (all motors draw varying amounts of power from a dead start that can vary from 3 to 10 times their running power). The inverter's of the second type as they generate a square pulse for a square wave that does not last as long as 180 degrees of the cycle are obliged to put out a higher voltage than the RMS value so as to integrate the same power as a sine wave does over the full 180 degrees of the cycle. If your inverter is of this second type you must be careful when powering equipment that include MOV's in the power supply as protection against spikes on the power line, as in many cases the voltage level may enter the protection range of the MOV and it may explode or in a worse case catch fire. I only trust an oscilloscope to give me the real values of voltage that any of these inverter's put out. Best regard's. JohnH848

It really is nice to have the pitching machine running in complete silence from the power box and rather than powered from a generator running in the infield.

I anticipate many years of use from this box for all kinds of tasks from powering my niece and nephew’s backyard play house to recharging the Razor motorcycles that I build (see ). The box is heavy to move therefore it will not be used for everything, but it is a nice option to have when needed. It is obviously much quieter than our generator and provides the same Watts of usable power. Of course the generator will provide power for as long as I have gasoline, but eventually I do need to plug in the batteries for a recharge of the battery box.

I hope that you found some value in this write up. Questions or Comments are always welcome. I will do my best to answer in a timely and accurate fashion.

Thank you. Schockmade

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