I ended up creating a contraption that housed a 12 volt battery, inverter, and battery charger inside a watertight ammo box.
Step 1: Materials
-Power Inverter: Cobra CPI 480, 400 Watt ($26)
-12 Volt Deep Cycle Led Acid Battery: Factory Second 35Ah Sealed GEL Battery ($45)
-12 volt Battery Charger: Bass Pro Shops XPS i 5 ($72)
These parts are not too specific. I listed what each part is, then what I used and the price I got it for. All the things above I purchased on sale or with some discount (thrifty shopper).
Check online for an inverter, I found mine at amazon on sale at half price.
Get a Factory Second Battery. It may sound like a bad thing, but to those of you who aren't too familiar, a factory second or 'blem' battery is one that is cosmetically damaged, but otherwise fine. Buying a battery this way saves you at least half, which is upwards of $50. Also, don't get a GEL battery like I did. Get an AGM battery if you can. GEL batteries take a special charger because they charge at a different voltage.
I lucked out with the battery charger. I bought it in store with a 20% discount, and it charges GEL and Regular Led Acid / AGM batteries.
-Ammo Can: Find one that fits all your components. Mine is a long low 40 mm ammo can. 17" x 6.5" x 10.5"
-8 Gauge Wire: *It's important to have a heavy gauge wire to handle the large current load from the battery to inverter!*
-8 Gauge Ring Terminals
-Heavy Amperage Switch (Mine is a 50A from radioshack)
-40 Amp Marine Circuit Breaker
-1/8" Carriage Bolts, Nuts & Washers.
-1/2" Angle Iron
-1/2" Flat Aluminum Stock
-3/8" Threaded Rod
-1/4 20 Nuts & Bolts (Battery Terminals)
Step 2: Safety
Batteries are incredibly dangerous if you are not careful. Ever feel how hot a AA or 9-volt battery gets when it shorts? Imagine what a car battery full of acid could be like. Read this http://rayvaughan.com/battery_safety.htm about exploding batteries.
Never create a short between the two terminals of a battery, or drop a tool on the terminals, or let both touch the sides of the box at the same time.
The power that the inverter outputs is incredibly dangerous. This is the same voltage and current that comes out of the wall at your house. The 400 Watt inverter I used outputs ~3.5 Amps. Currents as small as 0.005 A to 0.006 A across the body can kill you.
Here are the facts about lethal electricity: http://hypertextbook.com/facts/2000/JackHsu.shtml
Now, don't let any of this information be off-putting. Taking all the proper precautions when working with electricity and batteries (like wearing Safety Glasses for starters) can help avoid damage to the components and serious injury to yourself.
Step 3: Theory
Electricity - The flow of electrons.
What is important here is the electrical values. Voltage, Current (amps), Resistance, Watts
-Volt: Electrical Pressure
-Current: Electrical flow
-Resistance: Resistance to electrical flow
-Watt: Electrical Power. Product of Current and Voltage. (W = C*V)
This formula above is how I calculated the current of the inverter. Yes it says it on the box, but its nice to know yourself.
Ex. 400 W = C * 115v; C = ~3.5A
AC & DC (not the band)
AC: Alternating Current
DC: Direct Current
I could go in depth, but all you need to know is that AC is what the inverter outputs and what is in your house, and DC is what comes from batteries (don't worry, it can kill you too).
If you want to know more about electricity, google is your friend. A link to start you out: Electricity
The Power Pack
This isn't too bad to understand.
When you are home, the battery charger takes 110v AC current (I can say that now and you should know what it means) and turns it into 12v DC to charge the battery.
When you are away from home, the inverter takes the 12v stored in the battery and turns it into 110v AC power for you to use.
Everything is then housed in a watertight ammo can so you can take it with you.
Step 4: Locks
I took a trip to the hardware store. I saw little 1" U bolts and thought that they would work better than a gate latch. So I used them instead.
If you follow 646bph's instructable you should be fine.
Step 5: Construction: Battery
The bracket is 1/2" Angle Iron welded together to form a base for the battery to rest in. Threaded rod is welded to the outside, and eventually a piece of flat aluminum with holes drilled in it slides down on top of the threaded rod and is bolted down to clamp on the battery.
The whole bracket is JB Welded and then tack welded to the ammo can itself.
The battery itself will be dropped in last.
Step 6: Construction: Battery Charger
Make sure that there is space between the components and everything fits. Both the inverter and battery charger will get hot when in use, and you want to make sure there is adequate space for ventilation.
Step 7: Construction: Inverter
I ran into a few problems when mounting mine. My inverter has a "clamshell" shape that hangs over where the mounting holes are. This prohibits me from using a socket wrench to tighten the bolts. The other problem is that the two bolts in the corner opposite of the battery are completely in-accessible with any tool once the inverter is in place.
What I ended up doing was tightening the bolts down enough to hold it before putting the inverter in, then using loctite and fix them in place. Then I was able to slide the inverter in and pop the other two bolts in place to secure it. There is a picture to show this.
The other thing is that once the inverter is mounted, the wiring to it would be inaccessible. So before mounting it attach wires to the terminals and snake them where they need to go. Then (in the next step) it can be wired to the battery.
The last thing I did was make a ventilation hole for the inverter. When it is running full load, it gets very hot. And putting it in a crowded metal box is not the best for airflow.
We drilled a hole in the side of the box and mounted a firetruck 220v watertight outlet cover over it. The hole was placed right below the exhaust fan for the inverter.
Now when the it is running hot, the little door can be opened to allow increased airflow, and when it's not in use, it can be closed and remain watertight.
Step 8: Wiring
I later added (and you can see in a few pictures) a LED power indicator, and a 12 volt automotive outlet.
They also get wired between the switch and the negative terminal of the battery.
The automotive outlet is a cheap extension cable from walmart that I cut the male end off and wired in. One like this.
The battery charger is permanently wired to the battery as well. It has its own marked leads that are bolted to the terminals as well.
I whipped up a wiring diagram to help illustrate as well.
Step 9: Finished!
There you go. When you turn on the main power switch, the LED power indicator will light up as well as provide power to the inverter and 12volt automotive outlet. After that, bam. Portable Power.