Fast, Cheap, Easy and UGLY Portable Boombox!

Bluetooth capable, battery powered sound system that takes less than an hour to build!

Be gentle, this is my first instructable!

Step 1: Pick Your Head Unit

Ok, you need something to play your tunes. My idea was simple, use a car stereo (head unit) to provide audio inputs and amplifier. I picked up my head unit on ebay for £16 (new). Mine doesn't have a cd player, but uses sd cards and USB sticks. The lack of a cd drive makes the unit smaller, lighter and uses less power. You can use pretty much any head unit you like, so if you have one lying around, great! Car boot sales are awesome places to pick them up.

Step 2: Speakers...

This is a very personal choice. Personally, I wanted cheap, but decent sound. I happened to have some Panasonic satellite speakers left over from my home cinema system, so I used them. These had the advantages of being compact, powerful (125W max) and free!
It's a good idea to get speakers that can handle more power than your amp delivers (mine was 4x50w, so 50W per channel).
N. B. Just because the speakers are rated above the output power of the amp does not mean you cannot blow them! At max volume the amp may clip (starts to sound terrible), and this can apparently blow the speaker, even if they can handle more power. My volume goes up to 40,so I keep it at 38 or lower.

Try to match the impedance to the amp. My amps 4x50W rating was at 4ohms, but my speakers are only 3ohms. This means the amp can provide more power to them as it's easier to drive a current. This sounds ace, but more current means more heat and can fry your amp. Use other impedances at your own risk!

My system only uses 2 speakers ( front channels) at the moment. I'm planning on installing some larger ones in my garage that I can connect the rear channels to when I'm in there for more volume!

Step 3: Power!

Ok, so now you need power. 12v to be precise. For prototyping I used an old external hard drive power supply to provide the juice, then ultimately used a lead acid battery. You don't need much power. You'd think that an amp that can throw out 4x50W would need around 200W at max output, however this is not the case. I measured the current with my fluke while playing via bluetooth at full volume, and the current has just over half an amp. That means the head unit was drawing just 6W! My power supply was good for 25W, and never even warmed up suggesting that the current measurement was accurate. I also suspect the head unit is putting out no where near 50w per channel!
P. S. Xbox power supplies are 12v, and old pc supplies can be used with a bit of hacking.

When hooking everything up, I just used terminal block. You can solder it if you want something more permanent and neater.
Some head unit have an ignition wire which will need to be connected to the positive terminal for the unit to power up.

Step 4: Charging

If you're using a battery for power, you're gonna need to charge it. I used lead acid cos they're cheap. You can also get very cheap, wall wart style chargers for them (first pic) . If you go with that option, be prepared for 10-15hrs charging from flat.
You can get better chargers, and if you get a fancy one with separate constant current and constant voltage phases, and active cutoff, then you can actually charge lead acid batteries pretty quickly (1C is easily doable without damaging the battery). With wall wart style, do not exceed C/3 (so for my 7ah battery thats 7/3 =2.3Amps charging current). If you're in to RC stuff, your lipo charger (second pic) can probably charge it beautifully. That's what I use.
Avoid car battery chargers. They're designed for much higher capacity batteries and will likely wreck it.

Step 5: Bluetooth

Right, for bluetooth capability you have two options.
1. Pick a head unit that has bluetooth already.
2. Pick one that has a USB port and and aux in.

Mine was the second potion. No built in bluetooth, so I bought a bluetooth audio dongle from eBay (£5). This plugs into the USB port for power, then a small 3.5mm cable runs to the aux port (this sticks out the front on mine).
Just plug it in, connect to your phone and away you go!

Step 6: Add the Battery and Neaten It Up

I picked up a 7ah sealed lead acid battery at a car boot sale for £2. This is the perfect size for my build (physically) but may be a bit more capacity than is strictly necessary. After running for 2hrs, the open circuit voltage was still 12.6v, indicating over 75% capacity remaining. Assuming I want to leave at least 30% in the battery at all times (so as not to wreck it), I've got around 6 hours of battery life!
Update 20/4/14: real usage testing gives me a battery life of 7hrs exactly! This uses just 2.5ah meaning my 7ah battery is rather old and knackered. With a new one I'd be good for nearly 15 hours!

I made a plywood shelf for my battery and bolted it to my speakers using the mounting holes built in to them. The battery is tied down with a piece of steel strapping and the head unit is held in place with insulation tape!
Screwed on a handle from my local hardware store and boom! You're done!
Fugly as all hell, but really handy at a BBQ!

Once I got the parts together, total build time for this was under 45mins, and that includes hacking the power supply for testing. Reckon you could have this built in half an hour if you're quick.

Questions welcome,
Have fun!