These are commercially availible, but at a cost. DIY will make it easy and cheaper. Also it's more fun to make your own (unique) model.
Step 1: Materials
- USB-boost module(see below)
- Battery holder
- Charged battery (voltage depends on USB-booster)
The case is to your choice, should be clear after reading this 'ible.(Note: tins are conductive, so prevent shorts by insulation with paper or tape)
Just make sure the battery and boostcircuit fit nicely in or on the case.
The USB boost module the item you'll probably wont have near you, so you'll have to order it.
I bought mine at our University electronics store for 2 euro's.
Note to IPhone users:
An USB-connection has 4 wires: V+(5.1 volts), V- (earth, 0 volts) and 2 other wires. Iphones are the only phones who check the other 2 wires for a certain voltage to know how much current they can drain ("how fast they will charge"). It might be possible that these circuits don't add that voltage to those wires, thus preventing the IPhone from charging.
Step 2: Basic Principle
On the 2 low corners there are 2 holes with in- and in+. This is where you connect your battery to. As the boostcircuit will boost in incomming voltage to a 5.1V USB output, you'll connect your phone on the USB side and it will charge from the external battery.
On the in- you'll connect the negative wire of the battery. If you got a nice case for it, that will be the BLACK wire.
On the in+ you'll connect the RED, positive wire of the battery. If your case doesnt have an ON/OFF switch, take the time to interrupt the red wire with an toggle switch. This will make sure the battery doesn't drain when you arn't using the charger.
Like said before, make sure the metal parts won't short circuit by touching the wall of the case.
To sum it up:
BLACK/negative to in-
RED/positive to in+
Switch in RED to prevent idle draining.
PREVENT short circuits (with case)
Step 3: Putting It All Together
Make access possible to the USB-port from outside with a cable.
Add details to personal flavour.
Step 4: How Much Will My External Battery Charge My Phone?
Open your phone, and read what the battery says. It will say something about XXX mAh, X V, like 1300 mAh, 3.7V.
The product of these two will give the Energy in the phone battery. Also the used battery has these two ratings, though it's not allways printed at the battery, maybe an internet search will help there.
If you do find it, you can calculate the percentage of phone battery charged by the external battery.
100%*( External charge(mAh)* battery voltage(V) )*0.98 / ( Phone charge(mAH)*phone voltage(V) )
If you take 1 AA battery (sparkfun example), then you'll charge your phone for about 45% (phone dependent)
The Boost circuit has an efficiency of 98%, thats why the *0.98 comes in. I'll be assuming phones have an even better efficiency to charge, else an extra factor has to be added.
I hope you'll like my 'ible. Questions or other comments will be read.