Step 1: Materials
1 9V battery
1 9V battery connector
1 USB car charger
Step 3: Disassembly
Take apart car charger
Take everything out of it.
Leave only the main part.
Step 4: Testing
The way I would find out what part of the USB charger is positive and negative is by testing it.
On mine the spring was positive and the metal in the the sides are negative.
Get help for this step.
Step 5: Cutting
On your tin you are going to trace a outline of the front of the USB charger.
Then use a small drill to curt the hole.
WEAR SAFTY GLASSES
Step 6: SAFTY
Using electrical tape cover the entire inside of the tin
Step 7: Connecting
Using electrical tape connect the black wire of the 9V battery connector to what ever part of the charger you found to be the negative.
Connect the red wire of the 9V battery connector to what ever part you found to be the positive.
Then place the USB charger in the hole that you made in the tin.
When you are ready to charge connect the battery and when you are finished disconnect it.