I have a 5amp Deltran Battery charger. If I charge the car overnight, and leave it plugged in while coding, I can spend all day going through modules and not have the battery go flat at all. Remember to turn off your DRLs, and if your doors are going to be open, the interior lights. The main reason for having an actual power supply is if you would be trying to flash module firmware. I am no where near trying that, and probably won't ever do it, just too risky.
You can, if you go quickly, code a car without any power at all, presuming the battery is in good shape and you are only spending less than an hour or so.