One other thing to note. If you do use a power supply wart wart instead of a current supply the wall wart will get "really" hot!. If you are unsure put a fuse in series with the wall wart about twice the value you expect to charge with or slightly higher than the rated output of the wall wart whichever is lower.

Rodney's right about the accuracy but its still within about 10% at least it gives you an idea. Richard's right about testing it for DC voltage first also, though I usually just look on the charger itself.

Have fun. It's not as complicated as we've made it sound.

John