It still may not charge the battery, even if it's an ok voltage, and it might not have enough power to run the portable either (that design takes about 1.5 amps @ 7.5v). I'd suggest a battery that you can integrate the whole charger into the portable and only have to feed it some power (like 12v, if you used a AA car charger) to make it work. Then you power both the portable and the charger from the same adapter (just drop a 7808 in the line for the screen so you don't fry it. Only have it come into play when you plug in the charger.)