You need a charger with an Output line showing at lest 1.2 Amps or 1200 mA. (That's the normal charging current for a travel charger.) A 1 Amp charger probably won't be able to keep up with the phone's battery drain. (I had that problem in my car until I looked at the adapter [with strong reading glasses - where do they get this 0.25 point type?] and saw that it was 1 Amp output. Now, with a 2 Amp adapter, the phone stays charged. (The charger can be any output from 1.2 Amps up - a 30 Amp charger won't harm the phone (I use a 30 Amp 5 volt supply to charge phones all the time. Phones draw what they're designed to draw, as long as the charger can supply that much current.)
Run
Ampere when you're charging, to make sure you're getting 1200mA.