After finding out that fresh Duracell alkaline batteries can offer only few minutes of 2.5-watt RF transmission (FT817 would switch-off itself at PTT/FM or PTT/SSB within a short talk), tried to run it with my small, light and flat Toshiba portable PC power supply PA2450U: it works great ! And it runs at 100-140 Volt 50-60 Hz AC primary and outputs 15 Volt/3Amp DC, enough to run 817 with a full 5-watt RF power !