I'm traveling to Ireland and the UK next week, and plan to carry my Yaesu FT-817ND for a bit of radio work during the lulls in our trip. The rig transmits at a maximum of 5 watts, requiring a maximum of 2 amperes to operate. The rig accepts up to 16 volts DC. The bundled power supply offers just a meager 0.5 amperes at 12 volts, just enough to run the receive and internal battery charger. According to Trish's FT-817 QRP Page, the rig will switch to the internal battery on transmit if the external supply voltage is insufficient to carry the transmitter as would occur using a low-current source. That's great, but the 8 hour charge time versus the 1 hour or so of useful operating time makes the prospect of this hybrid solution not so good, even if I can bridge the receive time with the external source.
One of the great benefits of being a Resistor is access to a large collection of components to repurpose, and one such collection is our big box of wall-warts. I rummaged through the bin hunting for a supply that put out a nominal 12 to 16 volts and at least 2 amperes of current. I came across an old Toshiba laptop supply with a nominal 15 volts and 4 ampere capacity. This supply also has a nice world-friendly input range.
I'm not absolutely sure that the supply won't create enough switching noise to interfere with transmissions or the noise floor on HF, but a quick simplex test on FM with the internal charger running did not reveal any hum on the transmit signal. I'll work with it more, but I have little time before we depart.