While designed to supply huge current briefly to jump start a car, this one claims to have almost 90AHrs of capacity and also offers USB power, laptop power (at switchable 15 or 19V) and even a strange torch.
I thought this might be a great power source for portable operation at this weekend's QRP by the harbour. It arrived partly charged and I measured 15V at the output. The giant clip leads were removed and an Anderson Powerpole connector added but when I charged it and connected the KX3 it quickly complained that the voltage exceeded 16V and turned itself off. Nice feature there Elecraft!
These gadgets are basically a LiPo battery wrapped in some charging and voltage regulating electronics. I'm sure the 16V output is the result of 4*3.7V cells in series directly connected. One thought is to use an adjustable buck converter to efficiently bring this down to 13V but switching noise might be an issue.
The torch is rather fun but looks rather too much like police lights I think.
Update - built the voltage reducer
I have set up a buck converter to reduce the voltage to 13.8V. Here's the raw board.
Boxed up with sockets for input and output the KX3 radio seems happy and there is no obvious noise audible. These buck converters run at 180kHz and don't seem to produce much noise.
This buck converter isn't rated for much current. I can see a clean AM carrier transmitted but the voltage drops from 13.8 to 13.6 under that load. I'll try this on Sunday (but take alternate power).