The FT817 from Yaesu is still the most featured portable radio available. The Elecraft KX3 might have better HF performance, but for sheer spectrum coverage, it doesn't come close... unless you spend a few hundred more Euros for the 2W output 2m or 4m transverter. You will definitely have a world class radio, but it still won't do UHF, and would cost something between twice and three times what an FT817 would.
One place the 817 does fall down is its battery capabilities. The battery shipped with the radio is an internaally fitted 1400mAh NiMH that delivers in the region of 9.6v. This means you get a maximum of 2W out, and probably at most a couple of hours of average usage. Not great.
In the recent few years, battery technology has advanced pretty quickly. Lithium battery chemistry has given us batteries that have greater energy density, faster charge and much higher current draw. All of this benefits us that enjoy the more 'al fresco' operation. In the past, anything approaching a decent capacity could only be attained by carrying an external lead-acid or gelcell. Usually this would weigh at least what the radio does, and having an external battery takes from the most attractive 'shack in a very small box' aspect of the FT817.
Lithium Polymer batteries are used for everything these days, but the greatest range of size, capacity and voltage are available for the Radio Controlled model market. They are usuallly categorised as 1S, 2S 3S and so forth, with the number before the S denoting the number of cells. Each cell delivers in the ballpark of 4v, so a 3S giving a nominal 12v (actually 11.1v rated) would be perfect for our needs. The next criterion to satisfy is size... ideally something that would be very close to the internal NiMH unit shipped with the radio. Turns out there is an almost perfect match available from Hobbyking. I ordered 2, and after shipping was added, the overall cost was just more then €42 - pretty good, I thought. I already have a balanced LiPo charger, but this would add something like an additional €20 to the bill, but this would be a fixed cost, and would be useful for any other forays into LiPo power.
Once I received the batteries, I test-fitted them, and they were a perfect fit. I then chopped off one of the output plugs so I could replace it with a JST RCY connector, of which I have many, both male and female. They are cheap, sturdy and the ones I have are rated for 5A, which is more than enough.
Once the batteries were prepared, I made up an adapter from JST female to the little 5 way Molex on the FT817. I had a little 4 way Molex pigtail which I was able to sacrifice for this, since the rightmost (or leftmost depending on where you are standing) pin with the green wire, is a voltage sense for the charging circuit. Since I won;t be using the internal charger for the LiPo, since I don't want to lose it all to a fire, a 4 way Molex is perfect. A bit of soldering and a bit of heatshrink and I was ready to pack it all in and do a quick test.
Fits like a glove.
And once powered up, the radio sees the expected voltage, and is able to deliver the full 5W of power out.