I'm a reasonably happy owner of a Yaesu FT-817ND. It's a solid, reasonably inexpensive all-band all-mode portable radio solution for the SOTA activator. It's also very rugged, built like a tank. But it does have its downsides. Two related issues are the current consumption of the radio and the batteries which power it.
The radio does have a fairly high receive current consumption, around .3 amperes. This might not seem like much, but mix that with a reasonably amount of transmitting, particularly on CW (or data modes), and you'll see that the supplied internal battery packs aren't the best. The supplied battery is the FNB-72, rated at 9.6 volts and 1100 mAh. Later, Yaesu did a slight upgrade with the FNB-85, which raises the capacity to 1400 mAh. I used the -85 for about six months when I got frustrated with its capacity for multiple-peak activation days (around Big Bear Lake in California there are numerous easily-accessible high-value peaks. The XYL and I would bag a few, camp out, and then head out for a second day of activations. If I recall correctly, I once earned 56 SOTA points in a single weekend).
At any rate, the FNB series of batteries for the -817 use the Nickel-Cadmium (NiCd) chemistry, which is pretty ancient battery technology (We're talking the 1890s, people!).
I suppose this is understandable given that the FT-817 was first designed in the 1990s and first sold in the year 2000.
So I searched around the marketplace for a good replacement. The one I settled on was the W4RT Electronics OPP-817 battery pack and OFC-817 charger combination. This internal battery solution basically doubled the capacity of the batteries to 2500 mAh (although they now advertise that they increased this further to 2700 mAh). These batteries mount in the same compartment in the FT-817ND as the stock batteries. Rather, a new battery compartment door is supplied which has a charging jack for the OFC-817 charger. The W4RT batteries use the Nickel-Metal Hydride (NiMH) chemistry, which are have become lower-cost in recent years than the NiCd batteries.
These batteries have served me well for about 17 months, and have been through maybe 100 or 125 charge/discharge cycles. I always followed the charging instructions scrupulously, and the batteries did a decent job of providing power for the longer activations without increasing weight, bulk, or otherwise ill effect. But, on my activation of Old Rag (W4V/SH-012), I noticed that the voltage indicator in the 817 was rapidly declining. Thinking it was an anomaly, I recharged the batteries and, on a different weekend, I planned on activating Hazeltop (W4V/SH-004) and Fork Mountain (W4V/SH-003). Hazeltop went fine, but by the time I got to Fork Mountain, the radio would barely power up. It was confirmed in my mind: the W4RT batteries were finished.
So I looked around the marketplace and asked on the NA-SOTA forums. I considered other solutions, including homebrewing an external Lithium-Iron (LiFe) battery pack and charger, and using a setup much like Matt, K0MOS uses.
I settled on the HamSource WLB-817 pack, mainly because it offers a 3000 mAh capacity with higher voltage (approximately 11 volts) in an FT-817-friendly package. The chemistry is Lithium Ion Polymer (LiPo), which apparently is good for high-current applications (like the LiFe and NiMH batteries mentioned above). The WLB-817 seems to use the same charging methodology as the W4RT system, utilizing a replacement battery hatch door to facilitate the connection to the external charger. It is slightly more expensive than the W4RT batteries, too ($92 for the battery pack, charger, replacement battery hatch, and shipping & handling).
More on FT-817NDs and battery chemistry can be found on the KA7OEI FT-817 pages. And here. And here.
We'll see how this goes.
As for other "modifications" to the FT-817ND, I also use the W4RT 300 Hz CW filter and the Palm Peg Leg. (I like Palm a lot. I also use their Mini Paddles. And I'm trying to justify spending the money for their miniature Straight Key, too.)
Update: I've received some email as well as a reply on the NA-SOTA forums urging caution with the LiPo batteries, mainly because of the hazards associating with accidentally shorting them out, improper charging techniques, and the damage that they can do to equipment. Those are definitely valid considerations.
In the past I've been utterly scrupulous with regard to charging techniques, and with the safety issues involved in with the LiPos, I'll definitely continue to do so. Those batteries pack a lot of energy into a very small package.
Thanks for the emails and the replies elsewhere! -73, Nate N0PCL
Update 2: Here is another LiPo solution for the FT-817.