After getting some recommendations from the Arfcom Ham Forum, I ordered a 5,000 mAh battery for the FT-817ND, a battery tester/low voltage alarm, and a battery charger from Hobbyking.com, who sells it in for use in radio controlled cars.
In contrast, the Yaesu FNB-85 NiMH battery pack has a capacity of only 1,400 mAh.
Lithium-polymer batteries have been adopted by many hams for portable use because they pack a lot more amp hours into a smaller, lighter package than AGM or SLA batteries.
I’m really looking forward to getting the FT-817ND into the field.
I ordered the battery, charger, and voltage tester on Sunday 11/30, and received it from Hobbyking today, Wednesday, 12/3. When I got it I was confused at first because the Molex-type connector doesn't match the one on the radio. It turns out that it is the connector for charging the battery. The large red and black wires with bullet connectors are for powering other devices. I'm planning to cut off the bullet plugs and replace them with Anderson Power Poles.
Here it is, with Power Poles installed.