The FT-817 draws the same amount of current of 1.9A at 5 watts regardless if the voltage is 9.6V or 13.V
At voltage greater than 9.6V the extra voltage difference is just being dissipated inside the radio as extra heat. We do not need that so it is IDEAL to use a RC car battery that is a 9.6V nicad/nimh or even a 11.1V lipo battery.
12V SLA users can also benefit by using a voltage reducer to drop the 12v down to 10.4 volts.
My Buddy Phil AD5X has a great article (among others) which i built
works great on the Elecraft KX3 as well