[Elecraft] KX2 real life power

[Jim GM]

Just had to recharge battery for the first time.

I used just about 2.1 hours of battery time.  Real time I had the radio on for just over 7 hours calling CQ and just in the receive mode.  Output of my KX2 was 5 watts and the Battery Min was set for 9.0 V.

My question what is a better setting for this radio  8.7 V. to get my full 2.5 hours or is my thinking wrong?  If so why?

Jim K9TF