[Elecraft] KX2 output power drop

[VE2PID]

Hi folks .. A question about the KX2: I saw this on other posts, and 
also experienced it in portable operations: ''The KX2 was able to pump 
out a full 10 watts of power for a little over one hour, then as the 
voltage dropped, it reduced output to 5 watts.''. So it seems to be 
normal, but is it documented somewhere in the KX2 specs?

(FYI, I use an external rechargeable 12V 3000mAh Lithium ion Battery Pack.)

73, de Pierre VE2PID KX2 s/n 1464.