The Iris transmitter is a FlySky ER9x and the manual http://er9x.googlecode.com/svn/trunk/doc/ER9x%20Users%20Guide.pdf does not say anything about batteries, voltage etc.
Currently I am using 8 AA energizer rechargeable batteries which when fully charged give me about 11.1 - 11.2 v total.
How low can I let the voltage go before needing to recharge the batteries? Is 10.5 v a good cutoff?
Also, can you use other batteries such as a LifePo4 like this one https://www.hobbyking.com/hobbyking/store/__17955__HobbyKing_1500mAH_LiFe_3S_9_9v_Transmitter_pack_.html
What do other people use in the transmitter for batteries?