FT-100D lowest usable voltage

I am running my FT-100D (with WSJT-X etc) off of some 12v batteries. It 
works fine for 1-2 hrs, until at some point, the radio and rig control 
starts to do weird stuff - seems to be due to low voltage being fed to 
the radio.

Does anyone know from experience what the lowest voltage I should use to 
run my radio? Specs for the radio show 13.8v +- 10% (1.38v) = 12.42 but 
it seems like I've run below this voltage.

I am in the process of building/buying a low voltage alarm for my 
batteries (built ones are just a few $$ on ebay) and want to set it to a 
reasonable value.

Thanks, Roy K2RMA


-- 
***************************************************************
* No trees were destroyed in the sending of this message,
* however, a significant number of electrons were terribly inconvenienced
***************************************************************