Hi Fastsky,
Thanks for the info. What I done so far, till I can afford to buy the ESV, was to buy and install on my Hangar 9 Alpha 40 was the Micro unit Fail Safe by OFNA part # 91002. It tells me by a flashing light ( LED ) that my batteries are low, also it is set to turn my engine to idle when the radio signal is lost. On my Hangar 9 P 51 Mustang PTS I installed what you suggested an on board battery monitor. The one I got is from Venom, part # 0640 and I installed it on the servo deck so that all I have to do is to look through the canopy to check the batteries.
I have another question, what would be the best adapter that can be used to install on the Mustang to charge the on board batteries so that I don't take the wings off all the time?