There are quite a few trains of though about the devices. One is from the pattern folks that really rely on the KISS principal.. keep it simple stupid. (not meaning you.. just a term.. )

They contend that adding things add points of failure to your system. Most of them rely on a simple dual battery, dual switch, into separate RX channels as RCKen pointed out and shows in his drawing. They feel that any added point of failure will fail. Simple. They do not use the EMS device.

These guys have been at the pattern game for many years now, one has been flying RC and Control Line for around 40 years now and uses dual batterys on separate channels and each on a switch, for a long time now and does not have a lot of faith in added gizmos except perhaps a voltwatch.

I had a battery fail in flight on my Venus II, but I had two batteries set up as described. Before I took off for the third flight, both batteries checked out ok (quick look at the voltwatch). I took off, flew a tank, and landed. I then checked my batteries as I always do.. I check them before and after each flight.. and one checked bad. I swapped it out and recharged both to equalize them, and checked the bad battery. It had a bad cell. Happened in flight because, as I said, it was ok before the flight.

BTW, my process is this: I switch on switch #1 and check battery #1 on the voltwatch then switch off switch #1. I then switch on switch #2 and check battery #2 on the voltwatch. Then I switch on battery #1 so both are on and go flying (of course, if they check out ok).

So, yep, it can happen.. and will happen!! So, I use dual battery configs in most of my planes now, at least the larger ones.

CGr.