battery backup for model plane

I am an avid r/c plane flyer and would like some ideas on a battery backup system for a model plane.

would like to use 2 battery packs (4.8v) in such a way that pack a is the master pack and pack b is the backup.

if pack a should run 'low' i want pack b to switch in and totally isolate pack a (in case pack a has developed a fault), also when pack b switches in it will sound a loud buzzer / flash lights (time to land).

the best i have come up with is a relay driven by transistor / mosfet, the problem here might be the time that the contacts change over.

any ideas would be appreciated. I know very very little about electronics

thanx in advance

:roll: :roll: :roll:

What is the peak current load on the batteries?

i would guess at a possible 1.5A

Here's a place to start. You can play with the frequency and pulse width of the alarm by changing R7, R8 and C2. Switchover is designed to occur when Va drops to 4.0v. You can change this with R1 and R2. The assumption is that Vb is always greater than or equal to Va. If this is not true, you may need two more MOSFETs.
There are microprocessor supervisory circuits that appear to come close to meeting your needs, but they can't handle 1.5 amps.