Spyros,
unfortunately you will have to use the 3.6V battery if you have a Spectrum (colour screen) if you want to do more than a few dives on one battery. If it's a B&W screen the 1,5V AA should work ok but I still like the lithiums. I bought a set of 10 of these
Lithium Batteries for Rebreathers from DRE. They always work and it comes to $5.50 a battery which is pretty cheap. I'm sure the RBW store carries them too.
There are a lot of crap batteries out there which is why I always check mine with a volt meter but even then, if they are crap still, they can crash pretty fast.
Tibby