Recently purchased Redwing Yak 55 and trying to decide type of battery system to use. Have a DLE 35RA going in it and using digital servos Hitec 5645 along with Optical kill switch. Not a newbie, but in my Seagull Yak which is a gasser just used Nimh and never had a problem, just want to try something different. Suggestions would be appreciated...

I use LiIon batteries with a voltage regulator.....it's all about the discharge curve NiCads discharge pretty quick then slowly discharge down some numbers as a visualization let's say NiCad starts at 5 volts goes to 4.6 pretty quick then slowly discharges down.
The lipo, LiIon, and life batts let's say start at 7.2 discharges to 7 then holds 7, 6.9 until it suddenly drops to unsafe level.
the mAh rating of the battery is the capacity of the battery the more it is the longer you can go. Hope this helps

A123's are the equivalent voltage of a 5 cell NiCad/NiMH but charge faster, hold their charge forever without damage, and are rock solid batteries.

Thanks, going with battery redundency using a specktrum AR9000. Any thoughts on using 2S lipos, they wouldn't create a problem for my ignition and give consistent power to my servos? Sorry for the delay in responding to your suggestions been crazy busy with work.

Your servos aren't rated for 2s Lipo voltage. You would need a regulator for them, which adds weight and is another failure point in the system. The 2 cell LiFe/A123 batteries are the perfect solution, IMO. They are the lightest choice, can handle more current than your servos can ever draw, are durable, and hold their charge forever. I'll suggest going ahead and getting the round cell A123 instead of the flat cell LiFe batteries because their improved durability will hold up better to gasser vibration. The only real drawback to LiFe batteries is that you have to balance charge them which may mean buying a new charger and figuring out how to access the balance plug. I made a charge connector for mine that lets me charge through a standard 3 prong servo connector that I run to a charge port in the fuselage. It works great.

If They would work with the ignition also then will just need to decide what size for flight times to eliminate having to charge after three or more flights. I do believe in checking volts after each flight though, just to make sure nothing strange is going on with the battery

I put a 2600 in a 50cc size IMAC plane a couple of years ago. I can get 5 flights on it, but I did do a separate battery for the ignition. I would expect the 2600 would work fine in your bird for 5 flights also powering the ignition. I'm sure you know about using a BEC to separate the circuits, but it's worth mentioning for anybody else who reads the thread.

I'll second LIFE batteries. Valuehobby.com has phenomenal prices on them and they work great. Ive got them running ignition systems and receivers, no regulators, no issues yet. Charge once on Friday evening and fly all weekend.