Is there any advantage to a sub C 6 volt battery pack vs. a AA 6 volt pack having identical Mah rating? I can't think of any advantage. I believe there would be a slight disadvantage in the sub C pack because it would be a bit heavier (I think).

Other than weight, everything about the sub C cells is better. Most importantly is their ability to maintain voltage under heavy loads. AA cells are no match in this regard.
Pete

As pilotpete2 suggested, there is really nothing even similar between the two batteries. The sub C cells have a much lower impedance which as pete said greatly increases their ability to maintain voltage while under high current loads. Put the same load on the AA cells and they will fall on their face. Nixx batteries do not have a "C" rating like Lixx batteries but if they did, the sub C cells would have a much higher "C" rating than the AA. From there, the sub C cells are much more rugged. The AA batteries tend to lose capacity fairly quickly tapering off quite a bit after just a couple of seasons. The sub C cells can still maintain much of their capacity after 5 or more years. Even just looking at the batteries tells you something. The sub C cells are not bigger and heavier for no reason. They are larger by design.

With all that said, with the far superior A123 LiFe batteries that are now available and at very reasonable prices, Nixx batteries are no longer my "battery of choice".

Thanks for the info guys. Isn't it amazing how progress changes your perspective on things? Technology that worked so well for so long (AA rechargeable batteries, 72 MhZ radios) suddenly become much less desirable when new technology comes out (sub C battery packs, 2.4 Ghz radios). I believe what you guys are saying, but I am unsure how it happens. If two battery packs, one made up of AA cells, the other of sub C cells, have the same mah rating (say 2000 mah) how can one last significantly longer than the other? Mind you, I am not arguing with you--simply wondering how it happens.

OK, think of it this way. The capacity of a battery is a measure of how much total energy it can store and deliver into a load. In this case lets say both are rated at 2000 MAh. If these batteries had a "C" rating like Lixx batteries do, this would give us an indication of the rate these batteries can deliver that power into a load. They don't so I will make up some values here for illustration purposes. The smaller AA cells may only be able to deliver their energy into something like a 200MA load over a 10 hour period and still be able to deliver their full capacity. The larger sub "C" cells may be able to deliver their same amount of stored energy into something like a 1 A. load for two hours. Note that 200 MA spread over 10 hours equals the 2000 MAh capacity. Likewise, 1A. spread over 2 hours equals the same 2000 MAh capacity. In other words, the larger batteries can deliver their energy at a much higher rate than the smaller ones. Keep in mind these values are arbitrary just to make a point. Simply stated, the smaller batteries can not deliver their energy at the same high current rate that the sub "C" can and still maintain a normal voltage.

Lixx batteries all have a "C" rating or the rate they can safely be discharged at while still maintaining normal working voltages and without damaging the cells. It is unfortunate that Nixx batteries used for our purposes do not have these same ratings as it would make their capabilities and comparisons between different batteries more clear.

Basic battery 101 really hasn't changed that much over the years but the current demands of high torque servos coupled with more of them in the average airplane has changed making a reliable, higher "C" (discharge rating in this case) rated battery necessary. Thankfully, batteries rated for higher discharge rates are readily available these days and at reasonable prices.

I hope this helps a bit.

Edited to clean up some confusion.

I think there is some terminology confusion here... C rating with regard to Li batteries means the rate at which the battery can safely be discharged. C rating with regard to Nixx batteries means the capacity of the battery - how many mAh the battery holds. Unfortunately this hobby has used "C" for both parameters. Or am I mistaken?

At any rate, I have also noticed that the higher the energy density of a pack, the shorter the lifespan of the pack. For example, 1100 mAh NiCd AA cells last only 2 or 3 years while 700 mAh AA cells can last 10 years. This is one advantage of using sub C sized cells of the same capacity as AA sized cells. But the big advantage is the lower internal resistance of sub C cells and therefore their ability to source higher current.

I will add that in this hobby we tend to look down at old technology. Many of us want the latest greatest technology - we want to be on the leading edge. I think it is some kind of testosterone fueled dominant male thing. it is also encouraged by the manufacturers, who want us to keep buying new stuff. It doesn't mean that the old stuff was not satisfactory.

You are correct JPMacG regarding "C" ratings on Nixx batteries. I should have used a different term to illustrate my points. I went back and cleaned up the text a bit to reflect this. Thanks!

While I embrace new technology when it is time to replace the old stuff, I don't just abandon what works today in favor of something that is newer, just because it is newer. I have never been an "early adopter" of technology in my personal life. Not so much when I was still working but that is another story.

I have several older planes in my fleet and all of these still use 5 cell, NiCd batteries. A couple still use 72 MHz. radios. The batteries are still going strong so I see no reason to replace them quite yet. When their time comes, they will be replaced with A123.

I wasn't aiming my comment at you, Truckracer. Sorry about that. I was thinking of the guys in the two clubs where I fly. I too fly both new and old. I catch a lot of crap from other members every time I bring out my 72 MHz stuff, but it still works great.

Back when some of the first 2.4 radios came out and they were falling out of the skies, my older 72 radios were still going strong. When the then new radios were debugged, I bought into them and now own several. By waiting a year or so, I didn't loose any airplanes. I still hate to retire all my older 72 stuff when it still works so well. Its time will come though .....

Truckracer, I'm with you re: A123 packs. I converted my entire fleet, probably shaved a pound of weight collectively. Heck- I even did away with my Pb field battery and now have a 5000mAh A123 pack in my flight box. I shaved something like 8 lbs! No more lopsided schlepping to the flight line... haw

Thats an expensive flight box battery but you sure won't have to worry about running out of capacity. That replaces one big and very heavy Pb battery! I'll probably retire some of my older A123 to field box or starter duty one of these days. Most guys around here use LiPo batteries for that service but I'd prefer the greater safety of the A123.