Originally posted by distudio Sorry, as an engineer it's just hard not to comment. It's difficult to explain in few words but if you can bear with me.
I guess the easiest way to put it is yes both AA and L-ion battery systems differ in terminal voltage but if the host system is designed correctly this small variation can be accommodated such that there will be not be a performance deficit.
Where things get confusing is in the case of motors designed to drive heavy loads (such as a battery powered power hand tools). They do generally deliver more power as battery pack voltage is increased (and that's how they are marketed), however in this case the performance gain is primarily a function of the internal impedance of the batteries (the ideal battery will deliver infinite current however real batteries are current limited). Since power is a function of voltage x current if the current is effectively fixed the only way to increase power is to increase the battery voltage.
The AF motors in camera bodies just don't need to deliver the sort of power that would significantly constrain their design/operating voltage (as may be the case with powered hand tools). However when a system is poorly designed (like some of the early cameras which lack voltage regulation) battery voltage could affect motor speed (AF operation & shutter FPS) such that certain types of cells produce a marked increase or decrease in performance. Again this isn't just a function of cell voltage but of the cells internal impedance.
The biggest problem with the AA systems is that people can load them up with regular heavy duty or alkali cells, these really aren't suitable (due to their relatively high internal impedances) and performance will suffer noticeably relative to Ni-Cd, Ni-MH or Li/FeS2 (Energizer Lithiums). The beauty of a proprietary system (such as the L-ion battery in the K10D) is that there is limited variation in battery characteristics which makes it easier to design the host electronics.
I'm sure it's all as clear as mud now but as you can likely see there's a bit more to it than just terminal voltage, sorry again for the terse reply.
Cheers,
Rob, you are absolutely correct in your analysis.
The challenge is that the AA batteried cameras have been designed around either CRV3 or AA lithiums as the primary power source, which deliver a constant voltage higher than 6v when under load, usually around 6.5v. The use of AA NiMH batteries usually means that, even with freshly charged units, they deliver around 4.8v under load, which in real terms means that for a given motors impedance the power that motor can deliver under those conditions is about 60%, this significantly affects the AF performance. If the system was specifically designed for
only NiMH AA's, then the motor could be designed accordingly, but due to the variation of AA cell technology, there is just too much variation.
It is significant that Pentax have specified and supply AA Energizer (Li/FeS2) batteries with the K200D, I suspect that the performance using NiMH's will be significantly impacted, it will be interesting to check this out when I can evaluate a unit.
It can of course be argued that the camera should be designed around a 4.8v nominal supply and have regulators to control the voltage from higher voltage sources, however I suspect that the peak power levels encountered would mean fairly high power regulators and be wasteful of energy (heat) and also increase the current requirements and affect battery life/number of shots.
When using a proprietary battery system, the system performs in a much more predictable manner. That's why I eschew AA batteries and advocate the adoption of proprietary systems.
However I do concede that the best solution is to use a proprietary system with the ability to use AA's in an emergency. An easy way to do this with the K10D, for example, is to buy a 4 cell AA holder (or even larger cells) from a electronics store and have a flying lead to fit the external power socket and keep the batteries in your pocket. I find it just as easy to have a couple of spare cells and have a 12v charger in my car. I don't do long jungle treks away from power sources!