Actually, in terms of mAhs, my Sony rechargeable AAs hold just as much, if not more than many of the battery packs they offer.
Which says
absolutely nothing. I could (hypothetically speaking) produce two battery packs. One can provide 20,000 mAh, the other just 500 mAh. But if the former does so at a voltage of just 1 volt, and the latter at 60 volts, the latter holds more overall power (50% more than the former, to be precise, assuming constant current at constant voltage).
To add to the fun, though, there's also the question of what the load is like. It's no good having a 60 volt pack if the device needs 100 mA at 3 volts; the device will blow up when you connect the 60 volt pack, unless you have a DC-DC converter in the circuit somewhere to shift the 60 volts down to 3 volts.
When it comes to battery life, the only really useful apples-to-apples comparison is a set of batteries in a single given device. In that situation, you can compare AA NiMH batteries to a purpose-built lithium ion pack and get a legitimate result, but that result is good only for that particular camera. Different camera means a different result. Comparing mAh ratings is absolutely meaningless when you're also altering the type of battery. (so mAh can be useful to compare two different brands of NiMH AAs, for example, but not to compare NiMH AAs with a lithium ion battery.)
This is why, for example, NiMH batteries perform so much better than alkaline batteries in flashes and similar tasks: they have a lower internal resistance, so can provide a much higher current for a longer period of time. Doesn't necessarily mean they store more power, just that they are better suited to the job in question. There are other tasks (handheld torches spring to mind) where alkalines perform better because they need a higher voltage but lower current.
(Yes, I did a couple of years of electrical engineering before I dropped out to do computing instead. Does it show?
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