I know these are currently out of fashion but I'm still thankful they exist.
Let's remind ourselves of devices that use(d) these standardized batteries:
- Toys
- Digital cameras
- Torches
- Gadgets like fans
- Wireless keyboards
- TV remotes
Thanks to having a standardized system of batteries,
- You can use the same battery across several devices. This is a no brainer but it's very practical.
- Batteries can charge quicker thanks to being put in a dedicated charger and not being limited by USB cables. (But yes I concede that USB has been updated for faster charging over the years)
- Devices don't have down time when their battery is charging. To charge, the battery is removed from the device and can immediately be replaced with a fresh one.
- You'll never have to trash a device due to an expired battery. Just buy a replacement. And building on this...
- Any improvements in future battery technology can be retro-fitted into your existing devices. And there is a high incentive for future improvement, because...
- An accessible (due to easy replacement) and large (due to many devices) battery market is very attractive to competition.
If you look at the pros I listed, they all happen to be things that would be very useful for electric cars. So I think it would aid the adoption of electric cars if their batteries were standardized too.
Most 18650's don't go down to 2 volts. They should be considered "flat" at 3.4 volts - going any lower risks damaging the battery (unless you know what chemistry it uses, some can go lower safely).
Allowing a rechargeable AA to get down to 0.8V is also risky, if you go much lower than that you will damage the battery.
An 18650 is essentially the same voltage as three AA's. And the amount of charge level they store is closer to five or six. The size and weight, on the other hand, is a bit more than one AA.
But the biggest advantage, by far, is a lot of devices can be powered by a single 18650 but would require multiple AA's to have acceptable battery life (or a high enough voltage). As soon as you have multiple batteries thing start to get really complex. Your batteries will never be at exactly the same charge state and that can potentially damage the battery and the device.