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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.
Wait, do they not make AA-sized 18650 batteries?
18650 isn’t a specific type of battery, but a size. 18mm diameter, 65mm length, and 0 typically represents it being cylindrical in shape. 18+65+0
Heres a quick read
Besides being the wrong size by definition, AA batteries are expected to have 0.8V to 1.5V, while Lithium Ion cells (such as 18650) have a voltage range of 2V to 4.2V. That’s completely incompatible, you couldn’t even replace two AA batteries with a single Li-Ion cell.
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.
Couldn't it theoretically be fixed with a voltage regulator?
Don't they need a circuit protection to not over-discharge lithium batteries? Most AA devices would suck all the juice from the battery until it stops working.
Yes.
The other problem with lithium batteries like 18650s is that they need to be handled with care so manufacturers don't want users swapping them in and out like AA/AAA. This is why they build them into devices and have you charge them through a regulated port.
Well there aren't really different sizes of 18650s, if you chance the size it's no longer an 18650.
The flat vs button top issue can be annoying though
The problem with those is that the device loses the ability to sense the charge status of the battery, since the voltage remains the same until it’s empty.
NiMH AA's have the same problem.
But 18650 size is manufactured at much higher scales than other sizes, and therefore it's the cheapest, and therefore it continues to dominate.