Some of them are direct USB charge; AA and AAA.
This is a snip from this BLF review of Tenavolt: TENAVOLTS AA Lithium 1.5V rechargeable cells
[The output on the terminals of the supplied charger appears to be 5.25V without the cell present.
Actually, it’s basically passthrough voltage from whatever USB power source you’re giving it. Looks like it’s feeding 5V to the buck regulator, which then charges the internal Li-Ion cell.]
Some people seem to think you can charge them in a regular charger. I’ve tried numerous chargers on a few cells. Some you don’t have any success at all. A few of the USB charge type will seem to charge on a regular charger. Trouble is, with all the electronics ‘in the way’ it’s difficult to figure out if it’s actually working properly without putting them in the proprietary charger and see if more current gets added.
I have a few applications they are useful for. Mostly it’s just an interesting endeavor to mess around with them. Unless you get them at a good price, or you have an application that can actually put the voltage to good use, they re not a deal at all. I find the AAA kind of pathetic. There simply isn’t enough room in the tiny can for the electronics and the essential battery chemistry elements.