More and more devices now come kitted out with rechargeable lithium-ion batteries — you know, the ones that look like the old-style AA or C cell batteries, but are a slightly different size. The most common size is the now ubiquitous 18650, but there are loads of other sizes in use too, such as the 14500, 16340, and 26650.
These batteries are incredibly safe if treated properly, especially when you consider how much power they contain, and can last for many years and hundreds of discharge cycles before needing to be replaced.
Sometimes basic chargers can be “better” than clever chargers! Chiefly the issue being solved here is when lithium-ion batteries discharge down below their cut-off voltage point.