[smilespray]

It says the charging voltage is 8.2V, which is double the usual charging voltage for lithium cells.

Is it really safe to connect two 18650 cells in series like this?

I've read that you should manage each cell individually to reduce wear and/or risk of fire.

[RobotToaster]

Looks like he's using a BMS with a centre tap to balance the cells.

[smilespray]

I looked up the module on AliExpress:

https://www.aliexpress.com/i/1005004814091297.html

Anybody care to have a look and comment?

[RobotToaster]

I could only find a datasheet for the chip in Chinese, but from what I could get through google translate it looks like it can balance two or three cells in series.

[RF_Savage]

If the cells are matched it is less of a problem. But 2S battery management boards with additional features like low voltage cutoff and current limits are only a few euros in single quantities, so I see no reason not to use them.