The 18650 Lithium ion battery is inside of a solid enclosure that’s 4"x3"x1.5" and held into a battery clip. The safety/charging circuitry has overcharge protection, rapid discharge (short) protection which will shut off the power on the event.
I have had some “incidents” when I didn’t have the proper protection and nearly started a fire, but I’ve been using these for over 1 year now without incident or failure even after 4’ drop test.
I did just ask the vendor for UN/DOT 38.3 documentation on the batteries themselves.
My big goal was to create and sell 10 of these IR beam-break, WiFi race timer systems for dog agility EVER, so this is a lot of hassle I wasn’t expecting.