The battery run out in my best value for money watch, Gemius Army KM 101, bought for a total cost of under 1 dollar, Euro etc. The watch shows you that the battery is gone flat by continuously vibrating the seconds hand.
The caseback comes out easily, using a sharp blade or watch case opener.
Inside there is the plastic spacer used in many watches where the movement is smaller than the case (most watches..)
The watch uses a Z68 Zhong Chen Chinese movement, with a 377 battery.
For a watch this cheap it may be better to buy it again, rather than change the battery, but I have about 100 of these batteries, so I just replaced it with a new one.
Inside the built and the movement looked reasonable, with no nasty surprises. It is a cheap watch, but it works.
There is no gasket but the caseback fits in tight. I have got this watch wet whilst washing my hands with no issues.
I put it back together and it works fine again.