For whose who are concerned about CCC (cheap Chinese crap), don't worry, I intend to test it fully before using.
Yeah, C is fitting, I just tried those. C is very common but the intex air pumps use them so I luckily had some laying around

. Probably go with C first and see how that works. The only thing is how to connect it, I am not good with soldering so thinking about using electric tape, not sure if that's a good idea.
Yes, its simple quartz movement and yes its definitely strange, it runs out of battery much quicker than other clocks I have around the house, maybe because its are huge and heavy? not balanced at all for sure...