Yunus - I have had one of these, mounted to one of two bedside table stands, for over 5 years now and never had a problem.
Gunvault Multivault Safe: GS
The "battery power" needed to keep this thing "running" is so small, you could probably power it with a starchy potato in a dire emergency. We are talking seriously, SERIOULY low power threshold to run.
I change my batteries about twice a year, give or take a month if I forget to buy new batteries when the time is right. Never had a problem.
Now, this is not a true safe. If you want into this thing, with a screwdriver and a hammer, you are getting in. This keeps the honest, and little un-educated, hands out and keeps what is inside safe in a relative term.
If you want to go a little more serious, you can get one that goes between the wall studs in the closet and has biometric features. I forget the company that makes the one I am thinking of. Homak is one company. Securelogic is another one. Shop around.
Basically you cut a hole in your drywall, mount this safe to the existing studs. Flush it in nice and tight. Hang some shirts in front of it, or stack an empty box or two in front of it.
You need in, it's quick and easy, especially with the biometric versions.
Rest of the time, no one is the wiser.
Give it some thought and see what works for you.