Make sure the jumper just above the keyboard controller chip is set to INT. as that one switches between the on-board internal coin cell battery and the EXT. BATT. connector.

I have checked the jumper it is set right 1-2. The BIOS saves everything i set. And it also remembers the time but the time is not running when the computer is off. When i turn it off wait for a while and turn it on again. The time has still the value from the moment when the computer was turned off. Very strange.

I would try an external battery of 4.5v (like 3 AA) and see if it does same thing. That would rule out the onboard battery circuit from just barely keeping cmos alive but not good enough to run the clock as those are separate parts in the SIS 85c407 chip. If it does same thing then there is a problem with other parts. Just a thought....

Hi everyone, i tried the external 4.5V battery block of 3 AAA. And now it's working, the time keeps running when the PC is off. But strange that the onboard battery is not working.

Does anyone have an experience of how long the 3 AAA batteries last or how often you have to change them?

Great ! Those AA should last a few years, longer if you use the computer often. The cmos draws so little that basically the batteries will last about as long as their typical shelf life of about 5 years +/-
You could try using 99% Isopropyl alcohol and clean the coin cell contacts and also both sides of a new coin battery and try it again. Sometimes it does not take much dust or film for those cell sockets or the battery to not work proper.

If the external battery is working then the problem is somewhere in the path between the coin cell and the 85C407 chip. In addition to cleaning the battery holder contacts as Horun suggests, if you have a multimeter you can trace the route from the internal battery to the chip, look for broken connections, or blown resistors/diodes... I don't think there are any more complex components in the path.