A 8052 board counts, accumulates and keeps the production quantity by increasing its internal RAM (4-byte integer of its 256 byte RAM), despite of suddenly main power shutdown. This internal RAM is backup by 3 Volt Lithium battery

The question is not clear, you say that internal RAM is backed up by 3V battery, and on the other hand, you are asking for how to do this.

You can power up the micr with a battery (charged by mains) and put the micro in idle mode to save battery once you detect power fail. This way, internal RAM will retain the data.