Quote:
Can I just solder that battery circuit (VCC to GND) without touch on the original WRAM?
Nope, let's go back to square one. OK, originally there is a RAM chip that is powered by the NES. You want this RAM chip to be powered even when the NES is off (albiet at a lower voltage than when the NES is on).
Naive attempt: Wire a battery directly to the RAM chip.
Problem #1: Battery is fed +5V from NES, resulting in explosion/leakage.
Problem #2: When NES is off, battery is trying to power the cartridge and
entire NES, so it drains very quickly.
Solution to problem #1: Put a diode between battery and RAM chip. This blocks current from the NES into the battery.
Solution to problem #2: Put a diode between NES power and RAM chip's power pin. This blocks current from the battery into the NES.
You started out only solving problem #1. To solve #2, you have to disconnect the RAM chip's power pin from the board so you can put a diode between it and the original trace it was connected to. You can either cut and bend the pin up, or use a blade to cut the copper foil trace on the board.
Once you're done, I highly recommend verifying that your circuit actually performs its duties. When the NES is off, you should get around +3V on the battery itself, at least +2V on the RAM's power pin, and very close to 0V on the board where the pin used to connect (the NES power line). When the NES is on, you should
still get around +3V on the battery, and around +4.5V on the RAM's power pin.