When the battery fails, the RTC chip would be replaced. CMOS RAM can be cleared on the 1287A and 3287AMT chips by shorting pins 12 and 21. If there is a problem such as a forgotten password, the chip must be replaced. Also the Dallas 12887 and 12887A are similar but contain twice as much CMOS RAM storage. Although these are 24-pin chips, the Dallas chips may be missing 5 pins, these are unused pins.
Most chips have unused pins, though usually they are still present.
This is a rectangular 24-pin DIP chip, usually in a socket. The number on the chip should end in 6818. Short together pins 12 and 24. These are the pins on diagonally opposite corners - lower left and upper right. You might also try pins 12 and 20.