If the battery isn t removable, there may be a header on the motherboard for an external battery. If the battery is near the keyboard jack, that header is often at the back of the motherboard and to the left of the keyboard jack when facing the front of the computer. Many of them will have a jumper on them for jumpering the motherboard to use the battery on the motherboard or an external one. Removing the jumper may also work. I have not done that.
Some older computers have the battery and real-time clock built into a single unit.
They are usually black and rectangular in shape. Dallas Semiconductor made many of them. They are supposed to last about ten years. I have not done that either. The ooze will eat the traces on the motherboard and ruin it.