Bangaa Bishop
Those batteries are NOT proprietary, you might have to look at a jewelry store that has watch batteries in different sizes, or try a computer parts website likew newegg.com to find the right size.
That said, the older carts use the same RAM that stores the date and time on virtually all PCs. However, later carts used flash (NVRAM) memory that doesn't need the battery backup. This is more like the memory that carries the BIOS and setup program for your PC. This started happening during the life of the N64-Early N64 games used a battery, but later ones didn't have them anymore. I think Perfect Dark is one of the last ones that had a battery.
(someone correct me if i'm wrong here but i'm pretty sure about most of this)
That said, the older carts use the same RAM that stores the date and time on virtually all PCs. However, later carts used flash (NVRAM) memory that doesn't need the battery backup. This is more like the memory that carries the BIOS and setup program for your PC. This started happening during the life of the N64-Early N64 games used a battery, but later ones didn't have them anymore. I think Perfect Dark is one of the last ones that had a battery.
(someone correct me if i'm wrong here but i'm pretty sure about most of this)