It's 512 nybbles at $A000-A1FF--only the low 4 bits of each byte are valid. I think the high bits read as open bus. Here's the "load saved game" routine from Makaitoushi SaGa:
Code:
ROM0:04DC CD 19 05 call 0519 ; enable MBC2 battery RAM, hl = a000, de = cc00, b = 0
ROM0:04DF 2A ldi a,(hl)
ROM0:04E0 E6 0F and a,0f
ROM0:04E2 4F ld c,a
ROM0:04E3 2A ldi a,(hl)
ROM0:04E4 E6 0F and a,0f
ROM0:04E6 CB 37 swap a
ROM0:04E8 B1 or c
ROM0:04E9 12 ld (de),a
ROM0:04EA 13 inc de
ROM0:04EB 05 dec b
ROM0:04EC 20 F1 jr nz,04df
ROM0:04EE AF xor a
ROM0:04EF EA 00 00 ld (0000),a ; disable MBC2 battery RAM
ROM0:04F2 D9 reti
Why is the battery RAM laid out in this silly way? Because the RAM is inside the MBC2 chip itself, and the chip only has 4 data lines! MBC2 cartridges are limited to 16 ROM banks (2 megabits/256 kilobytes) for the same reason.