Battery Backup

A cartridge has no moving parts and no disk, so anything a game wants to remember between sessions has to be held in RAM that never loses power. Battery backup is the circuit that does it: a small lithium cell wired to the work RAM through a pair of diodes, so the console powers the chip while it is on and the battery keeps it alive, drawing a few microamps in its data-retention mode, while it is off. Nintendo Power explained it to players in 1991 as the thing that let “Game Paks never forget”: Memory Management Controllers brought “the use of battery backed up RAM that can save your game progress from one play session to the next”.

Fast facts

The circuit: RAM with a low-power retention mode; a 3 V lithium cell; a diode from each supply so “one cannot supply current to the other”; a resistor in series with the battery; a pull-up on the chip-enable line so the RAM ignores the bus while the console is off.
First on the Famicom: Family BASIC, June 1984, with 2 KiB of battery-backed RAM for the user’s programs.
First game cartridge: Seta’s Morita Shougi, April 1987, on the MMC1 with 8 KiB of battery-backed RAM.
Made famous by: The Legend of Zelda’s gold NES cartridge, 1987 — “it even features a built-in five year battery to save games!”
Nintendo’s guarantee: its MMC specification for licensees (1986, revised 1987) promised more than five years of data retention from the lithium cell; a 1989 revision, the MMC1B, was issued to improve it.
Game Boy: the cartridge header names the arrangement — MBC1+RAM+BATTERY, MBC2+BATTERY, MBC3+TIMER+BATTERY — and the MBC3’s clock runs from the same cell.
Today: a dead cell is replaced with a CR2032, ideally in a holder rather than under tape.

How it works

The NESdev wiki’s PRG RAM page gives the whole circuit in three sentences. “Most RAMs have a special low-power data retention mode, which decreases current consumption to few microamps”; one diode “ensures that the 3.3 V battery only powers the RAM, not the whole cartridge when in standby mode”, a second “keeps the 5 V from charging the battery”, and a resistor “ties /CE at high level when power is cut off” so that the floating bus of a powered-down console cannot corrupt the contents. On Nintendo’s MMC1 boards the same parts appear — “2 diodes and 2 resistors” — with the series resistor “as part of UL compliance”.

None of this needs the mapper’s help. The battery sits on the RAM’s power line, not on the MMC, which is why boards with and without the cell are otherwise identical: a solder pad on MMC1 boards selects whether the RAM is retained. What the mapper did was make the RAM worth having. Nintendo Power’s 1991 primer ties the two together — “Many of the classic games like The Legend of Zelda and Metroid became possible only after the MMC1 was developed” — and lists “an improved battery back up system” among the MMC5’s advances.

Firsts

The first Famicom cartridge to hold RAM through a power cycle was not a game. NESdev’s mapper history dates it to 21 June 1984: Family BASIC, “the first special cart Nintendo made for the system”, which “provides 2 KiB of battery-backed PRG RAM. It is the first cart (for a long while) to use either PRG RAM or battery backup.” The purpose was to keep the user’s BASIC programs. Nearly three years passed before a game did the same: “Seta’s Morita Shougi appears to be the first game to use the Nintendo MMC1 mapper. It’s also the first game cartridge with 8 KiB of PRG RAM and battery backup” (14 April 1987). Taito’s X1-005 followed that June, “the first mapper to provide a small amount of extra RAM on-die, which was almost always used for saving”.

In Japan the pressure to put a battery in the cartridge was weaker, because the Famicom Disk System had rewritable disks. Retro Gamer’s account is the plain one: “game data saved without the need for costly battery backup. Titles such as Metroid, which in the West had a complex password system, could incorporate save features in Japan because of this.” The Legend of Zelda was a 1986 disk game in Japan; the cartridge with the battery was the export version.

Zelda’s gold cartridge

That export cartridge is what made the technique visible. An Atari magazine, of all places, described it in March 1988: “The Legend of Zelda adventure game is the flagship of the Nintendo line. Packaged in a gold cartridge, it even features a built-in five year battery to save games!” By 1989 the phrase “battery-backed cartridge RAM” needed only Zelda as its example — ANALOG Computing faulted an Atari game because its high-score names were not “saved in battery-backed cartridge RAM, similar to The Legend of Zelda for the Nintendo Entertainment System” — and ACE could report that “arcade adventures are becoming more popular on the consoles, with battery backups now allowing games to be saved for a later date. No longer is play restricted to shoot-em-ups where each game is relatively quick.”

The five years were Nintendo’s own figure. The MMC specification it issued to licensees names the battery as a lithium coin cell and guarantees data retention for more than five years; in June 1989 a revised chip, the MMC1B, was announced to improve that retention. Plenty of the original cells lasted far longer, but the guarantee expired before the decade did.

What it was used for

Nintendo Power’s listing key from 1991 records, for every game, whether it used “a battery or password to save game play data”, and the magazine’s coverage shows the battery earning its cost in three ways. Progress in long games: Crystalis, where “a battery backed memory saves your progress”; Super Mario World, “built with a battery back-up which enables you to save your progress when you clear a Castle, Fortress, Ghost House or Switch Palace”, because “it’s next to impossible to make it through this long adventure in one playing session”. Records: “Since the F-Zero Game Pak has a battery back-up, it records the ten best times for each of the 15 tracks so that you can race for a place in F-Zero history.” And handheld saves: Metroid II on Game Boy shipped with “a battery backup to save game progress”.

The alternative stayed in use beside it. The magazine’s 1992 comparison of three Super NES golf games notes that “Jack Nicklaus Golf uses a password to save your position and True Golf Classics: Pebble Beach Golf Links employs a battery backup for saving data” — the choice was the publisher’s, made per game, and password systems were the cheaper answer for years after the battery was available.

Game Boy

On the Game Boy the arrangement is written into the cartridge header. The type byte spells it out — “03h MBC1+RAM+BATTERY”, “06h MBC2+BATTERY”, “0Fh MBC3+TIMER+BATTERY”, “1Bh MBC5+RAM+BATTERY” — and the GbdevWiki describes each. MBC1’s external RAM “is often battery buffered, allowing to store game positions or high score tables, even if the gameboy is turned off, or if the cartridge is removed from the gameboy”, in sizes of 2, 8 or 32 KByte. The MBC2 “includes 512x4 bits of built-in RAM (in the MBC2 chip itself). It still requires an external battery to save data during power-off though.” The MBC3 adds “a built-in Real Time Clock (RTC)” which “requires an external 32.768 kHz Quartz Oscillator, and an external battery (if it should continue to tick when the gameboy is turned off)” — the same cell that keeps the save keeps the clock, which is how a Game Boy game can know that a day has passed while it was switched off.

Replacing the cell

Nintendo’s five-year guarantee is long past for every cartridge it covered. The NESdev wiki’s battery-holder page is the repair guide: “Adding a battery holder to an NES Game Pak PCB with a dead battery lets a player use that Game Pak to save again.” It warns off the common shortcut — “many tutorials online … show one how to replace batteries by using electrical tape to wrap the new battery onto the cartridge and hold it in place. This method does not seem very stable” — and lists the parts: a CR2032 and a CR2032 holder soldered where the original tabbed cell sat. The save is lost when the cell is out, so a save worth keeping is backed up first, with a cartridge reader, before the old battery comes off.

Cartridge save memory