The Neo-Geo MVS arcade machine (I have a 2 slot unit) normally uses a Ni-cad battery soldered to the main board to support the 'backup ram'. This ram holds high scores and per-game settings when the system is powered off. As you will find from other sources, the original Ni-cad batteries have a tendency to fail and leak after a decade or so in service. If you've still got one in your system GET IT OUT NOW BEFORE IT DAMAGES YOUR BOARD!
Now, when I got my MVS the battery wasn't in bad shape and hadn't damaged anything. I removed it and replaced it with a remotely mounted Ni-cad battery pack from a cordless phone. After a little adventure with the backup ram, I decided that the remote solution for the battery, while workable, was kind of annoying.
I searched around a bit and found out that SNK appears to have designed the MVS motherboards to support a wide variety of possible battery configurations. The normal configuration is a small Ni-Cad soldered to the board, being trickle-charged when the system is on, and supplying power to the backup ram when the power is off.
It turns out that if you start looking at the board and the circuit, you can find a few interesting things:
1) The spot where the positive terminal of the battery is soldered down actually has two different through-holes available. This accommodates a variety of industry-standard batteries and battery holders.
2) The battery is hooked to the system voltage (and the backup rams) through a 470 ohm resistor AND a diode (the resistor and diode are in parallel). The diode is oriented so that current can flow OUT of the battery but not into it, so charging occurs through the 470 ohm resistor.
I scavenged a coin-cell battery holder from a defunct PC motherboard and soldered it into the appropriate holes on the MVS main board. I then removed the 470 ohm resistor and inserted a CR2032 battery into the holder. By removing the 470 ohm resistor, I prevent the main board from trying to charge a non-rechargeable battery, but by leaving the diode in place, I allow the battery to power the backup rams when the system is powered down.
I don't know how long this battery will last. I do note that the markings on the board indicate the original Ni-cad battery was about 50 millamp-hours, and the data sheet for the backup rams indicate that they can draw as little as 2 micro-amps when in low power mode. So I suspect it won't need a new battery for a while.
Here's a picture of the installed battery holder. You can also see where I removed the 470 ohm resistor behind it.