Repair leaked battery damage

Batteries are used in almost every brand and model of electronic pinball machine. They are required to keep track of settings (difficulty, number of balls, free play, ..) and audits (amount of games played, earnings, high scores and much more).
The problem is that batteries don't age well. Sooner or later batteries will start to leak. This can be because of age, because they're empty, or because of external influences (heat, humidity). Sometimes new batteries are defective and they start to leak a few days or week after installation. Whatever the reason is, you have to check on them because they will leak someday.
Once a battery starts to leak, the fluid inside will damage the printed boards and damage every component it comes in contact with. Because this is a slow process, it is not always discovered soon enough. The battery may keep enough of a charge so that the game still starts well. Usually only when a component on the board is damaged too much that the game does not start anymore, the owner may investigate what's wrong. Usually at this point then the damage is already too big to repair.

To make matters worse, the batteries are always located on the cpu board. That's the computer board that controls the whole game. If it is damaged your pinball machine will not play anymore. For some brand of replacement cpu boards have been made, but usually they are expensive (usually between $200 and $300). For some types of games, like the WPC-95 pinball machines (which popular games like Medieval Madness, Attack From Mars, Scared Stiff and Cactus Canyon are), new replacement cpu boards are just not available. Only used boards are available, but these games never show up cheap on ebay and can be parted out for their boards..
Prevention of the damage is thus required ! For more information about preventing battery damage, check my article about the Factory Settings Restored warning message.

Williams System 11 games show 'Adjustment Failure' (or 'ADJUST FAILURE' on games with smaller displays), WPC games warn for empty batteries with 'Factory Settings Restored'

Location of batteries in pinball machines

The exact type and location of the battery is different between brands and types of pinball machines, but it is always located inside the backbox.
More information: How to open the backbox of a pinball machine.

Bally and Stern electronic pinball machines have one rechargeable battery. It's at the bottom middle of the cpu board and looks like a large AA battery.
Gottlieb System80 games use a rechargeable DataSentry battery. It's located at the left side of the cpu board (but in some games this is mounted on its side), it looks like a black box of about 1inch wide, with DataSentry in white letters on it.
All Williams pinball machines (older electronic to the latest WPC-95) use three AA batteries (regular, non-rechargeable).
Data East, Sega and first generation of Stern games also use three regular AA batteries.
The latest generation of Stern games uses a lithium cell.

Except for the latest generation of Stern games, on all other brands and types you should remove the batteries, even when they have not leaked yet. Most rechargeable batteries kan just be desoldered and are not required to start the game. With games that use three AA batteries it's best to install a remote battery holder.

Stern games like Lord of the Rings use a battery holder has a slightly different orientation in comparison to Data East/Williams (those all have the + on one side). Stern battery holders have alternating poles, when you install a remote battery it's a different configuration.

Identifying and repairing battery damage

Battery damage on a WPC CPU (Popeye).
Notice all the green corrosion on the parts underneath the place where the batteries were.

Don't panic when you have discovered leaked batteries. Most of the time it is possible to fix the damage (maybe you can even do it yourself, or have it checked and repaired by a qualified pinball repair person). It is important not to ignore the problem and act as fast as possible.
The problem with leaked battery damage is that it has to be neutralised completely. Just removing the source (the bad batteries) is not enough to solve the problem. The existing spill will continue to spread and do it's damaging work until it is completely neutralised.
AA batteries leak a fluid which is a base, therefor you have to neutralise it with the opposite: an acid. Vinager is most common used to repair battery damage.
The procedure is to neutralise and remove all corrosion using a toothbrush and vinegar, and later rinse with either distilled water (and let dry long enough) or alcohol, to remove any residu left behind. With bad cases of corrosion, the damage must be removed using sandpaper or a small sandblaster / bead blaster.

This battery holder was installed in my own Bally Cirqus Voltaire pinball machine. After a few years the batteries has leaked. Luckily only the battery holder is damaged and not the expensive cpu board !

If you are lucky only the battery holder is damaged. If the damage is only on the springs or clips that keep the battery in place, you can try to neutralise that using vinager. But ideally you need to cut if off the board (or unsolder) and install a new remote battery holder (or solder new wires to the board).
Do inspect the board very well to make sure the damage is only on the battery holder and not on the board. It can be underneath the battery holder or under nearby ic's. Therefor you should really remove the battery holder to make sure the damage is not in the board itself.
Look at the backside of the cpu pcb. Sometimes (in very rare instances) the damage may not be immediately around the battery holder but only occur several inches around it. It will look like a wet, dark spot.

What most people underestimate, is that the leakage can even pass over electric wires and occur on other pcbs where the cpu is connected to. Make sure you inspect everything and don't miss corrosion on other part of the game. Sometimes even boards in the cabinet are affected (while the battery is located in the backbox) ! The damage on the cpu may only be small, but more damage may have happened somewhere else !

This small board is located at the bottom of the cabinet. The leaked base of the battery has travelled over the electric wires and damaged the components on this board too. There's almost 2 metre of wire between the battery and this board.

Often however the damage is discovered too late and the leakage has already spread from the battery holder onto the pcb. Depending on how long the damage has been able to work onto the pcb and the components, it will look different. The leaking base fluid will just look like a wet spot on the pcb. Neutralise it with vinegar.

When it has been able to work a longer time, the solder spots will all be dull. A good solder point should be shiny.
Battery damaged components will have dull solder points, IC legs will also be dull grey, .. Often the pcb will still work correctly and you can still neutralize the damage yourself. If you're lucky it'll not spread, if you're unlucky something can break down a long time later (sometimes a year later or longer). A professional repair person will therefor unsolder and replace anything with dull grey solder.

The worst damage is when you see green corrosion. This corrosion will not only attack clear metal (solder points and ic pins) but the components themselves. Even the bodies of resistors and diodes will be damaged and the components will fail. When your pcb looks this bad, you are probably not be able to repair it yourself. It's best to start looking for a replacement pcb or to send it off for professional repair, as it will fail in the future.

Battery damage on a Gottlieb System80 cpu board. The battery itself has been removed (notice the holes at the bottom center). The leaked base is still there (looks like dark moisture on the green pcb). Almost every component in this picture has corrosion and must be replaced. Components at the bottom have grey dull solder, the row of resistors at the center top have green corrosion on their legs, and the corrosion probably also got underneath the bank of dipswitches at the top right.

I once saw a Twilight Zone cpu board that did not boot anymore. The owner had the game for about 6 months, up to then it had always worked correctly. Upon close inspection, the problem was caused by leaked battery damage. While there was on first sight no trace of corrosion or that a repair had been done, there was still some green corrosion visible on the inside of IC legs (and parts underneath ICs).
The previous owner probably had correctly neutralized the damage (using vinager and a toothbrush) so no visible corrosion was left, but had not cleaned underneath all ICs. There it had just continued to do its damaging work for months. These ICs should've been desoldered and replaced too, although they still functioned at the time the battery damage was fixed.
If the damage is this bad, a professional repair person can still repair the board. But it's not an easy or cheap repair - all influenced components have to be desoldered. Using a small sandblaster all traces on the board are cleaned. Then new components are again soldered in place.