Pinball Battery Corrosion: How to Spot It, Stop It and Save Your Board

🔋 Pinball battery corrosion, in one paragraph

If your Gottlieb, Bally or Williams solid-state machine won't boot, resets on its own, or has forgotten every setting, a leaking backup battery is one of the first things to check. Almost every MPU (Main Processor Unit — the main brain board in the backbox) from roughly 1977 to 1989 carries a small battery to remember scores and settings. Over the decades that battery leaks, and the alkaline gunk it releases eats copper traces, chip legs and connector pins. The single most useful thing you can do to a working machine today costs nothing: open the backbox and get that battery off the board. The rest of this guide explains how to spot the damage, how to stop it, and what your options are once it has already started.

What the battery is actually doing there

Your machine keeps its high scores, audits and settings in a small memory chip — on Gottlieb System 80, Bally MPU and Williams System 3–7 boards that chip is nearly always a 5101 (a 256×4-bit static RAM). Static RAM is volatile: cut the power and it forgets everything within seconds. To hold those numbers while the machine is unplugged, the designers wired a battery straight to the 5101.

It was a sensible decision in 1980. The problem is that a battery left in place for forty years is not a memory backup — it is a slow chemical leak waiting to happen.

The same chip, the same battery, three different boards

The nice thing about this era is that the fault is almost identical across the big three manufacturers, so the fix is too:

• Bally MPU (AS-2518-17 and AS-2518-35), which also covers Stern M-100 and M-200: the 5101 sits at U8, with the battery mounted on the board nearby. The classic Bally battery-corrosion repair kits replace U8 along with a whole cluster of nearby resistors, capacitors, diodes and transistors — which tells you how far the damage usually reaches.
• Gottlieb System 80 / 80A / 80B: the MPU carries a battery pack (originally a rechargeable NiCad, very often swapped for AA cells by a previous owner) feeding the 5101. On the System 80 boards the charging circuit and the RAM are both within easy reach of the leak.
• Williams System 3 to 7: a holder of AA cells sits next to the CMOS 5101 at IC19. If those cells are flat or the holder is corroded, the game typically powers straight into audit/bookkeeping mode instead of booting normally — a useful tell.

🔍 How to tell if your board has it

You do not need an oscilloscope for this. Switch the machine off, unplug it, open the backbox and look at the MPU board with a torch. Battery corrosion has a very recognisable signature:

• A blue-green crust around the battery or its holder — that is corroded copper, and it is the giveaway.
• White or grey powder spreading out along the traces, sometimes with a faint chemical smell.
• Dark, lifted or broken traces, or chip legs that look furry, dull or eaten away.
• Green-tinged connector pins on the headers near the battery.

On the behaviour side, the symptoms that most often bring people to a repair forum are: the machine won't boot at all or hangs with a partial display; it resets or reboots during a game; it loses its settings and high scores every time it is switched off; or it boots into audit mode and won't leave. Any of those, on a machine of this age, means the battery is the first suspect.

⚠️ Why it is a slow-motion disaster, not a one-off fault

Here is the part that catches people out. A leaking battery is not like a blown fuse, where you replace the part and you are done. The electrolyte is alkaline, and once it is on the board it keeps reacting — it wicks along the copper traces, creeps under the chips where you cannot clean it, and works its way into the plated-through holes (the vias) that link the two sides of the board. That is why a board can look only mildly grubby on the surface and still be quietly dying underneath.

It is also why "I'll deal with it later" is the expensive choice. The difference between a five-minute job and a full board rebuild is often just how long the leak was left alone. If you take one thing from this article: the battery does not have to be leaking yet for removing it to be the right move.

🛠️ What to do right now (the cheap, honest fix)

Let's be straight about this, even though we sell replacement boards: if your board is still healthy, the correct fix costs a few euros and an hour of your time, and you do not need to buy anything from us to do it.

The job is simple in principle: get the battery off the board and give the 5101 its backup power from somewhere that can't damage anything. Desolder the old battery or holder, gently neutralise and clean any light corrosion (a cotton bud with a little white vinegar to neutralise the alkaline residue, then isopropyl alcohol to clean, then dry thoroughly), and fit one of the alternatives below. Here is how the common options compare:

For most people the remote holder or the supercapacitor is the sweet spot. The NVRAM route is elegant if your 5101 is in a socket rather than soldered down. Whichever you choose, the goal is the same: no battery on the board, ever again.

🧽 When the corrosion has already spread

If you have opened the backbox and found the blue-green crust already well established, be realistic about what you are looking at. Light surface corrosion caught early cleans up well. But once the electrolyte has travelled under the chips and into the vias, cleaning the surface only treats what you can see — the damage in the plated holes and beneath the ICs is exactly what you can neither reach nor fully inspect. Skilled repairers do save badly corroded boards, and it can be beautiful work, but "can be saved" and "will be dependable afterwards" are two different promises.

At that point you have three honest paths, and none of them is wrong:

• Repair it yourself or send it out. Great if you enjoy the process or want to keep the board original. Budget for the possibility of chasing a second and third fault later, because a forty-year-old board rarely fails just once.
• Keep the original and fit a modern replacement so the machine plays this weekend while the original sits on the bench as a no-deadline project. We walk through that trade-off in detail in repair or replace a pinball circuit board.
• Diagnose before you spend. If you are not sure how far the rot has gone, a diagnostic board such as the Lisy80 can tell you what is still alive on a Gottlieb System 80 before you commit to a repair or a swap.

Where a replacement board fits in

Modern replacement boards solve the battery problem by design — they are battery-free, so the failure mode is eliminated rather than managed. If your board is beyond a sensible repair, drop-in options exist for each system: the GottFA80_Plus for Gottlieb System 80/80A/80B, the WillFA7 for Williams System 3–7, and the BallyFA for Bally / Stern MPU machines. These are built on the open-source, non-commercial LISY project — a DIY effort we're glad to credit, not something we created. We test every board before it ships and support it afterwards; we'd genuinely rather you kept a healthy original alive than replaced it. To match a board to a specific title, see our guides for Bally / Stern MPU boards and Williams System 3–7 boards.

A quick word on machines you're buying

If you are shopping for a project machine — a Bally Eight Ball, a Williams Hot Tip, a Gottlieb Black Hole, or any of their stablemates — make "let me see the MPU board" part of the viewing. A photo of a clean board around the battery is worth more than any assurance about the playfield. And the moment a new-to-you machine comes home, before you even play it, open the backbox and check that battery. It is the cheapest insurance in the hobby.

❓ Frequently asked questions

How do I know if my pinball board has battery corrosion?

Power off, unplug, open the backbox and look at the MPU board near the battery. A blue-green crust on the copper, white powder along the traces, or furry-looking chip legs all point to battery corrosion. On the behaviour side, a machine that won't boot, resets during play, forgets its settings, or starts up in audit mode is a strong candidate.

Should I remove the battery from my pinball MPU board?

Yes. On any Gottlieb, Bally or Williams machine of this era, taking the original battery off the board and moving the backup power somewhere safe (a remote holder, a coin cell, a supercapacitor or an NVRAM module) is the single best preventative step you can take. Do it even if the battery isn't leaking yet.

Can a corroded pinball board be repaired?

Often, yes — light, recent corrosion cleans up well, and even badly damaged boards can be rebuilt by a skilled repairer. The catch is reliability: corrosion hides under chips and inside plated-through holes where you can't fully clean it, so a saved board isn't automatically a dependable one. Caught early it's a small job; left for years it can become a full rebuild.

What is the 5101 and why does it need a battery?

The 5101 is the small static-RAM chip that stores your scores, audits and settings on Gottlieb System 80, Bally MPU and Williams System 3–7 boards. Static RAM forgets everything the instant it loses power, so a battery keeps it alive while the machine is unplugged. That constant trickle of power is exactly why the battery is there — and why removing it means giving the 5101 a safer power source instead.

Will my machine lose its high scores if I take the battery out?

It will forget its stored settings and scores when unplugged unless you give the 5101 an alternative backup at the same time. A remote battery, a coin cell, a supercapacitor or an NVRAM module all keep the memory alive without the risk of a leak on the board. If you genuinely don't care about saved audits, you can run with no backup at all — the game will simply start fresh each time.

My machine forgets everything when I switch it off — is that the battery?

Almost certainly. A flat or removed backup battery (or a corroded holder that's no longer making contact) is the usual reason a machine of this age won't hold its settings. Fit a fresh backup power source, and if the board boots into audit mode on Williams System 3–7, that's the same story — dead backup power.

📚 Sources & further reading

• PinWiki — MPU Battery Replacement Options
• PinWiki — Gottlieb System 80
• pinrepair.com — Williams System 3–7 repair guide (Clay Harrell)
• The LISY project (Bontango)
• IPDB — Internet Pinball Database

Not sure how far gone your board is, or which route makes sense for your machine? Send us the model and a photo of the board and we'll give you an honest answer — including if the answer is that a €5 fix is all you need.