Battery Leak on a Pinball Machine: Damage, Dangers, and the Permanent Fix

A leaking battery is the number-one cause of CPU board failure in vintage pinball machines. Over time, the backup battery soldered to the CPU board leaks alkaline potassium hydroxide that eats through traces, components, and connectors. The good news: the damage can be cleaned and repaired, and there is a permanent fix to eliminate the problem for good — a battery-free replacement board. Here is everything you need to know.

Table of Contents

• Why is there a battery in a pinball machine?
• Why does the battery leak?
• The damage from a leaking battery: corrosion, NVRAM, and lost high scores
• How to spot a corroded board
• Cleaning and neutralizing corrosion, step by step
• Remote battery holder, capacitor, or battery-free board: which one to choose?
• The permanent fix: a battery-free replacement board
• FAQ

Why Is There a Battery in a Pinball Machine?

On electronic pinball machines (known as "solid state," starting around 1977), a small backup battery powers a volatile RAM chip when the machine is turned off. This memory stores the game settings, bookkeeping (audits), and high scores. Without it, the CPU board would lose everything each time the power is cut.

Depending on the manufacturer, this was either a 3.6V rechargeable NiCd battery pack or disposable alkaline cells. Gottlieb System 1 machines use a rechargeable "DataSentry" battery or a 3.6V NiCd AA cell. On Bally (and Stern-branded Bally) machines from 1977–1985, the battery is soldered directly to the middle of the MPU board (typically a three-cell AA pack). It is precisely this "on-the-board" design that causes problems decades later.

Why Does the Battery Leak?

A battery leak in a pinball machine is nothing unusual — it is the virtually inevitable fate of a twenty- to forty-year-old battery that was never replaced. Over time, the seal degrades and the electrolyte escapes. Two factors speed up the process:

• Age. Removing a rechargeable battery that is more than twenty years old is considered mandatory maintenance by experienced technicians.
• Lack of use. A machine sitting in storage and rarely powered on will see its battery discharge, swell, and eventually leak. The volatile gases and alkaline potassium hydroxide that escape then attack nearby components and connectors.

In other words, it is not a question of "if" but "when." Any original board still carrying its factory battery is a ticking time bomb.

The Damage From a Leaking Battery: Corrosion, NVRAM, and Lost High Scores

Alkaline electrolyte is extremely corrosive. Once it escapes, it first oxidizes the metal tabs on the battery holder, then wicks along the copper traces by capillary action. The consequences are progressive but serious:

• Corroded traces and components: telltale green or gray deposits, eaten-through traces, and attacked IC chip legs.
• Destroyed memory (RAM/NVRAM): the backup RAM chip and its socket are often the first casualties. The result: lost settings and high scores, and erratic game behavior.
• Corrosion spreading to connectors: on Bally machines, the corrosion can reach the J4 connector and creep underneath it. Because that connector carries 43V, even a small amount of corrosion can let the 43V leak onto the 5V bus and destroy every chip on the board.
• Damage spreading to other boards: an untreated source board can eventually damage every board it is connected to.

Let's be honest: a leaking battery is a purely electronic problem. A replacement board solves it permanently. However, it does not fix mechanical issues (coils, targets, mechanisms) or playfield wear — those are separate matters entirely.

How to Spot a Corroded Board

Before buying any used pinball machine, open the backbox and inspect the CPU/MPU board. Look for:

• white, green, or gray deposits around the battery or its holder;
• a swollen battery or a visibly aged battery still soldered to the board;
• darkened traces that are blistered or broken;
• tarnished or greenish connector pins.

On a Gottlieb System 1, a quick check is to measure the voltage across the backup RAM chip (5101): between 4.1V and 4.5V DC means the backup supply is fine. Below that, the battery is at the end of its life.

Cleaning and Neutralizing Corrosion, Step by Step

If the damage is limited, a board can be saved. Here is the method recommended by experienced repair technicians:

1. Remove the board from the backbox and immediately desolder the old battery or battery pack (dispose of it properly).
2. Identify damaged components (green/gray deposits). Clip their leads, remove them, and replace any chip or lead that looks suspect.
3. Check the connector pins: if they are green, they need to be replaced.
4. Sand the oxidized areas down to bright copper.
5. Neutralize the alkaline residue with a 50/50 mix of white vinegar and water, scrubbing with a toothbrush. This step is critical — without neutralization, the corrosion will come back.
6. Rinse with clean water, then with 99% isopropyl alcohol (which displaces the water and dries quickly).
7. Re-tin the sanded traces, reinstall components in quality sockets, and reset the audit memory.

This approach works, but it is time-consuming, messy, and best suited for people comfortable with a soldering iron. And the biggest catch: if you install a new battery, the clock starts ticking again — with the same risk of leakage a few years down the road.

Remote Battery Holder, Capacitor, or Battery-Free Board: Which One to Choose?

Three approaches exist to prevent a battery from ever leaking on your board again. Here are their pros and cons:

Solution	How It Works	Pros	Cons
Remote battery holder	AA batteries mounted off-board, connected by wires, with a blocking diode	No more leaks on the board; preserves memory for a long time	Requires soldering; batteries still need monitoring and replacement
Backup capacitor (1F)	A capacitor replaces the battery and recharges when the machine is powered on	Never leaks, never wears out	Machine must be turned on ~1 hr/month; initial charge ~8 hrs; hold time varies (one week to several months)
Battery-free replacement board	A modern FPGA-based board replaces the original electronics with no battery at all	Zero corrosion risk, Plug & Play, long-term reliability, memory retained with no maintenance	Higher upfront cost than a simple battery

The remote holder and the capacitor reduce the risk, but they still require either monitoring or regular maintenance. The only approach that eliminates the problem entirely is to take the battery out of the equation altogether.

The Permanent Fix: A Battery-Free Replacement Board

At Pinballs Store, our modern FPGA-based replacement boards faithfully reproduce the original electronics with no battery at all. Memory is retained by non-volatile circuitry — no battery means no leaks, no corrosion, and no more lost high scores. They are Plug & Play (no soldering required), ship with free technical support, and come with a 15-day satisfaction guarantee.

The right board depends on your machine:

• Gottlieb System 80 / 80A / 80B: the GottFA80_Plus all-in-one board replaces the CPU, driver, and power supply. See also the System 80 guide and the Lisy80 for diagnostics.
• Williams System 3 through 7: the WillFA7 board. Details in the Williams guide.
• Bally 1977–1985: the BallyFA CPU board for Bally machines. Learn more on the BallyFA product page.

For more on the choice between repairing and replacing a board, check out our article: Should You Repair or Replace a Pinball Machine Board?

FAQ: Battery Leak on a Pinball Machine

How do I know if my pinball machine's battery has leaked?

Open the backbox and inspect the CPU/MPU board around the battery. White, green, or gray deposits, a swollen battery, or darkened traces are clear signs of a leak. On a Gottlieb System 1, a backup voltage reading below 4.1V means the battery is at the end of its life.

Can a leaking battery destroy an entire board?

Yes. The electrolyte wicks along the traces and can reach the connectors. On Bally machines, the corrosion can let the 43V from the J4 connector bleed onto the 5V bus, damaging every chip on the board. Catching it early limits the damage.

How do I clean battery corrosion off a pinball machine board?

Desolder the battery, replace damaged components, sand down to bare copper, then neutralize the alkaline residue with a 50/50 white vinegar and water solution. Rinse with clean water followed by 99% isopropyl alcohol, re-tin the traces, and reset the audit memory.

Should I install a new battery after cleaning?

You can, but the risk of leakage will return in a few years. The alternatives are a remote battery holder, a backup capacitor, or — the permanent fix — a battery-free replacement board.

What if I lose my settings and high scores?

They are stored in the memory backed up by the battery — a dead or removed battery erases them. After a repair or the installation of a modern board, you will need to redo the settings. A battery-free board will then preserve the memory indefinitely with no maintenance.

Does a battery-free replacement board permanently solve the problem?

Yes, for the battery-related electronics. With no battery, there is nothing left to leak or corrode. However, it does not fix mechanical issues (coils, mechanisms, playfield wear), which require separate maintenance.

Sources & Further Reading

• Flipjuke — Gottlieb System 1: batteries and corrosion treatment
• FlipperFrance — Boards damaged by battery acid: what to do?
• Multibille — Repairing Bally 1977–1985 machines