Why Do Batteries Leak?

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You switch on your flashlight, and—nothing happens.

“Ah, the batteries must be dead,” you assume.

So, you open up your torch to replace the worn-out cells, but then your heart sinks. The‌re is ‌a‌ ‌white‌ ‌crusty‌ ‌mess covering the inside.‌ ‌The‌ ‌batteries‌ ‌have‌ ‌leaked,‌ ‌wrecking‌ ‌your‌ ‌flashlight.

A familiar story, right?

Leaking batteries cost you time, money, and inconvenience. What’s more, if you don’t take precautions, they can also cause injury.

So why do batteries leak? And is this unwanted situation avoidable?

Sit back, as I reveal the whole, surprising truth.

The main reasons why batteries leak:

  • Past their expiration date.
  • Incorrect insertion.
  • Subject to excessive heat.
  • Stored incorrectly.
  • Mixing old with new.
  • Combining different types.

Batteries—a Ticking Time Bomb

They appear innocuous—cylindrical cells that you just slot into your flashlight and power the bulb or LEDs. In short, they just make things work.

Yet, their casing hides an intensely chemically saturated environment, constantly in flux, which generates the energy to fuel your equipment. And, it’s these conditions which make them somewhat volatile—and prone to leakage.

To understand why these cells can spill their contents requires a quick battery function 101.

In brief, a battery converts its internal chemicals into electrical energy.

Typically, a battery consists of three parts—an anode (positive end), cathode (negative end), and an electrolyte—sometimes liquid, but in most cases, a dry powder.

The moment you connect the batteries to your torch and switch it on, the chemical reactions begin. While these may vary between battery types (alkaline, zinc chloride, etc.), the result is the same.

The chemicals cause an abundance of electrons to build up on the cathode—producing a shortage of electrons on the anode (the metal plate connected to the positive battery terminal).

These electrons hate being out of balance at either end and want to restore equilibrium. But the chemicals in the cell create a barrier preventing them from simply moving from one end of the battery to the other. Hence, they look for a different route.

In your flashlight, as the battery completes a circuit, the pathway involves passing through the bulb or LEDs. The electrons rush out of the cathode, lighting up the flashlight on their way as they flow back into the battery through the anode.

saveonenergy.com has some great animations on how electricity flows in batteries, which show this process in detail.

As long as you keep your flashlight switched on, this process will continue providing constant light—until the battery dies.

But, there’s an issue.

This chemical process causes high internal pressure in the battery. If it ruptures, the chemicals leak into your equipment—not only rendering the cell itself useless but also running the risk of wrecking your device.

Here’s a quick video explaining the working processes behind batteries:

Causes of Battery Leakage

It happens all too often.

I’m sure that you’ve seen the many YouTube videos of dropping a Mentos sweet into a bottle of cola (if you haven’t, take a look here). A chemical reaction occurs that causes an immense geyser due to the build-up of gas (in this case, carbon dioxide).

In a battery, a similar process happens, but instead of producing CO2, it creates a surplus of hydrogen. The seals at both the cathode and anode ends should be strong enough to withstand this gaseous force—however, sometimes they fail.

The leading causes behind battery leakage are:

Expiration Date

In your home, you no doubt have a battery drawer, cupboard, or box. But, do you have any idea how long those batteries have been in there?

Cells have an expiration date—and it’s not, as some people believe, a marketing trick to make you replace them frequently. Over time, batteries degradethe seals around the battery electrode terminals can lose strength, leading to leakage.

While some manufacturers will print an expiration date on every battery—commonly, you only see them on the original packaging. So, try to keep the cells inside their retail packs—or label them yourself with their use-by date.

Placing past-their-best cells in your flashlight can be playing Russian roulette with your prized flashlight.


Let’s assume in the above example you have some batteries kicking around in your drawer. They’re out of date—but as you haven’t placed them in your torch yet—no pressure is being created in the battery, right?

Unfortunately, no.

Batteries undergo a phenomenon known as self-discharge. In short, despite not being in a circuit, chemical reactions still occur inside the cell, draining its power and producing pressure. The rate of this discharge depends on the type of battery, its current charge level, and temperature.

So, even when in storage, your batteries can leak. If this happens, the chemicals emitted can speed up the degradation of any other cells they’re in contact with—possibly leading to a domino effect.


If you’re using your flashlight and the batteries die, you should immediately replace the cells and safely dispose of the expired ones.

But that doesn’t always happen.

If you don’t have a replacement set of batteries within easy reach, you may decide to pick up an alternative torch and resolve to sort the problem out later.

The issue is that the seemingly expired batteries still contain chemicals and gas. They’re continually trying their hardest to power your torch, leading to over-discharge. This places stress on the cells, which can lead to rupture and leakage.

Furthermore, as you sit there reading this article, you could be over-discharging batteries in your devices—even if they’re still functioning.

You’ve no doubt read the warnings on battery packaging for their ‘safe’ use. Often, they specify never to mix old and new batteries, nor to mix cell types.

Again, it’s easy to be cynical.

You may suspect that this is a strategic plan by manufacturing magnates to scare you into buying lots of new batteries from their brand. But, it has a basis in fact.

Combining different battery types, such as zinc and alkaline, or using a half-drained battery alongside a fully-charged one can cause issues. Dissimilar charge levels can place undue stress on one or more cells, leading to over-discharge, rupture, and leakage.

Incorrect Insertion

Your flashlight is dead, so you replace the batteries with a brand new set.

And it still doesn’t work.

If you’re anything like me, the first thing you surmise is that you’ve been somewhat of an idiot and inserted one battery the wrong way round. So you check, and if that’s the issue, relocate the offending cell.

You’ve been there, I know.

Unless you’re dealing with some seriously high amps, there’s no harm done. However, leave them in this positive-positive or negative-negative position for any length of time—and you have problems.

Not only will they rapidly discharge—it plays havoc with their chemical processes, which can lead to expansion, heat generation, and leakage.

Most often, this occurs after an electric device—such as a flashlight or remote control—is dropped and the cells fall out. You place them back in the compartment and assume all is well. But, you may have inserted them incorrectly.

Hence, always check that your equipment works immediately after any battery replacement.

Parasitic Drain

Once you’ve switched off your flashlight—you would suspect that it’s drawing no power from the cells.

Unfortunately, this isn’t always the case.

Features such as programmable switches, or handy ‘find-me’ LEDs, can be continually using cell energy. This places a constant pull on your batteries. While unharmful in the short term, if you don’t check these devices for a long time, the cell can drain and run the risk of over-discharge.


Like me, you’ve probably read your fair share of battery packaging. Ok, it’s not that thrilling, but you know the line—store in a dry, cool place.

It’s good advice.

Firstly, although movies are full of tropes of people experiencing electrocution in water—it’s a pretty awful conductor of electricity. It is, however, more efficient than air.

Therefore, if you place batteries in moisture-rich environments, this can create a circuit leading to battery drain, over-discharge and leakage.

Secondly, temperature increases the degradation of the battery materials and chemicals and increases discharge—again causing leaks.

What Happens When a Battery Leaks?

Once the seals of your batteries fail, the serious issues begin.

In the case of alkaline batteries, the most widely used type, they expel potassium hydroxide. This combines with carbon dioxide in the air to form potassium carbonate—white crystals, which then grow alongside the metal circuitry and cell terminals, having a corrosive effect.

Not only is this the end of your battery, but it can also mean the death of your device.

Similarly, carbon-zinc batteries expel zinc chloride, which has the same corrosive effect. Research indicates that rechargeable batteries, particularly Ni-MH (nickel-metal hydride), are particularly resistant to leakage.

How to Identify Battery Leaks

In severe cases, you don’t need to be a battery expert to see that your cells have leaked. However, noticing some of the signs can prevent further degradation and your flashlight or other equipment suffering damage beyond repair.

The leading indicators of battery leaks are:

  • Darkening of the plastic skin of the cell.
  • A rainbow appearance beneath the plastic battery skin.
  • White/brown crystallization on the contacts.
  • Unusually warm batteries.
  • Deformation or swelling of the cell casing.
  • Distinctive metal or egg smell.
  • Sticky batteries.
  • Cells are difficult to remove.

If you witness any of the above signals—it’s time to take action.

Remove and dispose of the batteries and check the cell compartment and contacts for any indications of leakage. If there are signs of any seepage, clean the area thoroughly with a cotton swab before inserting new batteries.

Are Battery Leaks Dangerous?

Whether removing leaking batteries from your torch or cleaning any residue, take care.

The leaking chemicals, particularly potassium carbonate, can cause some health concerns.

These include:

Skin Damage

If you get the potassium carbonate on your hands, it may cause some irritation and burns—particularly with prolonged exposure.

Should this occur, rinse the affected area with fresh water for around ten minutes, dry, and then apply an emollient. If you experience pain or you witness any deterioration to your skin—seek medical advice.

Eye Damage

Potassium carbonate can irritate the eyes. In severe cases, it may induce cornea damage and blurred vision.

In all cases, rinse immediately with water for 15 minutes and then consult with a physician.

Gastrointestinal Issues Naturally, a battery leakage will not tempt you to have a taste. However, should you get any chemicals on your hands when cleaning, it can make its way to your mouth, and problems can arise.

Therefore, always wear gloves when handling damaged batteries.

Ingestion can lead to:

  • Swelling of the throat which may impair breathing.
  • Drooling.
  • Intense stomach pain.
  • Diarrhea.
  • Pain in the chest.
  • Drastic fall in blood pressure.
  • Vomiting—often containing blood.

Should you accidentally consume potassium carbonate, immediately consume milk or water—unless you’re vomiting. Then, seek medical advice.

Breathing Problems

Inhalation may cause coughing, chest tightness, and breathing problems due to irritation of the respiratory tract.

In this circumstance, move to an area of fresh air. Should your symptoms continue, seek medical advice.

Right, enough of the scary stuff.

In the real world, you’d have to be extremely careless and be dealing with a massive battery leak to be putting yourself at any kind of dangerous risk. I’ve dealt with and cleaned numerous cell leakages in the past, and I’m still here to tell the tale.

Be sensible, take a few precautions, and you’ll be fine.

How to Clean a Battery Leak

Your flashlight isn’t working, you pop open the battery compartment, and your heart sinks as you see the signs of leakage.

Don’t despair.

This doesn’t mean that your favorite torch is now consigned to the scrapheap (although the batteries are).

Here’s how to cope with this unfortunate, but all too common situation:

1. Protection, Protection, Protection

To avoid the nasty effects of battery leakage—prepare yourself for battle.

Cover your hands with rubber or latex gloves. Ideally, a face mask and safety goggles can prevent any incursion of the chemicals into your hands or eyes. If possible, work in an area that provides excellent ventilation and with any breeze blowing away from your face.

2. Remove the Batteries

Take, not only the offending batteries from your equipment but also any cells which still appear in good condition. While they may superficially seem undamaged—and may even work when tested—the chances are they have begun to degrade.

Place the cells in a transparent plastic bag (so you can examine them for the next step). Ideally, double-bag for additional safety. Seal immediately to prevent any contamination of the air and then put aside for recycling.

3. Identify the Battery Type

As it’s highly unusual for Ni-MH batteries to leak, you’ll most likely be looking at a zinc-chloride or alkaline issue—and it’s important to know what you’re dealing with as it affects how to treat the problem.

A quick examination of the battery casing will let you know the type. Here are the two methods of resolution:

Alkaline Batteries

Unremarkably, the cell spills have an alkaline pH—so you need an acid to neutralize.

Get hold of a cotton swab and dip in vinegar or lemon juice. Coat the leak with this liquid. The alkaline-acid combination will then start to fizz—leave it alone until it stops.

Repeat the above procedure until no further reactions take place.

Begin to rub the affected areas with a fresh swab—if you need something a little more abrasive, an old toothbrush is ideal.

Zinc-Chloride Batteries

These cells leak acids—so you require an alkaline application.

Liberally cover the affected areas with baking soda—which again will promote a reaction. Repeat until the fizzing stops. If the leak has encrusted itself with little signs of liquid, rub with a paste made with baking soda and water—using a cotton swab as your application tool.

4. Dry 

Whether you’ve followed the zinc-chloride or alkaline cleaning method—this next procedure is the same for both types.

Remove any remnants of the leakage (and vinegar or baking soda) with a dry cloth or kitchen tissue. Ensure that no liquid remains, as this could cause future oxidation of the contacts.

5. Clean the Contacts

Should any residue remain on the casing or contacts, gently scrape with a plastic or wooden tool (such as a toothpick). If there are signs of severe corrosion on the contacts, sand with fine glass paper or a small metal file (ensure you’re still wearing a face mask and safety glasses).

The metal contacts can then be ‘polished-up’ using a standard pencil eraser.

Leave the battery compartment open for a couple of hours to ensure any remaining moisture evaporates and then test—with new batteries.

Here’s a quick visual guide on cleaning those scary leaks:

How to Avoid Battery Leaks

Following the appropriate procedure will, in most cases, mean your devices suffer no long-term damage from battery leakage. However, as the old saying goes, “An ounce of prevention is better than a pound of cure.”

Here are my top tips to ensure a leak-free life:

Read the Instruction Manual

Whether using a flashlight, radio, or clock—always read the manufacturer’s instructions.

Often, they will specify the best type of battery for that particular device. Using the incorrect variety may place undue stress on the cell—which, as we’ve seen, can lead to leakage.

Turn off Devices When Not in Use

With a torch, this is pretty obvious. You’re hardly likely to come into the house after searching the garden for your cat—and leave your flashlight on.

However, in other devices, especially children’s toys, it may not be so apparent that the equipment is still running.

A constant drain on the cells can lead to over-discharge and a leak.

Remove Batteries

Earlier in this article, I explained the concept of parasitic drain—a continuous trickle from the battery even when the equipment isn’t in use.

If you aren’t going to be using the electrical device for a while, remove the batteries. Not only will this reduce the chance of draining (and prolong the life of your cells)—but it also means that should a leak occur—your equipment isn’t compromised.

Don’t Mix and Match

Combining old and new, or zinc with alkaline is a recipe for disaster.

Store Safely

Being metal, heavy, and solid—it’s easy to consider batteries as robust objects.

They’re not—their temperamental and delicate—so treat them accordingly.

When storing your cells, do the following:

  • Keep in original packaging—it protects them from environmental damage, prevents contact with other batteries or objects (making a short circuit), stops you mixing old and new, and maintains the expiration date.
  • Ensure they’re cool and dry—preventing degradation.
  • Don’t throw them into drawers—contact with metal such as keys, paperclips, and pens can create short circuits and promote leakage.

Remove Dead Batteries

Your torch fails due to exhausted cells—but you don’t have any replacements in your home.

Don’t just put the flashlight back in the garage and resolve to buy some new batteries next time you’re at the store. Immediately remove those useless power packs and dispose of them appropriately.

An exhausted battery is likely to degrade, leak, and wreck your torch if not removed.

Buy Quality Batteries

It can be tempting to buy a battery pack from the dollar store—apparently containing so many cells, it could power a small country—but be careful.

Some manufacturers make poorly-made bargain-basement batteries with little concentration on quality. Substandard seals can mean that a seemingly money-saving purchase could cost you your flashlight.

Many top-end manufacturers provide a leak-free guarantee. Not only promising to replace any leaking batteries free of charge, but also replacement of any damaged devices.

Powering Down

Leaking batteries can wreck your torch and other electrical devices—the good news is, it’s avoidable.

Cell degradation occurs through misuse, incorrect storage, neglect or abuse. Follow the above tips, and you should be trouble and leak-free.

Yet, should the worst happen, you now have the technique to perform life-saving surgery on your seemingly terminal flashlight.