The white crust on an old AA battery is not acid
Open the back of a remote that has sat unused for a while and you may find it. A white, sometimes bluish crust has grown around one end of a battery. Almost everyone calls it battery acid.
In an ordinary alkaline AA or AAA, it is closer to the opposite. The liquid inside an alkaline battery is potassium hydroxide. This liquid, the electrolyte, is what lets current flow between the battery's two halves. Potassium hydroxide is a strong alkali, the chemical opposite of an acid. Energizer's technical handbook for its alkaline cells lists it as the electrolyte, and Panasonic describes it as a strongly alkaline solution.
The crust is the leaked liquid, turned into a salt by the air
When that liquid escapes, it meets the air, and the air contains carbon dioxide. The two react and form potassium carbonate, a white crystalline salt. Panasonic's battery FAQ names exactly this as the white crystals and powder on a leaking alkaline battery. The science explainer The Naked Scientists gives the same account.
The salt is still alkaline. Panasonic calls it a potent water-soluble alkaline substance that can cause chemical burns on skin. PowerStream, a battery company, takes a milder view and describes the converted residue as safe. The manufacturers' own safety pages are the more cautious of the two, and they treat the crust as something to handle with care.
A battery can leak after it is already dead
Most devices hold two or more batteries in a row, so the current passes through each one in turn. No two cells hold exactly the same charge, so one always runs out first.
The device does not stop the moment that happens. The cells that still have charge keep pushing current through the empty one. Energizer's alkaline FAQ puts it this way : as the weakest battery becomes exhausted, it is 'force discharged by the stronger batteries to very low or negative voltage levels which drastically increases the potential for leakage.'
So the usual picture can run backwards. It is not always the leak that killed the battery. Sometimes the battery leaked because it was driven on after it was empty. Manufacturers describe this route more directly than almost any other, though none of the sources says what share of leaks begin this way.
A single battery can be driven too far as well. Duracell notes that some devices keep drawing power from exhausted batteries, which may cause them to leak. Panasonic warns about batteries left for long periods in equipment that is not fully switched off and still draws a small current. They can run down and over-discharge, and that can cause leakage.
Mixing old and new batteries makes the gap worse
Anything that makes the cells in one device unequal makes it more likely that one of them is pushed past empty. Energizer says that mixing fresh and used batteries, different brands or different chemistries can significantly increase the potential for leakage, because their capacities do not match. Panasonic says the difference between a used and an unused battery grows during use, which leads to over-discharge and leakage. Duracell's FAQ says not to mix old and new batteries, different brands, or different types.
Energizer's list of what raises the risk for alkaline cells is short and specific : trying to charge them, mixing chemistries, mixing fresh and used cells, physical damage, long exposure to high temperature, and deep discharge.
Inside, the battery is slowly making gas
Why would draining a battery make it spill? Energizer's handbook says hydrogen gas is a byproduct of the chemical reaction in all alkaline batteries. Under normal use the amount is very low. Charging or short circuits produce much more.
Deep discharge matters here too. The same handbook advises device makers to cut off at 0.8 volts per cell, by which point about 95 percent of the usable capacity is gone. The reason it gives is the increased chance of excessive internal gassing when alkaline batteries are deeply discharged. This is Energizer's design guidance for its own cells, not a voltage at which a battery is known to leak.
Gas needs somewhere to go. Energizer's handbook describes a nylon seal that provides a safety venting mechanism. When pressure opens it, the liquid inside can come out with the gas. PowerStream describes the same sequence. Zinc inside the cell slowly breaks down water, the hydrogen builds pressure, and the vent opens and pushes electrolyte out.
This is one documented route, not the story of every leak. Panasonic also names heat, humidity, short circuits, impact and incorrect installation as things that can make a battery leak.
Batteries that were never used can leak too, but less often
Manufacturers describe leaks in normal storage as uncommon. Energizer's alkaline FAQ says alkaline batteries typically will not leak under normal storage and usage conditions.
In their accounts, the things that change this are heat, damp and time. Panasonic names high temperature and high humidity as the most significant factors for leaks in unused batteries, and also lists accidental short circuits and impact. Energizer's handbook adds that repeated swings between extreme heat and cold make the parts expand and contract and, over time, can break the seal. Panasonic also notes that a battery that was over-discharged and then left unused for a long time may leak later.
The chemistry never fully stops. Panasonic says a battery's chemicals change gradually even when it is not used. The Naked Scientists says that batteries keep reacting slowly even when nothing is connected, and that side reactions can eventually make them pop open. None of the sources says that every stored battery that leaks has followed one particular sequence. Energizer recommends storing batteries at room temperature with moderate humidity, and says the refrigerator is neither needed nor recommended.
Not every battery leaks the same thing
Alkaline cells, the common AA and AAA, hold potassium hydroxide, and their crust is potassium carbonate.
Zinc-carbon cells, often sold as heavy duty or general purpose, are different. Panasonic describes their electrolyte as a mildly acidic solution of zinc chloride or ammonium chloride, so here the leaked liquid really does lean acidic. Energizer says these cells also become more likely to leak when they are force discharged.
Rechargeable NiMH cells are not leak-free. Energizer's lithium handbook rates their leakage resistance 'Good', the same rating it gives alkaline cells. Panasonic warns that an over-discharged NiMH battery releases hydrogen gas. Duracell's cleanup advice groups NiMH leaks with alkaline ones.
Lithium AA cells use a different chemistry. Energizer's lithium FAQ says its lithium cells 'do not leak as alkaline batteries do', and its handbook rates their leakage resistance 'Superior'. The same FAQ says cells that have seen extreme abuse may vent and discolor the top cap. These are a manufacturer's statements about its own products.
Car batteries are where the old word fits. A lead-acid battery holds sulfuric acid. Even so, the crust on its terminals is not liquid acid but salts formed with the help of that acid. Jalopnik, a car news site, describes the bluish-green deposit on the positive terminal as copper sulfate.
Why vinegar works on AA leaks and baking soda does not
The chemistry decides the cleanup. A mild acid neutralizes something alkaline. To clean alkaline, NiCd and NiMH leaks off a device's contacts, Duracell's FAQ suggests vinegar or lemon juice diluted half and half with water, or boric acid in water, applied with a toothbrush or cotton swab. It says to wear household gloves and glasses, work in a well-ventilated area, and let the device dry completely before new batteries go in. This advice is for the device only, not for skin or eyes.
The familiar baking soda advice belongs to car batteries. Baking soda is a mild alkali, which is why Jalopnik recommends it for acidic terminal corrosion. Against an alkaline leak it has nothing to neutralize.
The manufacturers are direct about safety, and for skin and eyes their answer is plain water. Energizer calls alkaline battery leakage extremely caustic and says contact with bare skin should be avoided. If it touches skin, Energizer says to flush the area with plenty of water for 15 minutes. If the liquid gets into the eyes, Panasonic says not to rub them, to rinse with plenty of clean water and to get medical advice promptly, and it warns of the risk of chemical burns or blindness. Duracell says to contact a physician if battery chemicals reach the eyes.
The crust tells you what leaked, not always why
So the white crust in the back of the remote tells part of its story clearly. If the battery was alkaline, the liquid inside was an alkali, not an acid, and the air turned it into a salt. How the liquid got out is harder to read from the crust. One well-documented route is a leak after the battery died. Its neighbors or the device kept pulling current through it, gas built up, and a seal built to give way under pressure did just that. Heat, damp, age, a short circuit or damage can also start a leak, and the crust alone does not show which of these happened.
That is also why the manufacturers' advice is so plain. Duracell says to remove batteries as soon as they are depleted, and Energizer, Panasonic and Duracell all warn against mixing old and new.
