Dropping a battery into water triggers an electrochemical reaction: the battery’s voltage drives the surrounding water to split into hydrogen and oxygen gas, while the battery’s metal components begin to corrode. How dramatic that process gets depends heavily on the type of battery, how much charge it holds, and whether the water is fresh or salty. A single AA cell in a glass of tap water will bubble quietly and slowly rust; a lithium-ion cell from a phone or electric vehicle in saltwater can release toxic fumes, generate enough hydrogen to pose an explosion risk, and leach heavy metals into the environment.
Ordinary Alkaline Batteries in Water
When you drop a standard alkaline battery (the AA, AAA, C, or D cells in your junk drawer) into water, the battery essentially tries to use that water as an external circuit. A small electric current flows through the water between the positive and negative terminals, and that current breaks water molecules apart. Hydrogen gas forms at one terminal, oxygen at the other. The bubbles are usually tiny and slow because the voltage of a single alkaline cell is only about 1.5 volts, which isn’t much force.
The more visible problem is corrosion. Alkaline batteries contain a potassium hydroxide (KOH) electrolyte paste. In the presence of moisture, that electrolyte can leak through the battery’s seals. Research on zinc-air batteries, which share this alkaline chemistry, has confirmed that humid conditions alone can cause KOH residue to appear on a battery’s exterior surface.1Frontiers in Materials. Synergetic effect of temperature and humidity on the leakage of KOH electrolyte and related reliability of zinc-air batteries Submerge the battery entirely and you accelerate that leakage dramatically. KOH is a strong base, so the water around the battery becomes increasingly caustic over time. The battery’s zinc casing corrodes, the steel jacket weakens, and eventually the whole cell falls apart, leaving behind a slurry of metal oxides and alkaline paste.
For most people, this scenario plays out accidentally: a flashlight falls into a puddle, or old batteries sit in a damp garage for years. The practical risk is modest. Alkaline batteries carry limited energy, the gases produced are in very small quantities, and the corrosion happens slowly. The main concern is the caustic residue, which can irritate skin and damage whatever device the battery is sitting in.
Lithium-Ion Batteries React More Aggressively
The rechargeable lithium-ion cells in phones, laptops, power tools, and electric vehicles are a different story. These batteries operate at higher voltages (typically 3.6 to 4.2 volts per cell), carry far more energy, and contain organic solvent-based electrolytes that react badly with water. When water enters a lithium-ion cell, it can trigger internal short circuits and chemical reactions that generate heat, flammable gases, and toxic byproducts. Incidents involving lithium-ion batteries have been linked to water ingress as one of the pathways to failure.2ScienceDirect (Elsevier). A review of hazards associated with primary lithium and lithium-ion batteries
One of the clearest dangers is hydrogen gas. When lithium-ion battery packs are exposed to water, the metallic components corrode and produce hydrogen at a rate that increases with the mineral content of the water and the voltage of the battery pack. Research on submerged packs found that this corrosion also causes physical damage: connection plates can detach, the metal caps on electrodes thin out, and safety valves can perforate and fail.3Elsevier / Journal of Energy Storage. Hydrogen releasing law and in situ computed tomography investigation of structural damage of waded lithium-ion batteries In an enclosed space like a flooded vehicle battery compartment, that hydrogen can accumulate to flammable or explosive concentrations.
There is a counterintuitive twist, though. Research on using water submersion to prevent lithium-ion battery fires found that submerging fully charged cells in deionized (pure) water actually kept them stable and did not cause corrosion, while synthetic seawater caused rapid voltage discharge, electrode corrosion, and toxic effluent release. The seawater submersion did, however, effectively prevent thermal runaway when cells were subjected to extreme overheating.4ScienceDirect (Elsevier). Seawater submersion for cylindrical lithium-ion batteries thermal runaway prevention So water can simultaneously damage a battery and prevent it from catching fire, which is why firefighters sometimes flood burning EV battery packs with enormous amounts of water despite the chemical mess it creates.
Why Saltwater Accelerates the Damage
If you’ve ever heard that saltwater and electronics don’t mix, batteries are the extreme case. Dissolved salts dramatically increase water’s ability to conduct electricity, which means the electrochemical reactions happen faster and more forcefully. More current flows, more gas is produced, and corrosion eats through metal surfaces much more quickly than in pure water.
The gas issue gets more complicated in salt solutions. When enough voltage is present across battery terminals in a sodium chloride solution, the reaction at one electrode produces hydrogen gas, while the other electrode can produce either oxygen or chlorine gas. The chlorine formation is thermodynamically less favorable than simple water decomposition, and studies of lithium-ion batteries discharged in salt solutions found that any chlorine produced was in negligibly small amounts. Still, the possibility cannot be completely ruled out, and chlorine gas is toxic even in small quantities in a poorly ventilated space.5PubMed Central. Discharge of lithium-ion batteries in salt solutions for safer storage, transport, and resource recovery
This is relevant beyond lab experiments. Coastal flooding, storm surges, and vehicle submersion in brackish or ocean water all expose batteries to salt solutions. Electric vehicle battery packs that flood in hurricane zones face this exact chemistry, and emergency responders are increasingly trained to watch for hydrogen buildup around submerged EVs.
The Button Battery Problem
One of the most dangerous real-world scenarios involving batteries and moisture doesn’t happen in a pool or a pond. It happens inside a child’s body. Button batteries, the small disc-shaped cells found in hearing aids, watches, and toys, cause severe injuries when swallowed because they create the same electrolysis reaction against moist tissue that they would in a glass of water.
The tissue damage is fast and brutal. A button battery lodged against the lining of the esophagus generates an electric current through the surrounding moisture, producing hydroxide ions at the negative terminal. This creates a highly alkaline environment, with a local pH reaching 10 to 13, which is caustic enough to dissolve living tissue.6PubMed Central. Current management of button battery injuries Investigations into the mechanism have confirmed that the battery drives an electrolysis reaction in the moist environment: the positive electrode produces an acidic zone while the negative electrode produces a strongly basic zone, and the alkaline side causes deep, penetrating tissue destruction.7International Journal of Pediatric Otorhinolaryngology. Pathophysiology of esophageal impairment due to button battery ingestion
This is the same electrochemistry that makes a battery bubble in a glass of water, just with living tissue playing the role of the electrolyte. The injury can begin within minutes and lead to perforation of the esophagus within hours. It’s worth knowing that even “dead” button batteries retain enough voltage to cause harm. If you have small children in the house, keeping button batteries locked away and checking that battery compartments on devices have secure closures are basic precautions that genuinely matter.
What Leaches Into the Water
The battery itself suffers obvious damage in water, but the water also becomes contaminated. This matters for environmental scenarios: batteries dumped in landfills that flood, electric vehicles submerged in storms, and fire-suppression water runoff from battery fires.
When large lithium-ion batteries burn and firefighters douse them with water, the runoff is genuinely hazardous. Analysis of extinguishing water from battery and electric vehicle fire tests found high toxicity to aquatic organisms. The water contained nickel, cobalt, lithium, manganese, and fluoride at concentrations above surface water guideline values. It also contained per- and polyfluoroalkyl substances (PFAS, the so-called “forever chemicals”) at levels between 200 and 1,400 nanograms per liter, and flushing the battery with additional water pushed PFAS concentrations up to 4,700 nanograms per liter.8PubMed Central. Ecotoxicity Evaluation of Fire-Extinguishing Water from Large-Scale Battery and Battery Electric Vehicle Fire Tests That runoff, if it reaches a storm drain or waterway, carries a cocktail of contaminants that persist in the environment.
Even without a fire, battery waste in water is toxic to aquatic life. Experiments exposing organisms to water contaminated with phone battery materials found lethal effects on test species at relatively low concentrations.9Asian Journal of Biochemistry, Genetics and Molecular Biology. Experimental Assessment of the Toxicity Effects of Phone Battery Wastes on Aquatic and Terrestrial Bioindicators Heavy metals like cadmium, lead, cobalt, and nickel dissolve out of corroding battery components and accumulate in sediment and organisms. This is one reason proper battery recycling matters: tossing old batteries in the trash, where they can end up in a landfill exposed to rain and groundwater, creates a slow-motion version of the same contamination.
Effects on Fish and Aquatic Animals
Beyond the chemical contamination, a battery sitting in water produces an electric field, and aquatic animals can sense it. Some species are far more sensitive to these fields than you might expect. Research on freshwater fish found that species responded in distinct ways to electromagnetic stimuli. Lake sturgeon showed strong reactions, including body spasms, sudden stops, and pectoral fin flares, with some type of behavioral reaction occurring in 96% of trials.10Transactions of the American Fisheries Society. Behavioral Responses of Representative Freshwater Fish Species to Electromagnetic Fields European eels exposed to electric fields in water exhibited involuntary physiological responses such as twitching, loss of orientation, and muscle lock-up, along with behavioral changes like acceleration and rejection of the area.11Ecological Engineering. Behavioural response of downstream migrating European eel (Anguilla anguilla) to electric fields under static and flowing water conditions
A single household battery in a lake isn’t going to create a meaningful electric field at any distance, but the principle scales up. Submerged EV battery packs, underwater cable infrastructure, and improperly disposed industrial batteries can all generate fields strong enough to affect nearby fish behavior and migration. It’s another angle on why keeping batteries out of waterways is an environmental priority, not just a chemical contamination issue.
Batteries Designed to Work With Water
Given all the ways water destroys conventional batteries, it’s surprising to learn that some batteries are engineered to use water as a feature rather than a threat. Seawater-activated batteries, used primarily by the military and in maritime applications, are designed to remain inert until seawater flows through them. These batteries use a sodium-ion conducting membrane between two compartments, allowing sodium ions from the surrounding seawater to shuttle between the electrodes and generate electricity on demand.12PubMed. Rechargeable Seawater Batteries-From Concept to Applications The internal flow channel structures are specifically optimized to guide seawater throughout the battery’s interior for even performance.13Journal of Energy Storage. Seawater-activated battery module flow field simulation and structural optimization
On the laboratory frontier, researchers have developed aqueous batteries that use electrolytes at a neutral pH of 7.0, so benign that the electrolyte solution could be the same brine used to make tofu. One such design achieved over 120,000 charge-discharge cycles using covalent organic polymer electrodes with magnesium and calcium ions as the charge carriers, delivering a full-cell voltage of 2.2 volts.14Nature Communications. An aqueous battery using an electrolyte with a pH of 7 and suitable for direct environmental discard The explicit goal of that research was a battery safe enough to discard directly into the environment without contamination. It’s an early-stage technology, but it points toward a future where the phrase “battery in water” doesn’t automatically mean hazard.
How Underwater Batteries Stay Dry
For applications where conventional lithium-ion batteries need to operate deep underwater, like autonomous submarines, ocean sensors, and seafloor instruments, the engineering challenge is keeping water out rather than making it work with water. Two broad strategies exist. The first is brute-force pressure resistance: batteries are sealed inside casings made from high-strength materials like titanium alloys or ceramics, thick and rigid enough to resist the crushing hydraulic pressure at depth while preventing any seawater intrusion.15International Journal of Extreme Manufacturing. Manufacturing of lithium battery toward deep-sea environment
The second approach is more elegant: pressure compensation. Instead of building a container strong enough to resist the outside pressure entirely, engineers immerse the battery cells in an incompressible insulating fluid, usually oil, inside a container connected to a flexible membrane or piston. As the external water pressure increases with depth, the compensator transmits that pressure to the fluid surrounding the cells, keeping internal and external pressure equal. The enclosure only needs to be sealed and intact, not massively reinforced. One design using polycarbonate tubes to encapsulate lithium-ion cell stacks withstood pressures equivalent to 305 meters of seawater depth.16Journal of Power Sources. A rechargeable lithium-ion battery module for underwater use
These engineering solutions underscore how seriously the battery-water interaction is taken in professional contexts. At atmospheric pressure, water contact with a lithium-ion cell is a nuisance that leads to gradual corrosion. At 300 meters depth, where pressure is roughly 30 times what you feel at the surface, a single breach in a battery housing could trigger a cascade of short circuits, gas generation, and structural failure. The fact that these systems exist and work reliably is a testament to how well-understood the underlying chemistry is, even if the average person never thinks about it until they drop their phone in the toilet.