Why do batteries run out? Batteries run out because they store a limited amount of chemical potential energy, and the chemical reactions that separate charge and drive electrons through a circuit cannot continue indefinitely in the useful direction. As a battery powers a device, reactants are consumed or transformed, products accumulate, the difference in chemical potential falls, internal resistance can rise and the battery eventually can no longer maintain the voltage and current the device needs.
People searching for why batteries die, why batteries lose charge, how batteries store electricity, why rechargeable batteries wear out and what battery capacity means are asking a question that connects chemistry, electricity, materials science and everyday technology. A battery does not contain a little tank of electrons that gradually empties. Electrons already exist throughout the conductors. What the battery provides is an electrochemical push that makes charge move through an external circuit while ions move internally through an electrolyte.
The clearest explanation starts with energy conversion. The U.S. Department of Energy describes batteries as devices that store energy in chemical potential and release it on demand. During discharge, chemical reactions at two electrodes create a voltage difference. Electrons travel through the external circuit and do useful work while ions move inside the battery to preserve charge balance. The battery “runs out” when that reaction can no longer provide useful electrical output under the conditions the device requires.
The Short Answer: A Battery Runs Out When Its Chemistry Can No Longer Sustain Useful Voltage and Current
A fresh battery begins with reactants arranged in a high-energy chemical state. Connect a circuit and the battery creates conditions for oxidation at one electrode and reduction at the other. Electrons move through the wire because the two sides have different electrochemical potentials.
As discharge continues, the original chemical state changes. Reactants become products, ion concentrations change and the driving force becomes less favourable. The voltage may decline gradually or remain fairly flat until near the end, depending on the chemistry.
The device stops working when the battery can no longer keep its terminal voltage above the device’s minimum requirement while supplying the needed current. That can happen before every chemically available reaction has been exhausted.
A Battery Stores Chemical Potential, Not a Box of Electricity
The phrase “stores electricity” is convenient but incomplete. A battery stores chemical potential energy. The battery’s materials have been placed in a state from which a chemical reaction can release energy.
When the battery is connected to a circuit, that chemical reaction drives charge through the circuit. The energy carried to the device is electrical energy, but the stored source is chemical.
This is similar to fuel in one limited sense: the stored chemical state can be converted into another form of energy. The mechanism is different because a battery uses separated electrochemical reactions rather than combustion.
The Three Essential Parts: Two Electrodes and an Electrolyte
Most batteries contain two electrodes with different chemical tendencies and an electrolyte that allows ions to move between them. During discharge, one electrode is the site of oxidation and the other the site of reduction.
Electrons cannot usually cross the electrolyte directly in the same way ions can. They travel through the external wire instead. That external path is where electrical work happens: a motor turns, a screen lights or a processor computes.
Inside the battery, ions move through the electrolyte so charge does not build up indefinitely at one electrode. Electron flow outside and ion flow inside are parts of one coupled electrochemical process.
Why Electrons Flow Through the Device
The two electrodes create a difference in electric potential because their chemical environments favour different electron states. Connect them through a conducting path and electrons move in response to that difference.
The device is inserted into that path. Instead of electrons moving through a plain wire, they move through circuitry that converts electrical energy into light, motion, heat, sound or information processing.
For a broader electricity foundation, see Tell Me About Electricity | Charge, Current, Voltage, Circuits and Power.
Voltage Is the Push, Not the Amount of Charge Stored
Voltage describes electric potential difference. It tells us how much energy per unit charge can be transferred between two points. A battery marked 1.5 volts or 3.7 volts is telling us something about its operating potential, not directly how long it will last.
Two batteries can have the same nominal voltage but very different capacities. A tiny coin cell and a large cylindrical cell may both operate around similar voltages while storing very different amounts of energy.
Duration depends on capacity, energy content, discharge rate, device efficiency and operating conditions.
What Battery Capacity Means
Battery capacity is often stated in ampere-hours or milliampere-hours. In simplified terms, it measures how much electric charge the battery can deliver under specified conditions.
A 3000 mAh cell might theoretically deliver 3000 milliamperes for one hour or 300 milliamperes for ten hours, but real batteries do not scale perfectly because voltage, temperature, discharge rate and cutoff conditions matter.
For comparing stored energy across different voltages, watt-hours are more informative because energy depends on both charge and voltage.
Why a Battery Can Be “Empty” Before the Chemistry Is Completely Finished
Devices require a minimum voltage. A digital circuit may shut down long before the battery reaches a state of absolute chemical exhaustion because reliable operation below that threshold is impossible.
Under load, the battery’s terminal voltage can be lower than its open-circuit voltage because some energy is lost inside the cell. A battery that measures acceptable voltage with no load may sag sharply when a motor or processor demands current.
“Dead” therefore means unusable for the intended device, not necessarily chemically inert.
Internal Resistance Explains Voltage Sag
Real batteries are not ideal voltage sources. They have internal resistance arising from electrode materials, electrolyte, interfaces and transport limitations.
When current flows, some voltage is effectively lost across that internal resistance. Higher current causes a larger voltage drop. This is why a weak battery may still run a low-power clock but fail in a camera flash or motorised toy.
As batteries age or discharge, internal resistance often rises. The battery can become unable to supply bursts of power even when some energy remains.
Why High-Power Devices Drain Batteries Faster
A device that draws more current uses stored energy faster. A bright flashlight, wireless transmitter or motor generally demands more power than a small clock circuit.
High current can also reduce usable capacity in some battery chemistries because transport and internal-resistance losses become more significant. The battery heats more and the terminal voltage can reach the cutoff threshold sooner.
Battery life therefore depends on both how much energy is stored and how aggressively the device asks for it.
Why Batteries Get Warm
Some energy is dissipated as heat because current flows through internal resistance and because electrochemical processes are not perfectly reversible.
Moderate warmth can be normal under high load or charging. Excessive heat is different because temperature accelerates side reactions and can create safety risks, especially in high-energy rechargeable cells.
Battery systems therefore include thermal design, charge control and protective electronics. High stored energy is useful only when release remains controlled.
Primary Batteries and Rechargeable Batteries
Primary batteries are designed mainly for one discharge cycle. Their chemistry is not intended to be reversed safely and repeatedly by applying external electrical energy.
Rechargeable, or secondary, batteries use chemistries designed so that much of the discharge reaction can be driven backward. Charging pushes ions and electrons toward a higher-energy arrangement again.
The U.S. Department of Energy’s DOE Explains: Batteries describes this coupled movement of electrons through the external circuit and ions through the electrolyte during charge and discharge.
Why Rechargeable Batteries Do Not Last Forever
Rechargeable reactions are reversible only within limits. Every cycle can produce small irreversible changes: side products form, active material cracks or loses electrical contact, electrode surfaces change and electrolyte can degrade.
One cycle does little damage in a well-managed battery. Hundreds or thousands of cycles accumulate enough change to reduce capacity and increase resistance.
The battery still works, but it stores less energy and delivers it less effectively. That is battery ageing.
What Happens Inside a Lithium-Ion Battery
In a lithium-ion battery, lithium ions move between host materials at the two electrodes while electrons travel through the external circuit. During charging, external electrical energy drives the ions toward a higher-energy distribution. During discharge, the system moves back toward a lower-energy state and releases electrical energy.
The name “lithium-ion” covers many electrode chemistries. Different cathode and anode materials trade off energy density, power, cost, lifetime and safety.
The important common idea is reversible ion shuttling through an electrolyte combined with electron movement through the external circuit.
Why Lithium-Ion Batteries Age Even When You Do Not Use Them
Battery ageing has two broad components: cycle ageing from use and calendar ageing from time. Even while sitting, electrodes and electrolyte can undergo slow chemical reactions.
High temperature and high state of charge can accelerate some degradation pathways. This is why storage conditions influence long-term battery health.
A battery is a chemical system. Time continues to matter even when the external circuit is idle.
Why Cold Weather Makes Batteries Seem Weak
Low temperature slows chemical kinetics and ion transport. Internal resistance rises, so voltage drops more under load.
The battery may appear to lose capacity because the device reaches its cutoff voltage sooner. When the battery warms, some of the apparent capacity can return.
This is why a cold phone can shut down despite showing remaining charge and then operate again after warming. The chemistry became temporarily less capable of delivering power.
Why Heat Is Also Bad for Batteries
Warmth can improve short-term reaction rates, but excessive heat accelerates unwanted chemical reactions. Electrolyte decomposition, growth of interface layers and other ageing processes can speed up.
High temperatures also raise safety concerns in energy-dense batteries. Battery-management systems monitor temperature and limit charging or discharging when conditions become unsafe.
The ideal operating range is therefore neither very cold nor very hot.
Why Fast Charging Can Increase Stress
Fast charging moves ions quickly and raises current. That is convenient, but high rates can increase heating and concentration gradients inside the electrodes.
Modern battery systems manage this by changing charge rate as voltage, temperature and state of charge change. Charging often slows near the top because the cell has less margin for accepting energy safely.
A high-quality charger and battery-management system therefore do more than “push power in.” They control an electrochemical process.
Why the Last 20 Percent Can Behave Differently
Battery percentage is an estimate of state of charge, not a direct liquid-level measurement. Phones and laptops infer remaining charge using voltage, current history, temperature and models of the cell.
Near full or empty states, voltage behaviour and internal resistance can make estimation harder. A degraded battery can cause percentage to fall quickly because usable capacity is smaller than when new.
The displayed percentage is therefore a software interpretation of electrochemical state.
Why Devices Shut Down Before Zero Volts
Deep discharge can damage many rechargeable chemistries. A lithium-ion cell is not meant to be driven literally to zero volts during normal operation.
Protective electronics disconnect the battery when cell voltage reaches a lower safety threshold. The device calls this zero percent even though the cell still has a non-zero voltage.
This is another example of why user-interface “empty” and physical “no energy whatsoever” are different concepts.
Why Overcharging Is Dangerous
Rechargeable cells have safe voltage windows. Driving the chemistry beyond the upper limit can trigger unwanted reactions, heating and structural damage.
Modern devices use charge controllers to prevent this. When a phone reaches “100 percent,” the charger does not simply continue forcing maximum current into the battery forever.
Protection circuitry is one reason integrated batteries can be used safely by people who never think about electrode voltage.
Why Alkaline Batteries Leak
Primary alkaline cells contain a caustic electrolyte and sealed internal components. As cells age, discharge deeply or experience adverse conditions, pressure and corrosion can compromise seals.
Leaked material can damage battery contacts and electronics. Old cells should therefore be removed from devices that will sit unused for long periods, following manufacturer guidance.
A leaking battery is a chemical failure, not simply an electrical one.
Why Different Battery Chemistries Have Different Voltages
Cell voltage depends on the electrochemical potential difference between the two electrode reactions. Change the materials and you change the voltage.
Alkaline cells are commonly sold around 1.5 volts nominal. Nickel-metal hydride rechargeables are commonly around 1.2 volts. Many lithium-ion cells operate around 3.6 to 3.7 volts nominal, depending on chemistry.
Voltage alone does not tell us which chemistry is “better.” Safety, capacity, power, cycle life, cost and device design all matter.
Why Batteries Are Connected in Series
Connect cells in series and their voltages add. This lets designers build a pack with a higher operating voltage from lower-voltage cells.
A laptop or electric vehicle may contain many cells arranged in series groups to reach the voltage needed by motors and electronics.
Series connections also require cell balancing because one weak cell can limit the whole string.
Why Batteries Are Connected in Parallel
Connect compatible cells in parallel and the pack can provide more current and capacity while keeping roughly the same voltage.
Real battery packs combine series and parallel arrangements to meet both voltage and energy requirements.
Pack design therefore scales cell chemistry into a larger electrical system.
Why One Weak Cell Can Make a Whole Pack Seem Bad
Cells in a pack do not age perfectly identically. Small manufacturing differences, temperature gradients and usage history create variation.
In a series string, the weakest cell may reach its safe lower voltage first during discharge or upper voltage first during charging. The battery-management system must stop the pack to protect that cell.
Pack performance can therefore be limited by the least capable member, much like a chain limited by its weakest link.
Why Battery Management Systems Matter
Modern rechargeable packs include electronics that monitor voltage, current and temperature. More sophisticated systems estimate state of charge and state of health, balance cells and communicate with the device.
These systems protect the chemistry from conditions that accelerate degradation or create safety hazards.
The “battery” in a modern product is therefore often an electrochemical pack plus sensors, switches, algorithms and protective logic.
Why a Larger Battery Does Not Always Give Proportionally Longer Life
Increasing stored energy usually increases runtime, but devices can change behaviour when they have more battery. Larger screens, stronger processors and higher radio power may consume more energy.
Energy conversion also has losses. Voltage regulators, motors and power electronics are not perfectly efficient.
Runtime is a system property: battery energy divided by the device’s average power demand, modified by efficiency, temperature and cutoff behaviour.
Why Phone Batteries Seem Worse After Two or Three Years
Calendar ageing and charge cycles reduce capacity and increase internal resistance. The phone may still report a full charge, but “full” now represents a smaller amount of stored energy than when the battery was new.
At the same time, software updates and changed usage can alter power demand. More background activity or brighter settings can make runtime shorter even without dramatic cell degradation.
Battery life is therefore a combined story of chemistry and device behaviour.
Why Electric-Vehicle Batteries Need Thermal Management
An electric-vehicle pack contains large amounts of energy and operates across changing weather, acceleration and charging conditions. Temperature strongly affects performance, lifetime and safety.
Thermal systems heat cells when they are too cold and cool them when they are too hot. This keeps reactions within a range where the pack can deliver power without excessive degradation.
Battery engineering is therefore not only chemistry. It includes heat transfer, controls, electronics and mechanical protection.
Why Battery Energy Density Matters
Energy density describes how much energy can be stored per unit mass or volume. Higher energy density lets a phone run longer without becoming larger or lets a vehicle travel farther without adding as much battery mass.
But maximising energy density can conflict with cost, power, lifetime and safety. Engineering is about trade-offs, not one perfect number.
The best battery depends on the application. A stationary grid battery can tolerate more mass than an aircraft battery.
Why Power Density Is Different From Energy Density
Energy density asks how much total energy is stored. Power density asks how quickly energy can be delivered.
A battery designed for long endurance may not be optimised for massive short bursts of current. Another chemistry may deliver high power but store less total energy.
This is why comparing batteries using only capacity can hide important performance differences.
Why Batteries Self-Discharge
Even when disconnected, batteries can slowly lose charge through internal side reactions and leakage pathways. This is called self-discharge.
The rate varies dramatically by chemistry, temperature and battery condition. Some primary cells can retain useful charge for years; other chemistries lose a more noticeable fraction each month.
A disconnected battery is not chemically frozen in time.
Why a Battery Can Recover a Little After Rest
After heavy discharge, concentration gradients inside a cell can be uneven. When the load is removed, ions redistribute and the terminal voltage may rise.
This can make a “dead” battery appear to recover temporarily. The recovered voltage usually does not mean the original capacity has returned; it means internal conditions have relaxed.
Under a heavy load, the voltage may sag again quickly.
Why Battery Recycling Matters
Batteries contain metals, salts, plastics and electrolytes. Throwing large numbers into ordinary waste can lose valuable materials and create fire or contamination risks.
Recycling aims to recover materials such as lithium, nickel, cobalt, copper, lead or steel depending on chemistry, while handling reactive components safely.
The environmental value depends on collection, process efficiency and the chemistry being recycled, but material recovery becomes increasingly important as battery use scales.
Why Batteries Are a Systems Engineering Problem
A cell can have excellent chemistry and still make a poor product if cooling is inadequate, charging is badly controlled or mechanical protection is weak.
Conversely, intelligent control can extend useful life by avoiding the most damaging voltage, current and temperature conditions.
Battery performance emerges from chemistry plus electronics, software, mechanics, thermal design and user behaviour.
A Worked Energy Example
Suppose a battery stores 60 watt-hours of usable energy and a device consumes an average of 10 watts. An ideal calculation suggests six hours of runtime.
Real runtime may be lower because power demand changes, conversion is not perfectly efficient, temperature affects usable capacity and the device stops at a minimum voltage.
The simple calculation is still useful because it shows the governing relationship: runtime depends on stored energy divided by average power demand.
A Battery-Diagnosis Checklist
- Is the battery genuinely discharged, or is the device drawing unusually high power?
- Is the battery cold?
- Has capacity declined with age?
- Does voltage sag only under heavy load?
- Are contacts dirty or loose?
- Is the charger appropriate?
- Is one cell in a pack limiting the rest?
- Has the device’s software or usage pattern changed?
This separates electrochemical depletion from connection, temperature and system-level problems.
Common Myths About Batteries
Myth: A battery stores spare electrons
A battery stores chemical potential. The voltage difference drives electrons already present in the circuit.
Myth: Zero percent means zero electrical energy
Devices stop at a safe lower voltage before a rechargeable cell reaches literal zero volts.
Myth: Rechargeable means infinitely reusable
Side reactions and structural changes accumulate, reducing capacity and raising resistance.
Myth: A higher-voltage battery always lasts longer
Runtime depends on stored energy and power demand, not voltage alone.
Myth: Cold permanently removes charge
Cold often reduces the ability to deliver power temporarily; some apparent capacity returns after warming.
Frequently Asked Questions
Why does a battery die?
Because the electrochemical reactions can no longer maintain enough voltage and current for the device as reactants change, products accumulate and internal resistance rises.
Why can rechargeable batteries be charged again?
External electrical energy drives much of the discharge chemistry backward, restoring a higher-energy arrangement of ions and electrode materials.
Why do rechargeable batteries lose capacity?
Because small irreversible chemical and structural changes accumulate with time and cycling.
Why does my phone shut down in the cold?
Cold slows ion transport and increases internal resistance, causing voltage to sag under load.
What is the difference between mAh and Wh?
Milliampere-hours measure charge capacity. Watt-hours measure energy and include the effect of voltage.
Can an unused battery run out?
Yes. Batteries self-discharge and age chemically even while disconnected.
Where to Go Next
Battery behaviour makes more sense when linked to energy and electricity. Continue with Tell Me About Electricity and What Is Energy?. A battery is where those ideas meet chemistry.
Batteries run out because useful chemical free energy is finite. Discharge changes the materials that created the voltage difference. Rechargeable cells can reverse much of that change, but never perfectly forever. Battery life is chemistry, electricity, heat and control working as one system.
Why Voltage Falls Even When Some Chemical Energy Remains
A battery does not always stop working because every possible chemical reaction has finished. Devices require a usable voltage and current. As reactants are consumed and products accumulate, the chemical potential difference between the electrodes changes. Internal resistance may also rise. The battery can therefore reach a point where it still contains some recoverable energy but cannot maintain the voltage the device needs under load.
This is why a “dead” battery may show a slightly higher voltage after resting. When the load is removed, concentration gradients partly relax and the terminal voltage can recover. Put the battery back under a demanding load and the voltage drops again. The energy was not magically restored; the electrochemical system temporarily moved closer to equilibrium inside the cell.
Internal Resistance: The Hidden Reason Batteries Sag Under Load
Every real battery has internal resistance. When current flows, some voltage is lost inside the battery itself. A useful simple model is terminal voltage equals the ideal electromotive force minus current multiplied by internal resistance. That means a battery can look healthy on an unloaded voltmeter and still fail when a motor, camera flash or other high-current device is connected.
Internal resistance tends to increase as many batteries age, discharge, become very cold or develop less favourable internal chemistry. High current makes the effect more obvious because the internal voltage drop grows with current. This is why two devices can disagree about whether the same battery is “empty”: their current demands are different.
Why Cold Weather Makes Batteries Seem Weaker
Electrochemical reactions and ion movement depend on temperature. In cold conditions, reaction rates slow and internal resistance can rise. A battery that worked normally at room temperature may therefore deliver less usable power in the cold. The effect can be temporary; warming the cell within its safe operating range can improve performance because the chemistry and ion transport become faster again.
This does not mean batteries should be heated aggressively. Battery manufacturers specify safe temperature limits because overheating can damage cells and create safety risks. The useful principle is simply that battery performance is temperature-dependent because the battery is a chemical system, not an abstract container of electricity.
Why High-Drain Devices Empty Batteries Faster
A flashlight using a small current can continue operating from a cell that fails immediately in a high-power device. High drain increases internal losses and can push the terminal voltage below the device’s cutoff threshold. Some battery chemistries also deliver a smaller fraction of their nominal capacity when discharged very quickly.
This is why battery capacity should never be interpreted without conditions. A rating such as ampere-hours is measured under specified discharge conditions. Change the current, temperature, cutoff voltage or age of the cell and the usable energy can change.
Why Rechargeable Batteries Wear Out
Rechargeable cells reverse much of their discharge chemistry when an external power source forces current in the opposite direction. But the reversal is not perfectly lossless. Over many cycles, side reactions occur, active material can lose electrical contact, interfaces change and electrodes can develop structural damage. The battery gradually stores less charge or develops higher resistance.
Lithium-ion cells illustrate this clearly. During normal use, lithium ions move between electrode materials through an electrolyte while electrons travel through the external circuit. Protective interfacial layers are necessary for stable operation, but those layers continue to evolve. High temperature, prolonged high state of charge, deep cycling and high current can accelerate ageing under many conditions.
Cycle Life Is Not a Single Universal Number
When manufacturers describe cycle life, the number depends on how a cycle is defined and on test conditions. A battery repeatedly cycled through a narrow middle range may retain capacity longer than one repeatedly taken from very high charge to very low charge. Temperature and charge rate also matter.
A useful mental model is that battery ageing depends on both calendar time and use. A battery can age while sitting on a shelf, especially at high temperature or high state of charge, and it can age through cycling. The two processes overlap rather than forming separate clocks.
Why Phones Sometimes Shut Down Before Reaching 0%
The percentage displayed by a phone is an estimate produced by battery-management software. The system combines voltage, current, temperature, charge history and battery models to estimate state of charge. As a battery ages, the model can become harder to calibrate because capacity and internal resistance have changed.
A sudden shutdown at a displayed 10% does not mean the operating system simply “forgot” the remaining energy. Under load, the aged battery may suffer a voltage drop large enough to reach the device’s safety cutoff. The software estimate and the battery’s ability to deliver power have diverged.
Why Battery Percentage Is an Estimate, Not a Fuel Gauge
A car fuel tank can be measured geometrically. A battery’s state of charge is less direct because the stored energy is represented by chemical state distributed through electrodes and electrolyte. Voltage sometimes provides a useful clue, but the voltage–charge relationship is not always linear and changes with current, temperature and chemistry.
Battery-management systems therefore estimate rather than directly count a visible substance. They may integrate current over time, compare voltage with models and update the estimate after full or partial charge cycles. The percentage is useful, but it is not a direct microscopic measurement of “electricity left inside.”
Why Fast Charging Can Create More Stress
Fast charging pushes energy into a battery at a higher rate. Good charging systems control current, voltage and temperature carefully, often reducing charge rate as the battery approaches full state of charge. The reason is that electrochemical processes have physical limits: ions must move through electrolyte and enter electrode structures without creating damaging gradients or side reactions.
Modern batteries are designed to support specified fast-charging regimes, but faster is not free. Heat management and battery-management systems become more important. Users should follow device and manufacturer guidance rather than improvising chargers or thermal tricks.
Why Batteries Can Become Dangerous When Damaged
Batteries store energy in compact chemical systems. Physical damage, internal short circuits, manufacturing defects or severe overheating can release that energy rapidly. In lithium-ion cells, heat can accelerate side reactions, which generate more heat and potentially create a self-reinforcing thermal runaway process.
This is why swollen, punctured, leaking or overheating batteries should not be treated as ordinary rubbish or experimentally “tested.” The correct handling route depends on local battery-recycling and safety guidance. The physics and chemistry explain the risk; they are not an invitation to open or modify cells.
Why Different Battery Chemistries Behave Differently
Alkaline, nickel-metal hydride, lead-acid, lithium primary and lithium-ion batteries use different electrode materials and electrolytes. That changes nominal voltage, energy density, internal resistance, rechargeability, low-temperature performance, self-discharge and safety characteristics.
Calling all batteries the same because they all provide electricity is like calling all fuels the same because they all release energy. The shared function is real, but the mechanism matters. Device designers choose chemistries based on required power, energy, size, cost, lifetime and operating conditions.
Why a Battery Can Leak After It Is “Dead”
Discharged cells can continue undergoing slow chemical changes. In some primary batteries, corrosion or gas formation can damage seals and allow electrolyte to escape. Long storage inside a device increases the time available for these processes.
The practical lesson is to remove old batteries from equipment that will not be used for a long time and to follow manufacturer guidance for storage and disposal. A battery that no longer powers a device can still contain reactive chemicals.
Energy, Power and Capacity Are Different
Battery conversations often mix three different ideas. Energy is the total ability to do work. Power is the rate at which energy is delivered. Capacity, often stated in ampere-hours, is related to how much electric charge can be delivered under specified conditions. None of these quantities is identical to the others.
A battery may contain substantial energy but be unable to deliver high power because its internal resistance is large. Another battery may deliver enormous power for a short time but contain relatively little total energy. Understanding the distinction explains why starting a car and running a low-power sensor are different battery jobs.