A battery stores chemistry, not electricity
It is common to say a battery “stores electricity,” but that is not quite what happens. A battery stores energy in chemical form and converts it to electrical energy when a circuit is connected. The difference matters, because it explains most of what batteries can and cannot do.
Inside a typical cell there are three main parts:
- Two electrodes, usually called the negative and positive electrodes (the anode and cathode when discharging). They are made of materials that react differently with the same charged particles.
- An electrolyte, a material that lets charged atoms called ions move between the electrodes but does not let electrons pass through it.
- A separator, a thin barrier that keeps the electrodes from touching and short-circuiting the cell while still letting ions through.
When you connect a device, a chemical reaction starts at each electrode. At the negative electrode, the material gives up electrons. Those electrons cannot cross the electrolyte, so they travel through the external circuit, which is the wire into your device, and that flow of electrons is the electric current. Meanwhile, ions move through the electrolyte to keep the charge balanced. The electrons arrive at the positive electrode, where they take part in the second half of the reaction.
The reaction happens because the two electrode materials differ in how strongly they hold onto electrons. That difference sets the cell’s voltage. Different material pairings give different voltages, which is one reason batteries come in many chemistries.
Primary and rechargeable cells
In a primary battery, the reaction is not practically reversible. Once the reactants are used up, the cell is finished. In a rechargeable (secondary) battery, pushing current backward through the cell drives the reaction in reverse, restoring the original materials. Charging is therefore not “filling up” a container but running a chemical reaction in the opposite direction.
Reversal is never perfect. Small amounts of material are lost or changed with each cycle, which is why all rechargeable batteries slowly wear out. The article on lithium-ion batteries covers what that wear looks like in the most common type.
Capacity: how much energy is stored
Capacity describes how much energy a battery can deliver in total. You will see it written in two ways:
- Ampere-hours (Ah) or milliampere-hours (mAh), which measure charge. A small cell rated 2,000 mAh can in principle deliver 2 amps for an hour at its rated voltage.
- Watt-hours (Wh) or kilowatt-hours (kWh), which measure energy. This is the more useful number because it accounts for voltage.
To convert, multiply charge by voltage. As a hypothetical example, a 100 Ah battery at 12 volts holds 100 × 12 = 1,200 Wh, or 1.2 kWh.
Capacity is the size of the tank. A larger tank holds more, but it says nothing about how quickly you can empty it.
Power: how fast energy can move
Power is the rate at which energy is delivered, measured in watts or kilowatts. A battery’s power ability depends on how fast the chemical reactions can run, how easily ions move through the electrolyte, and how much resistance the cell has internally.
If capacity is a tank, power is the width of the hose. A battery can have a large capacity but a modest power rating, or the reverse. A battery built for a power tool may hold relatively little energy but release it quickly. One built for long, slow supply may hold a lot but deliver it gently.
A simple hypothetical shows how the two combine. Take a 10 kWh battery:
| Power drawn | Time to empty (ideal) |
|---|---|
| 2 kW | 5 hours |
| 5 kW | 2 hours |
| 10 kW | 1 hour |
Real batteries do not behave this ideally. Drawing power very quickly usually wastes more energy as heat and can reduce the usable capacity, so the actual times are somewhat shorter at high power.
Engineers often describe this relationship with the C-rate. A rate of 1C means a battery would be fully discharged in one hour, 0.5C in two hours, and 2C in half an hour. The same idea applies to charging.
Why energy and power trade off
Designers often have to choose which to favor. Thick electrodes packed with active material hold more energy, but ions take longer to move in and out, which limits power. Thin, porous electrodes allow fast movement but fit less material in the same space. Heat is also a limit: more power means more heat, and cells have to stay within a safe temperature range.
This is why a single “best battery” does not exist. A phone, a hearing aid, a car and a grid installation want different balances of capacity, power, weight, cost, lifespan and safety.
Energy density and other practical measures
When comparing batteries, a few measures come up repeatedly:
- Energy density describes how much energy fits in a given weight or volume. It matters most where weight is a problem, as with electric vehicles and wearable devices.
- Cycle life is how many charge and discharge cycles a battery handles before capacity falls noticeably.
- Efficiency is how much of the energy put in comes back out. Some energy is always lost as heat.
- Safety and cost, which often decide the winner in practice.
Older lead-acid designs are heavy and have lower energy density, but they are inexpensive and well understood, so they remain common for starting engines and backup power. Lithium-ion designs hold much more energy for their weight, which is why they dominate portable electronics and many smart home devices. Other chemistries exist, each with its own trade-offs, and researchers continue to explore new ones, including the approach discussed in the article on solid-state batteries.
From small cells to battery storage
The same principles scale up. Grid-scale battery storage is essentially many cells wired together, with electronics to manage charging, discharging and temperature. At that scale, the capacity-versus-power distinction becomes a daily design question. A site might be described as “50 megawatts, 200 megawatt-hours,” meaning it can deliver up to 50 MW at once and, at that rate, would run for four hours (200 ÷ 50 = 4).
Large installations may be used to smooth out the ups and downs of renewable energy, provide short bursts of power to keep the network stable, or supply backup power for facilities such as data centers. Which job a battery suits depends on whether the need is for lots of energy over time or a lot of power for a short moment.
Key points to remember
- Batteries store energy chemically and release it as electric current when the reaction runs.
- Capacity (Wh or kWh) is how much energy; power (W or kW) is how fast it can be delivered.
- Charging runs the reaction backward, and imperfect reversal causes gradual wear.
- Every design is a balance of energy, power, weight, cost, lifespan and safety, so the right battery depends on the job.