Batteries are measured using several different units, each describing a different aspect of performance. The most common are voltage (V), which measures electrical pressure; amp-hours (Ah) or milliamp-hours (mAh), which measure how much charge a battery can store; and watt-hours (Wh), which measure total energy. Beyond these basics, batteries are also rated by how fast they can discharge, how many charge cycles they last, and how their internal health changes over time.
Voltage: The Driving Force
Voltage is the most familiar battery measurement. It tells you how much electrical pressure a battery can push through a circuit, and it’s determined by the chemical materials inside the cell. A single lithium-ion cell has a nominal voltage of 3.6V, while a single lead-acid cell sits at 2V. A standard car battery uses six lead-acid cells wired together to reach 12V.
The word “nominal” is important here. It’s not the actual voltage you’d measure at any given moment. It’s a standardized reference point that represents the midway value during normal use. A fully charged lithium-ion cell actually reads about 4.2V, then gradually drops toward a cutoff of around 3.0V as it drains. The nominal 3.6V is roughly the middle of that range. Similarly, a fully charged and rested lead-acid cell reads about 2.1V, not the 2.0V nominal rating.
You’ll also see maximum charge voltage listed on battery specs. For most lithium-ion cells, this is 4.2V. Charging beyond that threshold risks damage or safety hazards, which is why chargers are designed to stop at a precise cutoff. Some newer lithium-ion formulations push that ceiling to 4.35V or 4.4V for extra capacity, but they require matched chargers.
Capacity: How Long a Battery Lasts
Capacity measures how much total charge a battery can store, expressed in amp-hours (Ah) for large batteries or milliamp-hours (mAh) for smaller ones. One milliamp-hour equals one-thousandth of an amp-hour. A battery rated at 3,000 mAh can theoretically supply 3 amps for one hour, or 1.5 amps for two hours, or 0.5 amps for six hours.
This is why your phone’s 5,000 mAh battery lasts longer than a pair of wireless earbuds with 60 mAh. It simply holds more charge. Car batteries are rated in Ah too, typically between 40 and 100 Ah, reflecting the much larger energy demands of starting an engine and running vehicle electronics.
Watt-Hours: Total Usable Energy
Capacity alone doesn’t tell you the full story, because it ignores voltage. A 3,000 mAh battery running at 3.6V stores less total energy than a 3,000 mAh battery running at 5V. To compare batteries on equal terms, you need watt-hours.
The formula is straightforward: multiply milliamp-hours by voltage, then divide by 1,000. A phone battery rated at 5,000 mAh and 3.7V stores about 18.5 Wh. A laptop battery at 5,000 mAh and 11.1V stores about 55.5 Wh. Watt-hours are the measurement airlines use to regulate batteries in carry-on luggage (the limit is typically 100 Wh), and it’s what electric vehicle manufacturers use to describe their battery packs, often in kilowatt-hours (kWh).
C-Rate: Charge and Discharge Speed
C-rate measures how quickly a battery charges or discharges relative to its total capacity. A 1C rate means the battery would fully discharge in exactly one hour. A 2C rate means it discharges in 30 minutes, delivering twice the current. A 0.5C rate means a slow two-hour discharge.
For a practical example, take a 10 Ah battery rated at 5C. That means it can safely deliver 50 amps of current (10 × 5), draining completely in about 12 minutes. Power tools, drones, and electric vehicles need high C-rates because they draw large bursts of current. A flashlight or remote control draws very little current relative to its battery size, so C-rate barely matters.
C-rate matters when you’re shopping for batteries for demanding applications. A battery with a low C-rate in a high-drain device will overheat, deliver poor performance, or fail prematurely.
Energy Density: Power Relative to Size and Weight
Energy density tells you how much energy a battery packs into a given weight or volume. Gravimetric energy density is measured in watt-hours per kilogram (Wh/kg), and volumetric energy density in watt-hours per liter (Wh/L). These numbers explain why different battery types dominate different applications.
Lithium-ion cells lead the pack at 150 to 190 Wh/kg, which is why they’re used in phones, laptops, and electric vehicles where weight matters. Nickel-metal hydride batteries come in at 60 to 120 Wh/kg, still common in some hybrid vehicles and rechargeable AA cells. Lead-acid batteries sit at the bottom, just 30 to 50 Wh/kg, but they’re cheap and reliable, which is why they still start most cars.
Cycle Life and Depth of Discharge
Cycle life counts how many times a battery can be fully charged and discharged before its capacity drops to 80% of its original rating. One cycle equals one full charge followed by a full discharge. If you drain your phone to 50% and recharge it, that counts as half a cycle.
The deeper you regularly discharge a battery, the fewer total cycles you get. Data for lithium iron phosphate (LiFePO4) batteries illustrates this clearly: draining to 100% each time yields roughly 3,000 cycles, while only discharging to 50% extends that to around 8,000 cycles. Limiting discharge to 30% can push cycle life past 12,000 cycles. This is why electric vehicle software often prevents you from using the very top and bottom of the battery’s range.
State of Charge and State of Health
State of charge (SoC) is the percentage readout you see on your phone or electric vehicle dashboard. Measuring it accurately is harder than it looks. The simplest method is voltage-based: measure the battery’s voltage and estimate how full it is. But this is unreliable for lithium-ion batteries because their voltage stays remarkably flat through most of the discharge curve. A battery at 80% and one at 30% might read nearly the same voltage.
More advanced systems use coulomb counting, which tracks exactly how much current flows in and out over time. This works well with lithium-ion cells because they’re chemically efficient, but the count drifts over time and needs periodic recalibration. Smartphones may show 100% when the battery is actually closer to 90%, and state-of-charge readings on new EV batteries can be off by as much as 15%. For a voltage-based reading to be accurate, a battery needs to rest unused for at least four hours, or 24 hours for lead-acid types.
State of health (SoH) is a longer-term measurement. It compares a battery’s current maximum capacity to what it held when new. Once that number drops to 80%, the battery is generally considered at end of life for its intended application, though it may still function at reduced performance.
Internal Resistance
Internal resistance, measured in milliohms (mΩ), reflects how much a battery resists the flow of its own current. A new, healthy battery has low internal resistance. As it ages, resistance climbs due to chemical degradation, corrosion, and material loss inside the cell. Higher resistance means less available capacity, more heat generation during use, and reduced ability to deliver high currents.
This is the measurement that best reveals hidden damage. A battery can show a normal voltage on a basic multimeter yet have dangerously high internal resistance. Dedicated battery testers measure resistance directly and flag cells that are degraded, while a standard multimeter only reads static voltage with no insight into what’s happening inside the cell. In one documented comparison, a car battery reading 12.3V appeared healthy by voltage alone, but a battery tester measured 110 mΩ of internal resistance, well above the 80 mΩ replacement threshold. That battery failed three days later.
Physical Size Standards
Batteries are also measured by their physical dimensions, standardized by organizations like the International Electrotechnical Commission (IEC) and the American National Standards Institute (ANSI). These standards define the exact diameter, height, and terminal layout for common sizes like AA, AAA, C, and D cells, ensuring that batteries from any manufacturer fit the same devices.
For cylindrical lithium-ion cells, the naming convention encodes the dimensions directly. A “18650” cell is 18mm in diameter and 65.0mm long. A “21700” cell, increasingly common in EVs and power tools, is 21mm wide and 70mm long. The larger the cell, the more active material it contains and the higher its capacity, though the exact rating varies by manufacturer and chemistry.