Battery energy and gravimetric energy density
Gravimetric energy density, often called specific energy, describes nominal stored energy per unit mass. It is central to runtime and weight comparisons for portable electronics, vehicles, aircraft, and storage systems.
The calculator multiplies nominal voltage by rated ampere-hour capacity, then divides by battery mass. Real usable energy is lower or different because voltage changes during discharge and performance depends on load, cutoff, temperature, aging, and conversion losses.
How to use the battery energy density calculator
- Choose a consistent boundary: Use either cell mass or complete pack mass consistently for comparisons.
- Enter rated values: Provide nominal voltage, rated capacity in mAh, and mass in grams.
- Calculate: Review nominal energy in Wh and gravimetric energy density in Wh/kg.
- Apply derating: Use measured discharge energy for engineering decisions where operating conditions matter.
Formula and variables
Voltage multiplied by ampere-hours gives nominal watt-hours; dividing by kilograms gives Wh/kg.
Specific energy = (V × mAh / 1000) / (mass g / 1000)- V — Nominal voltage
- Rated or representative battery voltage (V)
- C — Rated capacity
- Charge capacity (mAh)
- m — Battery mass
- Mass of the cell, module, or pack being compared (g)
Lithium-ion cell example
A cell is rated 3.7 V and 2,500 mAh and has a mass of 45 g.
- Voltage
- 3.7 V
- Capacity
- 2,500 mAh
- Mass
- 45 g
- Energy = 3.7 × 2,500 / 1,000 = 9.25 Wh
- Mass = 0.045 kg
- Specific energy = 9.25 / 0.045 = 205.56 Wh/kg
Result: Nominal energy is 9.25 Wh and gravimetric energy density is about 205.56 Wh/kg.
This is a nominal cell-level value; a protected pack generally has lower Wh/kg after enclosure, electronics, and interconnect mass are included.
Understanding your results
Compare like with like
Cell, module, and pack energy densities use different system boundaries. State the boundary whenever values are compared.
- Higher Wh/kg means more nominal energy for the same mass.
- It does not by itself describe power capability, cycle life, safety, cost, or usable depth of discharge.
Assumptions
- Nominal voltage is an adequate representative voltage.
- Rated mAh capacity applies at the relevant test conditions.
- The entered mass covers the same product boundary as the capacity.
Limitations
- Does not integrate the discharge voltage curve.
- Does not calculate volumetric energy density, peak power, heat generation, or remaining runtime under a variable load.
Common mistakes
- Using mAh as though it were Wh.
- Dividing by grams without converting to kilograms.
- Comparing cell-level energy density with pack-level values.
- Using maximum charge voltage instead of nominal voltage.
Practical use cases
Portable product design
Estimate how battery mass relates to nominal stored energy.
Technology comparison
Compare specific energy values using consistent cell or pack boundaries.
Frequently asked questions
How do I convert mAh to Wh?
Multiply mAh by nominal voltage and divide by 1,000.
Is energy density the same as power density?
No. Energy density describes stored energy per mass or volume; power density describes how quickly energy can be delivered.
Why is pack energy density lower than cell energy density?
Packs include enclosure, cooling, busbars, wiring, controls, protection, and structural mass.
Sources and review
- Battery University: BU-105 Battery Definitions and what they mean — Cadex Electronics. Accessed 2026-07-13.
- Batteries, Charging, and Electric Vehicles — U.S. Department of Energy. Accessed 2026-07-13.
Reviewed 2026-07-13.