Battery Efficiency Calculator

Calculate coulombic efficiency, energy efficiency, round-trip efficiency, expected output energy, required input energy, and battery energy loss. The calculator keeps charge, energy, and system-boundary concepts separate so results stay meaningful.

Battery efficiency depends on measurement boundary, voltage, C-rate, temperature, internal resistance, aging, BMS behavior, and external conversion losses. Define the boundary before comparing one efficiency value with another.

Engineering tool

Battery Efficiency Calculator

Calculate coulombic efficiency, energy efficiency, round-trip efficiency, energy loss, and required input energy.

Charge Input

Charge capacity measured during charging.

Charge Output

Charge capacity measured during discharge.

Coulombic Efficiency

95 %

Result console

Efficiency: 95%
Decimal ratio: 0.95
Input charge: 3Ah
Output charge: 2.85Ah
Charge loss: 150mAh
Formula used: ηQ = Qout / Qin × 100%

Measurement boundary matters

Battery-only efficiency, battery plus BMS efficiency, and full system round-trip efficiency can produce different values. Define the boundary before comparing results.

Define the measurement boundary before interpreting efficiency. Battery-only, battery plus BMS, charger-to-battery, and full system round-trip efficiency are not directly interchangeable.

Formula reference

Battery Efficiency Formulas

Battery efficiency formulas use charge in Ah, energy in Wh, and efficiency as a 0 to 1 ratio internally. Percentage input and output are converted around the calculation.

ηQ = Qout / QinηE = Eout / EinηRT = ηcharge × ηdischargeEout = Ein × ηEin = Eout / ηEloss = Ein − EoutQloss = Qin − Qout

Variable definitions

ηQ: Coulombic efficiency, output charge divided by input charge
ηE: Energy efficiency, output energy divided by input energy
ηRT: Round-trip efficiency over a complete charge/discharge cycle
ηcharge: Charging-stage efficiency
ηdischarge: Discharging-stage efficiency
Qin: Input charge in Ah
Qout: Output charge in Ah
Qloss: Charge loss in Ah
Ein: Input energy in Wh
Eout: Output energy in Wh
Eloss: Energy loss in Wh

Worked Examples

Coulombic Efficiency

Input charge = 3000 mAh, output charge = 2850 mAh

ηQ = 2850 / 3000 × 100% = 95%; charge loss = 150 mAh

High Coulombic Efficiency

Input charge = 100 Ah, output charge = 99 Ah

ηQ = 99%; charge loss = 1 Ah

Energy Efficiency

Input energy = 120 Wh, output energy = 102 Wh

ηE = 102 / 120 × 100% = 85%; energy loss = 18 Wh

Round-Trip Efficiency

Charge efficiency = 95%, discharge efficiency = 92%

ηRT = 0.95 × 0.92 = 0.874 = 87.4%; loss = 12.6%

Expected Output Energy

Input energy = 2 kWh, efficiency = 90%

Output = 1.8 kWh; loss = 0.2 kWh

Required Input Energy

Required output = 500 Wh, efficiency = 80%

Input = 500 / 0.8 = 625 Wh; loss = 125 Wh

Joule Conversion

Input energy = 36,000 J, output energy = 32,400 J

36,000 J = 10 Wh; 32,400 J = 9 Wh; efficiency = 90%; loss = 1 Wh

Output Greater Than Input

Input energy = 100 Wh, output energy = 105 Wh

Mathematical efficiency = 105%; review warning is shown instead of clipping to 100%

Measurement Boundaries

Battery-Only Efficiency

Measures energy or charge at the battery terminals only. Charger, inverter, DC-DC converter, and wiring losses are outside the boundary.

Battery + BMS Efficiency

Includes protection circuits, current sensing, balancing, BMS quiescent current, and pack-level internal wiring.

System Round-Trip Efficiency

May include charger, battery, BMS, DC-DC converter, inverter, wiring, and auxiliary loads. Do not compare it directly with battery-only efficiency.

Engineering Notes

What Is Battery Efficiency

Battery efficiency is useful only when the input, output, units, and measurement boundary are clearly defined.

Coulombic Efficiency

Coulombic efficiency compares Ah in and Ah out. It does not include the voltage difference between charge and discharge.

Energy Efficiency

Energy efficiency compares Wh in and Wh out and is better for real energy-loss analysis.

Round-Trip Efficiency

Round-trip efficiency is multiplication, not averaging. 95% × 92% = 87.4%.

Battery Energy Loss

Energy loss usually appears as heat through internal resistance, polarization, wiring, BMS, and conversion losses.

Internal Resistance

Internal resistance causes I²R heating and voltage sag, especially at higher C-rate.

Voltage Hysteresis

Charge and discharge voltage curves are not identical, which can reduce energy efficiency even when coulombic efficiency is high.

Battery Chemistry

Lithium-ion, LiFePO4, lead-acid, and NiMH efficiency behavior differs and should be checked against manufacturer data.

Temperature Effects

Temperature changes internal resistance, usable capacity, charge acceptance, and energy efficiency.

C-Rate Effects

Higher C-rate can increase heating and reduce measured energy efficiency.

Battery Aging

Aging raises internal resistance and can lower practical energy efficiency.

BMS Losses

Current sensing, protection MOSFETs, balancing circuits, and control electronics may add pack-level loss.

Cell Balancing Losses

Passive balancing intentionally burns energy as heat to equalize cells.

Charger Efficiency

Charger efficiency is separate unless the measurement boundary includes the charger.

DC-DC Converter Efficiency

Converter loss should be counted only when the system boundary includes the converter.

Inverter Efficiency

Inverter efficiency is part of system efficiency, not battery-only efficiency by default.

Output Greater Than Input

A result above 100% usually points to SOC mismatch, unit errors, sensor error, or incomplete test cycles.

Common Mistakes

Confusing coulombic and energy efficiency

High Ah efficiency does not guarantee equally high Wh efficiency because voltage and heat matter.

Using Ah as Wh

Ah is charge capacity. Wh is energy. Do not compare them directly.

Averaging charge and discharge efficiency

Round-trip efficiency is multiplication, not simple averaging.

Forgetting percent conversion

90% must be used as 0.9 inside formulas, not 90.

Mixing battery and charger efficiency

Battery efficiency and charger efficiency are different unless the measurement boundary includes both.

Ignoring BMS and wiring loss

Pack-level measurements may include BMS, balancing, connector, cable, and sensing losses.

Mismatched SOC endpoints

Initial and final state of charge must be consistent for a meaningful efficiency test.

Clipping output greater than input

Do not force results to 100%. Show the math and review the measurement setup.

Support reference

FAQ

What is battery efficiency?

Battery efficiency compares useful output with input over a defined measurement boundary. The boundary may be battery-only, battery plus BMS, or a larger system.

What is coulombic efficiency?

Coulombic efficiency compares output charge with input charge using Ah or mAh. It does not directly include voltage differences.

What is energy efficiency?

Energy efficiency compares output energy with input energy using Wh, kWh, or joules. It is better for estimating real energy loss.

What is round-trip efficiency?

Round-trip efficiency is the combined efficiency of a complete charge and discharge cycle. It is calculated by multiplying charge efficiency by discharge efficiency.

What is the difference between coulombic and energy efficiency?

Coulombic efficiency compares charge. Energy efficiency compares energy, so it includes voltage behavior, internal resistance, and heat loss effects.

How do I calculate battery energy loss?

Subtract output energy from input energy. For an efficiency-based estimate, output energy equals input energy multiplied by efficiency.

Why is round-trip efficiency lower than charge efficiency?

Round-trip efficiency includes both charge and discharge stages. For example, 95% charge efficiency and 92% discharge efficiency gives 87.4%, not the average.

Can battery efficiency be greater than 100%?

A result above 100% usually indicates inconsistent measurement boundaries, different initial and final SOC, unit errors, sensor error, or incomplete cycles. The calculator shows the mathematical result and flags it for review.

Does C-rate affect battery efficiency?

Yes. Higher C-rate can increase voltage sag, internal heating, polarization, and conversion losses, reducing measured energy efficiency.

Is charger efficiency included in battery efficiency?

Only if the measurement boundary includes the charger. Battery-only efficiency should not include charger, inverter, or DC-DC converter losses.