Engineering Reference

Battery Capacity, Energy and C-Rate Units Reference

Quick-reference battery voltage, Ah and Wh capacity, C-rate, runtime, series and parallel relationships, SOC, SOH, internal resistance, and datasheet ratings.

Reading Time: 14 min
Format: Parameter lookup
Updated: September 26, 2026

Quick Battery Parameter Lookup

Battery parameter selection lookup
Need to know	Parameter	Unit or notation
Representative electrical potential	Nominal voltage	V
Charge capacity	Capacity	Ah, mAh
Stored or deliverable energy	Energy	Wh, kWh, J
Current relative to capacity	C-rate	C
Remaining charge estimate	State of charge	SOC, %
Aging or condition metric	State of health	SOH, %
Load-related voltage drop	Internal resistance or impedance	mΩ, Ω
Cells in a series string	Series count	Ns
Parallel strings or cells	Parallel count	Np

Core Parameter Table

Battery datasheet parameter lookup
Parameter	Symbol	Unit	Meaning	Depends on	Common mistake
Nominal voltage	Vnom	V	Representative rated voltage used for naming and nominal energy	Chemistry, product definition and test basis	Treating it as constant terminal voltage
Charge / maximum voltage	Vmax	V	Upper voltage limit defined by the manufacturer	Chemistry, temperature and charge method	Using it as nominal voltage
Cutoff / minimum voltage	Vcutoff	V	Lower endpoint used for discharge or protection	Load, temperature and manufacturer limits	Assuming all rated capacity remains above another cutoff
Charge capacity	Q	Ah, mAh, C	Quantity of electric charge under stated test conditions	Rate, cutoff, temperature, age and test method	Calling Ah energy or output current
Energy	E	Wh, kWh, J	Energy associated with voltage and delivered charge	Voltage profile, usable capacity and losses	Comparing batteries by Ah alone
C-rate	C-rate	C	Current normalized to rated capacity	Capacity basis and manufacturer rating	Treating C-rate as capacity
State of charge	SOC	%	Estimate of remaining charge relative to a defined usable or rated basis	Estimator, current integration, voltage, temperature and history	Using terminal voltage as a universal exact SOC
State of health	SOH	%	Condition metric relative to a defined new-battery baseline	Capacity, resistance, power capability and chosen definition	Treating SOH as SOC
Internal resistance / impedance	Rint, Z	mΩ, Ω	Load-dependent voltage-drop and loss model or measured impedance	SOC, temperature, age, frequency and method	Treating it as one fixed resistor
Continuous current	Icont	A	Current permitted continuously under stated conditions	Temperature, cooling, cells, interconnect and protection	Replacing it with a pulse rating
Peak / pulse current	Ipeak	A	Short-duration current allowed for a specified pulse	Pulse duration, duty cycle, voltage and temperature	Using it as a continuous rating
Cycle life	Ncycle	cycles	Cycles to a specified retention or end-of-life criterion	Depth of discharge, rate, temperature and charge limits	Treating it as a chemistry-wide constant
Self-discharge	—	%/month	Stored-charge loss while not powering the external load	Temperature, SOC, chemistry and time	Confusing it with system standby current

Voltage Terminology

Nominal is representative

Nominal voltage is a rated value for identification and nominal energy arithmetic. It is not the terminal voltage at every SOC, current, temperature or age.

Actual terminal voltage

Terminal voltage reflects chemistry, SOC, load current, internal impedance, temperature, aging and recent charge or discharge history.

Maximum is not nominal

A charge or maximum voltage is an upper limit under a specified charging method. It must not replace nominal voltage in ordinary naming or energy comparisons.

Chemistry tables are separate

Chemistry-specific nominal, charge and cutoff ranges belong to REF-BAT-002. The exact product datasheet always governs.

Charge Capacity: Ah and mAh

Ampere-hour measures electric charge capacity, not energy and not instantaneous output current. 1 Ah = 3600 C and 1000 mAh = 1 Ah. Therefore 2500 mAh = 2.5 Ah.

Capacity is condition-dependent

A datasheet capacity normally applies at a stated discharge rate, cutoff voltage and temperature, and may also specify charging and rest conditions.

Ah is not amperes

A 5 Ah rating does not mean the battery may safely deliver 5 A. Continuous and pulse current limits are separate specifications.

Energy: Wh and J

1 Wh = 3600 J. With an explicit voltage basis, E(Wh) ≈ V × Q(Ah) and Q(Ah) ≈ E(Wh) / V. Nominal voltage produces a nominal energy estimate, not guaranteed deliverable energy.

Capacity and energy comparison
Battery	Voltage basis	Capacity	Nominal energy	Interpretation
A	3.7 V nominal	5 Ah	18.5 Wh	Higher Ah, lower nominal energy
B	12 V nominal	3 Ah	36 Wh	Lower Ah, higher nominal energy
Example pack	12 V nominal	100 Ah	1200 Wh = 1.2 kWh	100 Ah recovered only when dividing by 12 V

Actual deliverable energy also depends on the discharge curve, load, cutoff, temperature, age and system efficiency. mAh alone cannot compare energy at different voltages.

C-Rate

Current relationship

I = C-rate × Q(Ah). For a 5 Ah battery, 0.2C = 1 A, 1C = 5 A and 2C = 10 A.

Ideal time relationship

The reciprocal gives an idealized rate interpretation: 1C ≈ 1 h, 0.5C ≈ 2 h and 2C ≈ 0.5 h. It does not guarantee runtime or establish a safe current.

Allowed charge and discharge C-rates are product-specific and condition-dependent. Peukert-type behavior is relevant to some batteries, notably traditional lead-acid applications, but must not be applied universally.

Series and Parallel Relationships

Ideal matched-cell series and parallel relationships
Configuration	Voltage	Ah capacity	Nominal energy	Key caveat
Single cell	Vcell	Qcell	Vcell × Qcell	Use the exact product ratings
Ns in series	Ns × Vcell	Approximately Qcell	Voltage adds; Ah does not
Np in parallel	Approximately Vcell	Np × Qcell	Ah adds; voltage does not
Ns × Np	Ns × Vcell	Np × Qcell	Ns × Np × cell Wh	Nominal arithmetic for matched cells

This is arithmetic, not construction guidance. Real parallel and series packs require compatible chemistry, cell and SOC matching, protection, balancing and manufacturer-approved architecture.

4S example

Four 3.6 V, 2.5 Ah cells in series give 14.4 V nominal, approximately 2.5 Ah and 36 Wh nominal.

3P example

Three matched 3.6 V, 2.5 Ah cells in parallel give approximately 3.6 V, 7.5 Ah and 27 Wh nominal.

4S3P cross-check

A 4S3P pack gives 14.4 V, 7.5 Ah and 108 Wh nominal. Twelve cells at 9 Wh each also give 108 Wh.

Limits remain separate

Pack current and energy capability are constrained by cells, interconnects, protection, thermal conditions and imbalance, not arithmetic alone.

Runtime Estimates

Current-based

runtime(h) ≈ Ah / A. A 10 Ah battery at a constant 2 A gives an idealized 5 h estimate.

Energy-based

runtime(h) ≈ usable Wh × η / load W. 120 Wh at 20 W gives 6 h ideal, or 5.4 h with an assumed 90% conversion efficiency.

These estimates are not guarantees. Use a realistic SOC window, load profile, cutoff, converter efficiency, temperature and aged capacity. Rated capacity can change with discharge rate and test method.

Internal Resistance and Voltage Sag

A first-order Thevenin estimate uses ΔV ≈ I × Rint, Vterminal ≈ Voc − ΔV during discharge, and Ploss ≈ I²Rint. Battery impedance is not a fixed resistor; SOC, temperature, aging, frequency and measurement method change the result.

Sag example

At 5 A and 50 mΩ, ΔV ≈ 0.25 V. From 12.6 V open circuit, the simple model estimates 12.35 V terminal voltage.

Loss example

At the same point, 5² × 0.05 = 1.25 W. Real electrochemical and dynamic effects can differ from this approximation.

SOC, SOH and Usable Capacity

Battery condition terminology
Term	Meaning	What it is not	Conditions to preserve
SOC	Remaining charge estimate relative to a defined basis	A universal exact terminal-voltage lookup	Chemistry, load, rest, temperature and estimator
SOH	Condition relative to a defined new-battery baseline	Remaining charge at this moment	Capacity, resistance, power capability and chosen criterion
Depth of discharge	Fraction removed from a defined full state	Always identical to 100% − SOC under every convention	Definition, SOC window and measurement method
Usable capacity	Capacity available inside selected operating limits	Automatically equal to nameplate capacity	Cutoff, rate, temperature, age and reserve margin

Continuous, Peak and Life Ratings

Continuous current

Check temperature, voltage sag, cooling, connections and protection. Ah capacity alone does not define a continuous-current limit.

Peak current

A pulse rating is valid only for its duration, duty cycle, starting SOC, temperature and voltage-limit conditions.

Cycle life

Cycle life is tied to a retention criterion and test profile. Depth of discharge, rates, voltage limits and temperature materially affect it.

Maximum ratings

Do not combine independent maximum voltage, current, temperature and duration limits as one valid operating point.

Conversion and Formula Lookup

Battery unit and arithmetic lookup
Relationship	Equation	Condition
mAh to Ah	Ah = mAh / 1000	Exact unit conversion
Ah to coulombs	C = Ah × 3600	Exact charge-unit relationship
Wh to joules	J = Wh × 3600	Exact energy-unit relationship
Ah to Wh	Wh ≈ V × Ah	Requires an explicit voltage basis
Wh to Ah	Ah ≈ Wh / V	Requires an explicit voltage basis
C-rate current	I = C-rate × Ah	Arithmetic; allowed current remains product-specific
Current runtime	h ≈ Ah / A	Idealized first-order estimate
Energy runtime	h ≈ usable Wh × η / W	Define usable energy and efficiency
Voltage sag	ΔV ≈ I × Rint	First-order resistance approximation

Common Interpretation Mistakes

Confusing Ah with Wh.
Comparing energy by mAh alone.
Treating nominal voltage as constant terminal voltage.
Treating Ah as output current.
Treating C-rate as capacity.
Claiming 1C guarantees exactly one hour.
Ignoring capacity test conditions.
Treating calculated runtime as guaranteed.
Adding Ah in series.
Adding voltage in parallel.
Assuming arbitrary cells can be paralleled safely.
Treating internal resistance as fixed.
Treating the IR sag model as exact battery behavior.
Using terminal voltage as a universal exact SOC.
Confusing SOC with SOH.
Using peak current as continuous current.
Treating rated energy as fully usable energy.
Treating cycle life as a chemistry-wide constant.
Using maximum charge voltage as nominal voltage.
Assuming one C-rate is safe for every cell.
Applying Peukert's law universally.
Ignoring temperature, cutoff or system efficiency.

Datasheet Lookup Workflow

1Identify the exact cell or pack and chemistry.
2Check nominal, charge and cutoff voltage definitions.
3Check capacity and its rate, cutoff and temperature conditions.
4Convert Ah and Wh only with an explicit voltage basis.
5Check continuous and pulse-current limits separately.
6Confirm the capacity basis used for C-rate.
7Check operating and charging temperature ranges.
8Check impedance measurement conditions.
9Review cycle-life test conditions.
10Define usable SOC window and reserve.
11Verify protection and balancing requirements.
12Use the manufacturer datasheet for operating limits.

Safety and Scope Boundary

This page explains ratings, units and nominal arithmetic. It is not a charging, pack-construction or repair procedure. Do not bypass protection, charge damaged cells, improvise charging methods or connect cells outside manufacturer-approved pack and protection guidance.

Support reference

FAQ

What is the difference between Ah and Wh?

Ah measures electric charge capacity. Wh measures energy and includes voltage. Batteries with the same Ah rating can have different energy when their voltages differ.

How do I convert mAh to Ah?

Divide by 1000. For example, 2500 mAh equals 2.5 Ah.

How do I convert Ah to Wh?

Multiply ampere-hours by an appropriate voltage: nominal Wh is approximately Vnom multiplied by Ah. This is a nominal estimate when nominal voltage is used.

Can I convert Wh to Ah without knowing voltage?

No. Ah equals Wh divided by voltage, so the voltage basis must be known or explicitly assumed.

What does nominal battery voltage mean?

Nominal voltage is a representative rated value used for identification and nominal calculations. Actual terminal voltage changes with SOC, load, temperature, age, chemistry and recent history.

What is battery C-rate?

C-rate expresses charge or discharge current relative to capacity. For a 5 Ah battery, 0.2C corresponds to 1 A and 1C corresponds to 5 A as arithmetic relationships.

Does 1C always mean exactly one hour of runtime?

No. One hour is an ideal reciprocal-rate interpretation. Actual runtime depends on usable capacity, cutoff, rate effects, temperature, aging and losses.

What changes when cells are connected in series?

For ideal matched cells, series connection adds voltage while the Ah capacity of one series string remains approximately the cell Ah rating.

What changes when cells are connected in parallel?

For ideal matched cells, parallel connection adds Ah capacity while voltage remains approximately the cell voltage. Safe implementation requires manufacturer-approved matching, protection and pack design.

What is battery internal resistance?

It is a simplified resistance or impedance measure associated with voltage sag and internal loss. Its value depends on SOC, temperature, age, frequency and measurement method.

Why does battery voltage drop under load?

A first-order estimate is ΔV = I × Rint. Real battery polarization and electrochemical dynamics make the simple resistance model approximate.

What is the difference between SOC and SOH?

SOC estimates remaining charge relative to a defined basis. SOH estimates aging or condition relative to a defined new-battery baseline.

Is terminal voltage an exact measure of SOC?

No. Voltage-to-SOC behavior depends on chemistry, load, temperature, rest time, hysteresis and age. Use the product's specified estimation method.

Is peak discharge current the same as continuous current?

No. Peak current applies only for stated pulse duration, duty cycle, voltage and temperature conditions; it is not a continuous rating.

Why can actual runtime differ from Ah divided by current?

Rated capacity is condition-dependent, and real runtime also reflects load profile, cutoff, voltage variation, converter efficiency, temperature, aging and discharge-rate effects.

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