Battery calculator

Battery C-Rate Calculator

C-rate compares current with battery capacity. A 100 Ah battery at 20 A is running at 0.2 C; the same 20 A is 2 C for a 10 Ah battery.

C-rate0.2 C

Calculation breakdown

Calculation breakdown
Ideal equivalent durationBefore chemistry, voltage and high-rate losses	5 h
Current	20 A
Capacity	100 Ah

C-rate = Current (A) ÷ Capacity (Ah)
= 20 A ÷ 100 Ah
= 0.2 C

Before you rely on this

Do not treat the calculated C-rate as an allowed rate. Check continuous, pulse, temperature and BMS limits for the exact battery.

Method

How the number is reached

C-rate normalizes current by capacity so batteries of different sizes can be compared. 1 C would move the rated amp-hour capacity in one hour under an ideal linear model; 0.2 C corresponds to five hours.

Real runtime is not simply 1/C. Lead-acid capacity falls at higher discharge current, voltage limits intervene, and manufacturers publish separate continuous and pulse limits.

C-rate = Current (A) ÷ Capacity (Ah)
Ideal duration (h) = 1 ÷ C-rate

Symbols

C: Current as a multiple of rated capacity

Worked examples

The same maths, applied

Example

20 A from a 100 Ah battery

A 100 Ah bank supplies a steady 20 A load.

C-rate = 20 A ÷ 100 Ah
C-rate = 0.2 C
Ideal duration = 1 ÷ 0.2 = 5 h

Result: 0.2 C, equivalent to five ideal hours.

Limits of the model

What it assumes, and where it stops

Assumptions

Capacity and current are expressed at the same battery-bank boundary.

The duration is an ideal ratio, not a delivered-runtime prediction.

Not covered

C-rate alone does not account for voltage, temperature, age, cutoff voltage or Peukert behavior.

Safe charge and discharge rates come from the specific battery and BMS documentation.

Sources

Where these figures come from

BMV-700H — Battery capacity and Peukert exponentVictron EnergyDefines C-rate and Peukert behavior, gives the lead-acid C20 example and documents the exponent range and limitations.
VE.Bus BMS NG — Battery current and discharge-floor settingsVictron EnergyManufacturer guidance for maximum and recommended C-rates, BMS current limits and minimum state-of-charge floors.

Next steps

Related calculators

Peukert's Law Calculator
Lead-acid batteries deliver less than their rated amp-hours when discharged faster than the test rate. Peukert's law estimates that reduction from the battery's rated current and exponent.
Battery Charger Size Calculator
Choose a charging window and SOC target to estimate the DC amps a charger must supply. The model reserves time for current taper near full charge and adds any loads running while charging.
Battery Runtime Calculator
Enter your battery and your load to estimate runtime. The calculator applies depth of discharge, battery health, conversion losses and — for lead-acid only — the Peukert effect, and shows each step of the energy budget.

All battery calculators →

Last meaningful update: 2026-08-19. This date changes only when the model, the sources or the guidance change.