Battery State of Charge Calculator

A battery's State of Charge (SoC) tells you how much usable energy remains — similar to a fuel gauge — and measuring it accurately requires accounting for both battery chemistry and temperature. Enter your Battery Voltage (open circuit), select your Battery Type (Lead Acid, Lithium-ion, NiCd, or NiMH), and provide the ambient Temperature to get your State of Charge percentage. Secondary outputs include the Temperature Compensated Voltage and an overall Battery Condition rating. Also try the UPS Runtime Calculator.

V

Open circuit voltage measurement

Temperature Unit *

Ambient temperature for compensation

Results

State of Charge

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Temperature Compensated Voltage

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Battery Condition

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Frequently Asked Questions

What is battery state of charge (SOC)?

State of charge is the percentage of battery capacity that remains available. A 100% SOC means the battery is fully charged, while 0% means completely discharged. See also our Lead-Acid Battery Calculator.

How do I measure battery voltage for SOC calculation?

Measure the open circuit voltage when the battery has been at rest for at least 2 hours without charging or discharging. Use a quality digital multimeter for accurate readings.

Why does temperature affect battery SOC calculations?

Battery voltage changes with temperature. Cold temperatures reduce voltage readings while warm temperatures increase them, so temperature compensation ensures accurate SOC calculations.

What's the difference between SOC calculations for different battery types?

Each battery chemistry has different voltage curves. Lead acid batteries have different voltage-to-SOC relationships compared to lithium-ion or nickel-based batteries.

At what SOC should I recharge my battery?

For lead acid batteries, recharge at 50% SOC to prevent damage. Lithium-ion batteries can be discharged deeper, typically to 20-30% SOC safely.

How accurate are voltage-based SOC calculations?

Voltage-based SOC calculations are reasonably accurate for rested batteries but can vary by ±5-10% depending on battery age, temperature, and load history.