How do I check battery SOC from voltage?

Measure battery voltage with no load after a 30-minute rest period. Then compare to the voltage-to-SOC table for your battery chemistry and system voltage. LiFePO4 at 12V: 13.0V ≈ 50% SOC. Lead-acid AGM at 12V: 12.3V ≈ 50% SOC. Always apply temperature correction in cold weather.

How does temperature affect battery voltage readings?

Cold temperatures cause batteries to read lower voltage even at the same SOC. A LiFePO4 battery at 0°C reads about 0.05V lower than at 25°C. Lead-acid is more affected — 0°C causes ~0.15V lower resting voltage. This calculator applies a temperature correction to give a more accurate SOC estimate.

What is a good SOC to keep lead-acid batteries at?

Lead-acid batteries (AGM and flooded) should ideally stay above 50% SOC to maximize cycle life. Discharging to 20% SOC or below causes sulfation that permanently reduces capacity. Keeping lead-acid above 70% SOC with shallow cycling maximizes lifespan — typically 500-1000 cycles vs. 200-400 cycles with deep discharge.

Battery SOC Monitor Calculator

Enter your battery voltage, chemistry, and temperature — get estimated state of charge, remaining capacity, runtime, and a full voltage-to-SOC lookup table.

Battery parameters

Battery chemistry?

System voltage (cells in series)?

Current voltage reading?

Temperature unit?

Current temperature (°F)?

°F

Current load?

Battery bank capacity?

Battery state of charge

50.0% SOC

Good — normal operating range

Remaining capacity50.0 Ah

Est. runtime at 10A load5.0 hrs

Temp-corrected voltage13.00 V

Battery temp25.0°C

Voltage-to-SOC lookup (12V system)

SOC %	Voltage (V)	Status
0%	11.8V	Empty
10%	12.0V
20%	12.2V
30%	12.5V
40%	12.8V
50%	13.0V	<-- you are here
60%	13.1V
70%	13.2V
80%	13.3V
90%	13.4V
100%	14.6V	Full

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How to Use This Calculator

Select your battery chemistry and system voltage

Start by selecting your battery chemistry. LiFePO4 (lithium iron phosphate) has a very flat voltage curve — voltage stays nearly constant from 20% to 80% SOC, making it hard to estimate by voltage alone. Lead-acid batteries have a steeper curve and are highly temperature-sensitive. NMC (nickel manganese cobalt) has the steepest curve, making voltage a more reliable SOC indicator. Then select your system voltage — 4 cells in series = 12V, 8 = 24V, 16 = 48V.

Enter voltage, temperature, and load

For the most accurate reading, measure voltage at the battery terminals with no load attached and let the battery rest for 30 minutes after charging or discharging. Enter the actual battery temperature — not air temperature. Cold batteries show artificially low voltage even at full charge. Enter the current load in amps to get an estimated runtime calculation.

Read the lookup table

The 10-row voltage-to-SOC table shows the full discharge curve for your battery type at your system voltage. Your current reading is highlighted in green. Use this table to understand what voltage corresponds to each charge level — especially useful for lead-acid batteries where resting voltage is the primary SOC indicator.

The Formula

Temp-corrected voltage = Measured V - (TempDelta × TempCoeff × NominalV) SOC = Interpolated from voltage-to-SOC table (linear between table points) Remaining Ah = Battery Ah × (SOC / 100) × Cold derating factor Runtime (hrs) = Remaining Ah ÷ Load (A) Cold derating = max(50%, 1 - (25 - TempC) × 1%) when TempC < 25°C

The voltage correction accounts for temperature effects on battery chemistry. Lead-acid batteries lose ~1% capacity per degree Celsius below 25°C — a 100Ah battery at 0°C behaves like a 75Ah battery. LiFePO4 is less affected but still loses 5-15% capacity at freezing temperatures. The interpolation uses linear interpolation between the 10 voltage breakpoints in the chemistry table.

Example

Mark — 12V LiFePO4 system reading 13.0V at 50°F

Mark has a 12V LiFePO4 200Ah battery bank in his off-grid cabin. The temperature is 50°F (10°C) and his inverter is drawing 10A. The BMS reads 13.0V.

ChemistryLiFePO4

System voltage12V (4 cells)

Measured voltage13.0V

Temperature50°F (10°C)

Load10A

Capacity200Ah

Result

Temp-corrected voltage12.97V

Estimated SOC~48%

Cold derating (10°C)85% of rated

Remaining capacity~81 Ah

Runtime at 10A~8.1 hrs

RecommendationCharge soon

At 48% SOC in cold weather, Mark has about 8 hours of runtime at his current load. The cold derating means his 200Ah bank is only delivering ~170Ah effective capacity. He should plan to charge before SOC drops below 20% — approximately 3 more hours at this load rate.

FAQ

Why is voltage a poor SOC indicator for LiFePO4? ▼

LiFePO4 has an extremely flat discharge curve — voltage stays between 13.2V and 13.0V from about 80% SOC down to 20% SOC on a 12V system. This means a voltage difference of just 0.2V spans 60% of the battery's capacity. By the time voltage starts dropping noticeably (below 12.8V), you're already below 20% SOC. For accurate LiFePO4 monitoring, use a battery management system (BMS) with coulomb counting, which tracks amps in and out over time. Voltage is only reliable when the battery has rested for at least 30 minutes after charging or discharging.

How does cold weather affect battery SOC readings? ▼

Cold temperatures cause two effects: (1) Lower resting voltage — a fully charged battery at 0°C reads lower voltage than the same battery at 25°C, causing the calculator to underestimate SOC. The temperature correction adjusts for this. (2) Reduced usable capacity — cold slows the electrochemical reactions, reducing how much energy you can actually extract. Lead-acid loses 30-40% capacity at 0°C; LiFePO4 loses 10-20%; NMC loses 15-25%. This is why your RV or boat battery seems "dead" on cold mornings even though it showed full charge the night before.

What is the minimum SOC I should discharge to? ▼

Recommended minimum SOC by chemistry: LiFePO4: 10-20% — these batteries tolerate deep discharge well and most BMS units cut off at 10%. Lead-acid AGM: 50% — discharging below 50% regularly shortens lifespan significantly; below 20% can cause permanent sulfation. Lead-acid flooded: 50% — same as AGM; these are the most damage-prone at deep discharge. NMC: 10-20% — tolerates deep discharge but high-frequency deep cycling reduces cycle life. For maximum lead-acid lifespan, stay above 70% SOC with shallow cycling.

Should I measure SOC under load or with no load? ▼

Always measure with no load for the most accurate SOC. Under load, voltage sag causes artificially low readings — a battery at 70% SOC may read 0.3-0.8V lower while powering a large inverter. After removing load, voltage recovers in 30 seconds to a few minutes depending on battery type. For lead-acid, a full 30-minute rest gives the most accurate "resting voltage" reading. LiFePO4 recovers faster (1-5 minutes). If you must measure under load, add approximately 0.2-0.5V to your reading to estimate true SOC.

What is the difference between SOC, DOD, and remaining capacity? ▼

SOC (State of Charge): percentage of energy currently stored, from 0% (empty) to 100% (full). DOD (Depth of Discharge): how far you've discharged from full — DOD = 100% - SOC. A battery at 30% SOC has been discharged to 70% DOD. Remaining capacity (Ah): the actual amp-hours available before the battery reaches its minimum allowed SOC. For a 100Ah battery at 60% SOC with a 20% minimum: remaining = (60% - 20%) × 100Ah = 40Ah usable. These three metrics are related but serve different purposes in system design and monitoring.

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