Battery State of Charge Calculator

Estimate battery state of charge from measured voltage, chemistry curve, bank voltage, load correction, temperature derating, and reserve level for smart home backup systems.

📌Real Battery Presets

🔋Battery Voltage And Load Inputs

Battery chemistry profile Choose the voltage curve that best matches the battery being measured.

Nominal battery bank voltage Use the nominal voltage of the complete battery bank, not one block in a string.

Rated battery bank capacity (Ah) For parallel batteries, enter combined Ah. Series strings keep the same Ah.

Measured terminal voltage (V) Rested voltage is best. Under-load and charging readings are corrected below.

Measurement condition Discharge readings are corrected upward; charge readings are corrected downward.

Current at reading or load (A) This current is used for voltage correction and runtime estimate.

Estimated bank resistance (mΩ) Approximate voltage sag correction: amps x mΩ / 1000.

Battery temperature (°F) Capacity derating is referenced to 77°F / 25°C.

Reserve SOC to keep (%) Lead-acid is often planned around 50%; LFP commonly uses 10% to 20% reserve.

Estimated State Of Charge 0% Interpolated from corrected voltage

Usable Capacity Above Reserve 0 Ah Effective Ah x usable SOC window

Usable Energy Above Reserve 0 Wh Usable Ah x nominal bank volts

Estimated Runtime 0 hr Usable Ah / entered amps

Calculation Breakdown

⚙Battery SOC Spec Grid

12.7V Typical full rested flooded 12 V

12.8V Typical full rested AGM 12 V

13.6V Typical full rested 4S LFP

77°F Reference capacity temperature

50% Common lead-acid planning reserve

20% Common LFP planning reserve

mΩ Resistance input for sag correction

Wh Usable Ah times nominal voltage

📊12V Resting Voltage State Of Charge

SOC	Flooded Lead-Acid	AGM / SLA	Gel Lead-Acid
100%	12.73 V	12.80 V	12.85 V
90%	12.62 V	12.65 V	12.68 V
80%	12.50 V	12.50 V	12.52 V
70%	12.37 V	12.37 V	12.40 V
60%	12.24 V	12.24 V	12.27 V
50%	12.10 V	12.10 V	12.15 V
40%	11.96 V	11.96 V	12.00 V
30%	11.81 V	11.81 V	11.85 V
20%	11.66 V	11.66 V	11.70 V
10%	11.51 V	11.51 V	11.55 V
0%	10.50 V	10.50 V	10.50 V

Lead-acid voltage SOC is most useful after the battery has rested. Manufacturer data should override generic reference curves.

📈Lithium Resting Voltage Reference

SOC	4S LiFePO4 12.8V	16S LiFePO4 51.2V	13S NMC 48V
100%	13.60 V	54.40 V	54.60 V
90%	13.40 V	53.60 V	53.30 V
80%	13.28 V	53.12 V	52.00 V
70%	13.20 V	52.80 V	50.96 V
60%	13.12 V	52.48 V	50.05 V
50%	13.08 V	52.32 V	49.14 V
40%	13.04 V	52.16 V	48.36 V
30%	13.00 V	52.00 V	47.58 V
20%	12.92 V	51.68 V	46.54 V
10%	12.80 V	51.20 V	44.85 V
0%	11.60 V	46.40 V	41.60 V

🌡Temperature Capacity Reference

Battery Temperature	Lead-Acid Capacity	Lithium Capacity	Planning Detail
32°F / 0°C	About 80%	About 92%	Cold cabinets need extra reserve
50°F / 10°C	About 90%	About 97%	Common garage derating point
77°F / 25°C	100%	100%	Most ratings use this reference
95°F / 35°C	About 97%	About 98%	Heat reduces long-term battery life
113°F / 45°C	About 92%	About 95%	Hot equipment closets need caution

📐Common Smart Home Battery Examples

Use Case	Typical Battery	Planning Load	SOC Detail
Alarm panel standby	12 V 7 Ah SLA	0.3 to 1 A	Resting voltage is usually stable
Router and ONT backup	12 V 35 Ah AGM	1.5 to 4 A	Correct voltage while powering gear
PoE cameras and NVR	12 V 100 Ah LFP	8 to 18 A	Flat curve makes reserve important
24 V solar battery shelf	24 V 200 Ah flooded	10 to 40 A	Use rested voltage after charging
48 V network rack	51.2 V 100 Ah LFP	5 to 25 A	Check BMS SOC when available

🧪Chemistry Comparison Table

Battery Type	Voltage Curve	Reserve Habit	Best SOC Method
Flooded lead-acid	Moderate slope	Keep near 50%	Hydrometer plus rested voltage
AGM / SLA	Moderate slope	Keep near 50%	Rested voltage with load correction
Gel lead-acid	Moderate slope	Keep near 50%	Rested voltage and low charge rates
LiFePO4 / LFP	Very flat middle	10% to 20%	Shunt/BMS plus voltage estimate
NMC / NCA lithium	Sloped curve	10% to 20%	Voltage estimate is more responsive

✅Battery SOC Calculation Tips

Measure after rest when possible: Charging raises terminal voltage and active loads pull it down. The calculator applies a resistance correction, but a rested reading is still the cleanest voltage-based SOC input.

Match the curve to the battery: A 12 V AGM, 12 V flooded battery, and 12.8 V LiFePO4 pack can show similar labels but very different voltage-to-SOC behavior.

When monitoring an battery, a person might use a voltmeter to take voltage readings of the battery. However, voltage readings doesnt necessarily indicate the remaining time that the battery will last. Many batteries appear to provide a more linear view of their remaining life, but batteries dont always function in this linear view due to there complex chemical compositions.

To understand the nature of a battery, it is essential for the understanding of the relationship between the voltage, the temperature, and the electrical load of a battery. A voltmeter does not indicate the amount of energy left in a battery. Instead, it measures the electrical pressure of the battery at a specific time.

Voltage Alone Does Not Show How Long a Battery Will Last

If the battery is resting, the voltage will be more different then if the battery is providing power to an electrical device. When measuring the voltage of a battery that is under heavy electrical load, the voltage will drop. This drop in voltage indicate that the battery will fail to meet the demands of the device that it is powering.

This drop in voltage is referred to as voltage sag. One must account for the effect of voltage sag when determining the true voltage of a rested battery. Depending on the chemistry of the battery, the voltage of a battery can even drop in different manner.

For example, lead acid batteries will have a predictable discharge curve. This means that the voltage of the battery will drop in a steady manner as the capacity of the battery diminishes. In contrast, lithium batteries will have a flat discharge curve.

This means that the voltage of a lithium battery will remain the same over an extended period. Due to the same voltage of a lithium battery over time, it is possible for a battery that is seemingly full to be nearly empty. To account for this, a reserve limit must be used with lithium batteries as the voltage isnt a reliable indicator of the amount of charge that a lithium battery have left.

The temperature of a battery can alter the way that the battery function. This is due to the chemical reactions that the battery use to produce the power that it provides. When the chemicals in a battery move slow due to low battery temperatures, the chemical reactions that occur within the battery will also take place at a slow rate.

A slow rate of chemical reaction mean that the battery will not be able to provide as much power as it should. If a battery remains in a cold environment, the battery will have a lower capacity than it should have. For example, the battery might provide twenty percent less capacity with the same amount of cold storage than a battery that remains at room temperature.

Thus, the temperature of a battery can alter the amount of energy that it can provide. A battery should have a reserve capacity set aside to allow for the lifespan of the battery. If a lead-acid battery is discharged to zero percent, the plate in the battery will be damaged.

This damage to the battery plates can lead to a shortened lifespan of the battery. Due to this shortened lifespan, many people only use fifty percent of the capacity that a lead-acid battery can store. A lithium battery can have more of its capacity used than a lead-acid battery can.

However, a reserve should still be used for the same reasons as lead-acid batteries to allow for error in the electrical load of the device. In order to calculate how long a battery will last, it is necessary to understand the current draw of the equipment that will use the battery. If the current draw of the battery is in amps and the battery has a number of usable amp-hours, one can calculate the amount of time that the battery will last.

It is essential to use the usable capacity of the battery rather than the total amount of capacity that the battery can hold. The total capacity is the amount of energy that the battery can provide with no load on the battery. However, the usable capacity of the battery is the amount of energy that the equipment can use after providing for the reserve capacity of the battery and after accounting for the temperature of the battery.

Another way to determine the state of a battery is to compare the voltage of the battery to reference tables that display the expected voltage given the chemistry of the battery. If the voltage of a rested battery is less than the voltage indicate in the reference table, there is a chance that the battery has issues with cell failure or has issues with sulfation. By checking the voltage of a battery once a month, the person can ensure that the battery is in proper functioning condition.

By correcting for voltage sag and taking into account the temperature and chemistry of the battery, a person will be able to plan for the life of their battery rather than guess as to how long the battery will last.