Lithium Battery State of Charge Calculator

Estimate lithium battery SOC from measured pack voltage, chemistry curve, load or charge correction, rated capacity, and the reserve you want to keep unused.

⚡Real Battery Presets

🔋Battery Measurements

Lithium chemistry curve

The calculator interpolates against the selected resting voltage curve.

Series cells in pack

Examples: 4S = 12.8V LFP, 13S = 48V NMC, 16S = 51.2V LFP.

Measured pack voltage (V)

Use terminal voltage measured with a multimeter or battery monitor.

Measurement condition

Load voltage is corrected upward; charge voltage is corrected downward.

Current at reading (A)

Use discharge amps for load, or charge amps when charging. Use 0 if rested.

Estimated pack resistance (mΩ)

Pack mΩ corrects voltage sag or rise: amps x resistance / 1000.

Rated pack capacity (Ah)

Enter nameplate Ah or tested usable Ah at the pack voltage.

Reserve to keep unused (%)

Usable energy is calculated above this reserve SOC.

Estimated state of charge

Corrected from pack voltage

Usable capacity above reserve

Formula: Ah x (SOC - reserve)

Usable energy

Formula: usable Ah x nominal volts

Time at entered current

Runtime or charge time by condition

⚙Spec Comparison Grid

3.2V

LFP nominal cell

3.6V

NMC nominal cell

2.3V

LTO nominal cell

30 min

Preferred rest time

📊LiFePO4 Resting Voltage Reference

SOC	Cell voltage	4S 12.8V pack	16S 51.2V pack
100%	3.40 V	13.60 V	54.40 V
90%	3.35 V	13.40 V	53.60 V
80%	3.32 V	13.28 V	53.12 V
70%	3.30 V	13.20 V	52.80 V
60%	3.28 V	13.12 V	52.48 V
50%	3.27 V	13.08 V	52.32 V
40%	3.26 V	13.04 V	52.16 V
30%	3.25 V	13.00 V	52.00 V
20%	3.23 V	12.92 V	51.68 V
10%	3.20 V	12.80 V	51.20 V
0%	2.90 V	11.60 V	46.40 V

📈NMC, LiPo, and LTO Voltage Reference

SOC	NMC cell	LiPo cell	LTO cell
100%	4.20 V	4.20 V	2.80 V
90%	4.10 V	4.12 V	2.65 V
80%	4.00 V	4.03 V	2.55 V
70%	3.92 V	3.95 V	2.47 V
60%	3.85 V	3.87 V	2.40 V
50%	3.78 V	3.80 V	2.35 V
40%	3.72 V	3.74 V	2.30 V
30%	3.66 V	3.69 V	2.25 V
20%	3.58 V	3.62 V	2.15 V
10%	3.45 V	3.50 V	2.00 V
0%	3.20 V	3.30 V	1.80 V

🧪Chemistry Comparison Table

Chemistry	Nominal cell	Typical full charge	SOC behavior
LiFePO4 / LFP	3.2 V	3.65 V charge, about 3.40 V rested	Very flat middle curve; small voltage changes matter.
NMC / NCA / 18650	3.6 V to 3.7 V	4.20 V	Sloped voltage curve; voltage SOC estimate is easier.
LiPo	3.7 V	4.20 V	Similar to NMC, but load sag can be large at high C rate.
LTO	2.3 V to 2.4 V	2.70 V to 2.80 V	Wide cycle life; lower voltage per cell.

📐Common Pack Examples

Pack type	Series cells	Nominal voltage	Typical use
12V LiFePO4	4S	12.8 V	RV, marine, small solar storage
24V LiFePO4	8S	25.6 V	Solar carts, inverters, workshop backup
48V LiFePO4	16S	51.2 V	Server rack battery and home backup
48V NMC e-bike	13S	46.8 V to 48.1 V	E-bike and compact mobility packs
6S LiPo	6S	22.2 V	Drone, RC, and portable high-current loads

💡Calculation Tips

Voltage correction: The calculator estimates open-circuit voltage as measured voltage plus current x resistance during discharge, or minus that value during charge. Enter 0 A for a rested pack.

Flat LFP curve: LiFePO4 voltage changes slowly between about 40% and 80% SOC. A shunt-based battery monitor is more precise, but voltage still gives a useful field estimate.

Reference curves are practical rested-voltage approximations at normal room temperature. Cell age, temperature, calibration, BMS balancing, and recent charge or discharge history can shift real readings.

Monitoring the voltage of a lithium battery can be difficultly. The voltage reading of a lithium battery are not always an accurate representation of the amount of energy the battery has left. From the voltage monitor of a battery, a voltage number can be displayed, but the voltage number isnt going to convey to a person whether there is a small amount of energy or a large amount of energy left in the battery.

This is due to the fact that lithium batteries does not work like fuel tank. Additionally, the voltage levels of lithium batteries dont drop visibly as the energy levels drop from the batteries. The chemical process of the batteries hides the energy levels behind a voltage curve.

Why battery voltage does not show true charge

The voltage reading of a lithium battery can be inaccurate due to a phenomenon known as voltage sag. Voltage sag will occur when a heavy inverter is running. Due to this phenomenon, the voltage from the battery will drop instant.

This may make the voltage of the battery appear as if the battery is nearly empty with its current power output. However, the voltage will bounce back to its normal reading after a person turns off the heavy inverter. Thus, rested voltage is a more accurate representation of the voltage of a battery than the voltage that is measured while the battery is under load.

To measure rested voltage, the battery should sit idle for a period of time. The rested voltage will be more accurate because the chemical components of the battery will have had time to stabilize. The chemistry of the lithium battery can change, so it is important for a person to understand the chemistry of the battery they are using.

For instance, lithium iron phosphate batteries, or LFP batteries, is stable chemistries. However, LFP batteries have a flat voltage discharge curve. A flat voltage discharge curve mean that the voltage output by an LFP battery will remain the same throughout a portion of the batterys energy output.

Thus, the voltage as measured by a multimeter will show a very small difference in voltage whether the energy level of the battery drop from 80% to 40%. Therefore, although LFP batteries will provide steady power output, it can be difficult for individuals to understand the remaining energy level of an LFP battery. Chemistries such as NMC and LiPo batteries is different than LFP batteries.

NMC and LiPo batteries have a sloping voltage discharge curve. This make it easier for individuals to understand when the voltage levels of the battery are dropping with the energy being used. However, NMC chemicals are more sensitive to discharging to 0% energy compared to LFP batteries.

If a person completely discharges an NMC battery, it can cause permanent damage to the chemistry of the battery. Due to the different chemical reaction of each battery type, it is important for individuals to understand the chemistry of the batteries that they are using to avoid damaging them. A tool can be used to estimate the charge level of a battery.

The calculation tool can calculate the theoretical rested voltage of the battery. This calculation can take into account the current load on the battery as well as the internal resistance of the battery pack to account for voltage sag. In this tool, a person can input their battery measurements.

The tool will perform the calculations for the person. Additionally, the tool will display the voltage of the battery as a percentage that individuals can use. This percentage is a more usable percentage of energy compared to raw voltage measurement.

The percentage that the battery can display indicate the state of charge. In addition to percentage, however, a battery reserve should be established. Many individuals dont wish to run their batteries down to 0% energy levels.

Running a battery to 0% will put stress on the battery. Additionally, the battery management system on the battery may shut the battery off sudden when the battery reaches 0%. For many people, using the bottom 20% of the batteries as a reserve will provide an additional layer of safety for the battery.

This percentage can also be accounted for in the calculation tool so that the person knows the usable power of the battery. Knowing the remaining runtime of a battery is more important than the remaining amp hours that a battery has. If a person knows the number of hours of runtime that a battery currently has at full load, they can decide whether or not they need to turn off an appliance such as a space heater.

Thus, knowing the runtime of the battery allows individuals to plan the energy use of the battery. However, voltage estimates are never perfect. The voltage can change due to temperature shift in the battery and the age of the battery.

As batteries age, their internal resistance increase. Higher internal resistance creates more voltage sag. However, for most applications, the voltage-corrected estimate is all that most users will need in order to ensure that there essential appliance such as lights and refrigerators continue to run.