State of Charge Calculator

Use chemistry settings, capacity, voltage, aging, and thermal adjustments to estimate real battery charge and reserve-aware usable energy.

Battery chemistry

Rated capacity (Ah)

Remaining charge (Ah)

Measured pack voltage (V)

Cells in series

Nominal cell voltage (V)

Full cell voltage (V)

Empty cell voltage (V)

Battery temperature (C)

Reference temperature (C)

Temperature coefficient (% per C)

Age loss (%)

Self discharge (%)

Coulombic efficiency (%)

Reserve margin (%)

Coulomb estimate weight (%)

Formula used

Effective Capacity = Rated Capacity × (1 − Age Loss) × Temperature Factor

Coulomb SOC = ((Remaining Charge × Coulombic Efficiency) ÷ Effective Capacity × 100) − Self Discharge

Voltage SOC = ((Cell Voltage − Empty Cell Voltage) ÷ (Full Cell Voltage − Empty Cell Voltage)) × 100

Blended SOC = (Coulomb SOC × Coulomb Weight) + (Voltage SOC × Voltage Weight)

Usable SOC = Blended SOC − Reserve Margin; Usable Energy = Effective Capacity × Nominal Pack Voltage × Usable SOC

This model combines chemical behavior, charge inventory, thermal drift, and aging losses, producing a more realistic battery estimate than voltage alone.

How to use this calculator

Select the battery chemistry closest to your pack.
Enter rated capacity and your current measured charge.
Add pack voltage and the number of series cells.
Review or adjust full, empty, and nominal cell voltages.
Enter temperature, age loss, self discharge, and efficiency.
Set reserve margin and the coulomb weighting preference.
Press calculate to show results above the form.
Export the summary as CSV or PDF if needed.

Example data table

Chemistry	Rated Capacity (Ah)	Remaining Charge (Ah)	Pack Voltage (V)	Cells	Age Loss (%)	Usable SOC (%)
Lithium Ion	100	62	14.80	4	8	59.86
Lead Acid	200	110	12.42	6	12	44.38
LiFePO4	150	108	13.10	4	5	66.28
NiMH	24	15	13.60	10	10	48.54

Frequently asked questions

1. What does state of charge mean?

State of charge shows the remaining battery capacity as a percentage of usable capacity. It is similar to a fuel gauge, but chemistry and temperature can shift the estimate.

2. Why blend voltage and charge data?

Voltage reacts quickly to load and temperature, while coulomb counting tracks stored charge over time. Blending both methods usually reduces error when either signal alone is imperfect.

3. Why does chemistry matter?

Each chemistry has a different voltage curve, efficiency, and self discharge profile. Using the correct chemistry gives a more realistic relation between cell voltage and remaining charge.

4. How does temperature affect results?

Cold and heat can reduce available capacity and distort voltage readings. The temperature adjustment lowers effective capacity when the battery operates away from its reference condition.

5. What is reserve margin?

Reserve margin is a safety buffer you keep unused. It protects cycle life, avoids deep discharge, and helps maintain performance under sudden load spikes.

6. Can I use this for pack maintenance?

Yes, it is useful for routine estimation, maintenance planning, and quick comparisons. For critical control systems, pair it with calibrated battery management hardware and logged testing.

7. Does age loss equal battery failure?

No. Age loss simply reduces effective capacity from the original rating. A battery may still work well, but deliver less runtime and reach low voltage sooner.

8. What export options are included?

The page can export a CSV summary and a printer friendly PDF. These options make it easier to document inspections, maintenance checks, or battery comparisons.