Battery Capacity & Voltage
How much charge is left in the battery? By measuring voltage, you can estimate the state of charge (SOC). This guide explains how it works with interactive charts and tables for common battery configurations.
Fully charged cell
4.2 V
Nominal voltage
3.6–3.7 V
Discharge cutoff
3.0 V
The basics
A lithium-ion cell voltage varies depending on how much charge remains. A fully charged cell is at 4.2 V, while a discharged cell (at a safe discharge cutoff) is around 3.0 V. Between these endpoints, you can use voltage to estimate how much capacity is left.
To calculate the voltage of a full battery pack, multiply cell voltage by the number of cells in series (S). For example:
It is important to understand that the relationship between voltage and state of charge is not linear. The curve is relatively flat in the middle range (30-80%) but drops quickly near the ends. That makes voltage-based SOC estimation most reliable in the middle of the charge cycle.
Discharge curve
The chart below shows a typical discharge curve for an 18650 lithium-ion cell. You can switch between a single-cell view and full battery-pack views with different configurations.
Discharge Curve – 18650 Lithium-Ion Cell
The graph shows how voltage drops as a typical 18650 lithium-ion cell discharges. The curve is relatively flat in the middle (30–80 %) but drops quickly at the ends.
Note: Note: The curve shows voltage at low load (at rest). Under high load, voltage drops further due to the battery's internal resistance. The discharge curve may also vary slightly depending on cell chemistry and temperature.
Note: the curve is flat in the middle
Between about 30% and 80% SOC, voltage drops very little (from ~3.8 V to ~3.6 V). This means small measurement errors can create large differences in estimated capacity in this range. On the other hand, when voltage starts dropping quickly, you know the battery is almost empty.
Calculate SOC from voltage
To estimate battery state of charge from a measured voltage, follow these steps:
- Measure resting voltage - Let the battery rest for at least 10-30 minutes after charging or use to get a stable value.
- Calculate cell voltage - Divide the measured voltage by the number of cells in series. Example: 75 V ÷ 20S = 3.75 V per cell.
- Compare with the discharge curve - Use the table or chart to find the corresponding SOC. 3.75 V corresponds to about 60%.
Example: 72 V system
Measured voltage: 78.0 V
Cell voltage: 78.0 V ÷ 20 = 3.90 V
Estimated SOC: ~75-80 %
Voltage tables for common configurations
Here are complete tables for the three most common battery configurations in electric vehicles. Values are based on a typical lithium-ion discharge curve at room temperature and resting voltage.
48 V system
13S - Nominal: 48.1 V
| SOC | Cell | Pack | Status |
|---|---|---|---|
| 100% | 4.20 V | 54.6 V | Fully charged |
| 90% | 4.08 V | 53.0 V | Excellent |
| 80% | 3.93 V | 51.1 V | Good |
| 70% | 3.82 V | 49.7 V | Good |
| 60% | 3.75 V | 48.8 V | OK |
| 50% | 3.70 V | 48.1 V | OK |
| 40% | 3.65 V | 47.5 V | Low |
| 30% | 3.60 V | 46.8 V | Low |
| 20% | 3.50 V | 45.5 V | Very low |
| 10% | 3.30 V | 42.9 V | Critical |
| 0% | 3.00 V | 39.0 V | Empty |
52 V system
14S - Nominal: 51.8 V
| SOC | Cell | Pack | Status |
|---|---|---|---|
| 100% | 4.20 V | 58.8 V | Fully charged |
| 90% | 4.08 V | 57.1 V | Excellent |
| 80% | 3.93 V | 55.0 V | Good |
| 70% | 3.82 V | 53.5 V | Good |
| 60% | 3.75 V | 52.5 V | OK |
| 50% | 3.70 V | 51.8 V | OK |
| 40% | 3.65 V | 51.1 V | Low |
| 30% | 3.60 V | 50.4 V | Low |
| 20% | 3.50 V | 49.0 V | Very low |
| 10% | 3.30 V | 46.2 V | Critical |
| 0% | 3.00 V | 42.0 V | Empty |
72 V system
20S - Nominal: 72.0 V
| SOC | Cell | Pack | Status |
|---|---|---|---|
| 100% | 4.20 V | 84.0 V | Fully charged |
| 90% | 4.08 V | 81.6 V | Excellent |
| 80% | 3.93 V | 78.6 V | Good |
| 70% | 3.82 V | 76.4 V | Good |
| 60% | 3.75 V | 75.0 V | OK |
| 50% | 3.70 V | 74.0 V | OK |
| 40% | 3.65 V | 73.0 V | Low |
| 30% | 3.60 V | 72.0 V | Low |
| 20% | 3.50 V | 70.0 V | Very low |
| 10% | 3.30 V | 66.0 V | Critical |
| 0% | 3.00 V | 60.0 V | Empty |
Why 52 V instead of 48 V?
52 V batteries (14S) have become popular because they provide about 8% higher voltage than 48 V (13S) with the same cell count, which delivers more power and better performance in high-load scenarios. Many controllers labeled "48 V compatible" actually handle up to 60 V and work very well with 52 V batteries.
SOC calculator
Use the calculator below to quickly estimate state of charge from measured voltage and your battery configuration.
SOC Calculator – Estimate Charge Level
Enter your measured battery voltage and configuration to get an approximate estimate of the charge level (SOC).
Or enter a custom number of cells in series:
SMeasure at rest (not under load) for best accuracy.
Enter a voltage above to see estimated charge level.
Obs: Note: Results are estimates based on a typical lithium-ion cell discharge curve. Actual SOC may vary depending on cell chemistry (LFP has a flatter curve), temperature, age, and whether the battery was recently under load.
Practical tips
Measuring while riding? Adjust the voltage upward
If you check voltage while riding (for example on the display) and have just stopped accelerating, it shows a lower voltage than the true resting voltage. This is because the battery is still "recovering" from the load.
Rule of thumb: Add 2-5 V to the displayed voltage during or right after riding to estimate resting voltage.
Example: The display shows 71 V after hard acceleration on a 72 V system -> actual resting voltage is probably around 74-76 V (~50-60% SOC instead of ~30%).
The harder you accelerate and the higher current that is drawn, the larger the difference becomes. During light cruising the difference is smaller (~1-2 V).
Avoid deep discharge
Regularly discharging the battery below 3.0 V per cell (or below 20% SOC) significantly shortens its lifespan. Most BMS units have a built-in discharge cutoff, but it is good to be aware of this.
Summary
- A lithium-ion cell varies between 3.0 V (empty) and 4.2 V (full)
- Multiply cell voltage by the number of S cells to get pack voltage
- The curve is flat in the middle - voltage-based SOC is a rough estimate
- While riding: add 2-5 V to the displayed voltage to estimate resting voltage