Battery discharge losses calculator
This calculator estimates voltage sag, resistive heating, energy loss, usable energy, effective capacity, and remaining charge for constant-current battery discharge studies.
Enter battery and discharge inputs
The form uses a three-column layout on large screens, two columns on smaller screens, and one column on mobile devices.
Example data table
These examples illustrate how rising current and resistance increase voltage sag and heating losses.
| Chemistry | Pack Voltage | Current | Capacity | Total Resistance | Time | Voltage Drop | Loss Power | Loss Energy | Efficiency |
|---|---|---|---|---|---|---|---|---|---|
| Lead-acid | 12.00 V | 20 A | 100 Ah | 0.022 Ω | 2.0 h | 0.44 V | 8.80 W | 17.60 Wh | 96.33% |
| Li-ion | 14.80 V | 30 A | 50 Ah | 0.015 Ω | 1.5 h | 0.45 V | 13.50 W | 20.25 Wh | 96.96% |
| LiFePO4 | 25.60 V | 60 A | 80 Ah | 0.010 Ω | 1.0 h | 0.60 V | 36.00 W | 36.00 Wh | 97.66% |
Formula used
Battery discharge loss is modeled as resistive heating plus voltage sag under load. The calculator also adjusts usable capacity with Peukert behavior and manual derating.
Pack Voltage = Cell Count × Nominal Cell Voltage Total Resistance = (Battery Resistance + Cable Resistance) ÷ 1000 Voltage Drop = Discharge Current × Total Resistance Loaded Voltage = Pack Voltage − Voltage Drop Loss Power = Discharge Current² × Total Resistance Loss Energy = Loss Power × Discharge Duration Reference Current = Rated Capacity ÷ Rated Hours Effective Capacity = Rated Capacity × (Reference Current ÷ Discharge Current)^(Peukert − 1) × Derating Factor Remaining Capacity = Available Capacity − Consumed CapacityThese equations are best suited to screening studies, comparisons, and early design checks. Measured cell impedance, temperature behavior, and voltage curves improve accuracy further.
How to use this calculator
- Choose the battery chemistry or keep custom settings.
- Enter cell count and nominal cell voltage for the pack.
- Provide battery and cable resistance in milliohms.
- Enter discharge current, rated capacity, and rated hours.
- Add the expected discharge duration in hours.
- Set the initial state of charge and reserve target.
- Adjust the Peukert exponent and derating if needed.
- Press Calculate losses to show the result above the form.
- Use the export buttons to save the current result as CSV or PDF.
Frequently asked questions
1. What does this calculator measure?
It estimates voltage drop, resistive heating, useful energy, loss percentage, effective capacity, remaining charge, and runtime under constant-current discharge conditions.
2. Why does current affect losses so strongly?
Resistive heating follows the current-squared rule. Doubling discharge current can roughly quadruple heat loss when total resistance stays unchanged.
3. What is the Peukert exponent doing here?
It reduces effective capacity at higher discharge current. This matters most for lead-acid batteries and less for many lithium chemistries.
4. Should I include cable resistance?
Yes. Cable and connector resistance also waste power and increase voltage sag, especially in high-current packs, long runs, and compact enclosures.
5. What does capacity derating represent?
It lets you manually reduce usable capacity for temperature effects, aging, safety margin, or other application-specific limits not modeled directly.
6. Why is loaded voltage lower than nominal voltage?
Current flowing through internal and cable resistance creates a voltage drop. The load therefore sees a smaller terminal voltage during discharge.
7. Is this suitable for detailed battery validation?
It is best for engineering estimates and comparisons. Detailed validation should also use measured impedance, temperature data, and full discharge curves.
8. When should I use the reserve state of charge?
Use it when your design must preserve emergency energy, protect cycle life, or avoid deep discharge below a chosen operating threshold.