Lithium Cell Calculator

Inputs

Example data table

Formula used

How to use this calculator

1. Select a preset, or choose Custom for manual entry.
2. Enter cell voltage limits and rated capacity in mAh.
3. Set series and parallel counts for your pack layout.
4. Choose Current mode for a fixed draw, or Power mode for devices.
5. Adjust usable depth-of-discharge and efficiencies for realism.
6. Add resistance and derate factors to estimate sag and heating.
7. Press Calculate to view results above the form instantly.
8. Use CSV or PDF buttons to export your latest report.

Lithium cell sizing for practical packs

Lithium cells are characterized by voltage limits, rated capacity, and internal resistance. This calculator combines those with series-parallel layout to estimate pack energy, usable energy, runtime, voltage sag, heating loss, and a simple CC/CV charge time.

1) Voltage windows and chemistry choices

Chemistry sets the operating window. A typical NMC or NCA cell is near 3.7 V nominal, 4.2 V maximum, and about 3.0 V minimum. LFP is often 3.2 V nominal with a 3.65 V top. LTO is lower, near 2.4 V nominal and 2.7 V maximum.

2) Series and parallel scaling

Series increases voltage; parallel increases capacity. The model uses Vpack = Vcell × Ns and Cpack(Ah) = (Ccell(mAh) ÷ 1000) × Np. A 3s2p pack keeps the 3s voltage but doubles amp-hours versus 3s1p, which usually improves runtime.

3) Usable capacity, DoD, and efficiency

Runtime depends on usable capacity, not nameplate capacity. Depth-of-discharge limits protect cycle life, while coulombic efficiency captures charge/discharge losses. A derate factor can represent cold conditions or aging. The calculator multiplies these factors to compute usable amp-hours and usable watt-hours for a realistic planning baseline.

4) Current mode versus power mode

Some loads draw near-constant current, but regulated electronics can behave like constant power. In power mode, load power is converted to an equivalent input current using nominal pack voltage and converter efficiency. Current can rise as voltage falls, shortening runtime compared with a fixed-current assumption.

5) Voltage sag and internal resistance

Internal resistance causes voltage sag and heat. Pack resistance is approximated as (Rcell × Ns) ÷ Np, and sag is ΔV = I × Rpack. Increasing parallel strings reduces sag, keeps under-load voltage higher, and helps avoid early cutoff. High sag also signals higher stress on wiring, connectors, and cells.

6) C-rate and heating loss screening

C-rate equals discharge current divided by pack capacity in amp-hours. Higher C-rates usually require high-power cells and better thermal management. Resistive loss is estimated as Ploss = I² × Rpack, which helps screen designs for hot connectors and efficiency loss. Large loss values suggest lowering current or increasing Np.

7) Cutoff setting and runtime realism

Cutoff voltage prevents over-discharge and protects safety margins. Conservative cutoffs reduce usable energy, while aggressive cutoffs can accelerate degradation. Because sag lowers voltage during peaks, cutoffs may be reached earlier than expected. Using realistic minimum voltage and resistance values produces runtime estimates that align better with field behavior.

8) Charging time and documentation workflows

Most lithium charging uses a constant-current stage followed by a constant-voltage taper. The calculator estimates charge time using charger current, an adjustable CC share, and a taper approximation. While simplified, it supports charger sizing and downtime planning. CSV and PDF exports help standardize lab notes and design reviews.

FAQs

1) Why does runtime change when I reduce DoD?

DoD limits the portion of capacity you actually use. Lower DoD preserves cycle life and keeps voltage more stable, but it reduces usable amp-hours, so the calculated runtime decreases accordingly.

2) What internal resistance should I enter?

Use a value from the datasheet or measured at your expected temperature and state-of-charge. Typical cells can range from about 15 to 80 mΩ, depending on chemistry, size, and age.

3) Why does my pack hit cutoff early at high current?

High current increases voltage sag across internal resistance. The under-load voltage can fall to the cutoff threshold sooner, even if capacity remains. Increasing Np or lowering current usually improves this.

4) When should I use power mode?

Use power mode for devices that regulate their output and keep power roughly constant, such as DC-DC supplied electronics. The calculator converts power to input current using nominal pack voltage and efficiency.

5) Is the charge time estimate exact?

No. It is a planning estimate based on a CC share and a simplified taper current. Real charge time depends on charger limits, temperature, cell balance, and the end-of-charge current threshold.

6) How do I model cold-weather performance?

Cold typically reduces available capacity and increases resistance. Use the capacity derate to reflect lost capacity and consider increasing resistance to simulate additional sag under load.

7) Can I use this for large battery systems?

Yes, for early sizing and comparisons. For large systems, validate with manufacturer data, BMS limits, wire resistance, and safety standards. Use the warnings as prompts for deeper checks.