Lithium Ion Battery Capacity Calculator

Calculator Input

Cell capacity

Enter single cell capacity in mAh.

Nominal cell voltage

Common lithium ion value is 3.6 V or 3.7 V.

Full charge cell voltage

Many lithium ion cells use 4.2 V.

Cutoff cell voltage

Use the datasheet cutoff value.

Series cells

Series cells raise pack voltage.

Parallel cells

Parallel cells raise amp hour capacity.

Depth of discharge

Use percent of capacity allowed for discharge.

System efficiency

Include inverter, wiring, and converter losses.

Aging capacity remaining

Use lower values for older packs.

Temperature derating

Cold operation often reduces usable capacity.

Reserve margin

Keep spare capacity for safer planning.

Load type

Choose how the load is entered.

Load value

Enter watts or amps based on selected load type.

Desired runtime

Used for recommended parallel group sizing.

Maximum continuous C-rate

Use the cell datasheet continuous rating.

BMS standby current

Enter standby drain in mA.

Self discharge per month

Enter monthly percentage loss.

Storage time

Use zero for immediate use.

Single cell weight

Used for estimated cell weight.

Example Data Table

Pack	Cell Capacity	Series	Parallel	Nominal Voltage	Pack Ah	Gross Wh	Typical Use
3S2P	3000 mAh	3	2	11.1 V	6 Ah	66.6 Wh	Small electronics
4S3P	3000 mAh	4	3	14.8 V	9 Ah	133.2 Wh	Portable tools
7S4P	3500 mAh	7	4	25.9 V	14 Ah	362.6 Wh	E-bike modules
13S5P	3000 mAh	13	5	48.1 V	15 Ah	721.5 Wh	Large battery packs

Formula Used

Cell amp hours = Cell capacity mAh ÷ 1000

Pack amp hours = Cell Ah × Parallel cells

Nominal pack voltage = Nominal cell voltage × Series cells

Full charge pack voltage = Full charge cell voltage × Series cells

Cutoff pack voltage = Cutoff cell voltage × Series cells

Gross energy = Pack Ah × Nominal pack voltage

Usable factor = DoD × Efficiency × Aging × Temperature × Reserve factor

Usable Wh = Gross Wh × Usable factor

Available Wh = Usable Wh − Self discharge loss − BMS drain

Runtime hours = Available Wh ÷ Load watts

Load C-rate = Load current ÷ Pack Ah

Recommended parallel groups = Required pack Ah ÷ Cell Ah, rounded up

How to Use This Calculator

Enter the rated capacity of one lithium ion cell.
Add nominal, full charge, and cutoff cell voltages.
Enter the number of series and parallel cells.
Add discharge depth, efficiency, aging, temperature, and reserve values.
Choose whether the load is entered in watts or amps.
Enter desired runtime for recommended parallel sizing.
Add storage losses if the pack will sit unused.
Press the calculate button and review the result above the form.
Use CSV or PDF export to save the calculation.

Why Capacity Matters

A lithium ion battery pack is more than a group of cells. It is a planned energy source. Capacity tells how much charge the pack can store. Energy tells how much work it can support. Both values matter when you size a device, inverter, tool, robot, light, or backup system.

Pack Structure

This calculator separates cell data from pack data. You enter cell capacity, cell voltage, series count, and parallel count. The tool then estimates amp hours, watt hours, usable energy, and runtime. It also includes losses. Real packs rarely deliver the full label rating. Discharge depth, converter efficiency, aging, temperature, reserve margin, self discharge, and BMS drain all reduce available energy.

Series and Parallel Logic

Series cells raise voltage. Parallel cells raise amp hour capacity. A 4S3P pack has four cells in series and three in parallel. The total cell count is twelve. The pack voltage follows the series count. The pack amp hour rating follows the parallel count. Energy is found by multiplying pack voltage by pack amp hours.

Runtime and Load Checks

The load input can be entered as watts or amps. A watt load is direct. An amp load is converted using pack voltage. The calculator then estimates runtime from usable watt hours. It also checks the load current against the selected C rate. This helps show whether the pack is comfortably sized or under stress.

Safe Design Notes

Use conservative values for important projects. Leave a reserve. Reduce capacity for cold operation. Add aging margin if the pack must still work after many cycles. Use the result as an engineering estimate, not as a safety certificate. Final designs should follow cell datasheets, protection circuit limits, wiring ratings, thermal testing, and local electrical rules.

Better Pack Planning

A good pack design balances energy, power, safety, cost, and service life. Higher capacity can extend runtime. Higher voltage can reduce current. More parallel cells can reduce stress. Better margins can improve reliability. This calculator brings these choices together in one clear result.

Testing Notes

Documentation matters. Record the cell model, temperature, load profile, and cutoff voltage. Compare calculated runtime with a measured test. Update assumptions after testing. Clear records help maintenance teams review failures early.

FAQs

What is lithium ion battery capacity?

It is the amount of charge a cell or pack can store. It is often shown in mAh or Ah. For energy planning, watt hours are more useful because they include voltage.

How do series cells affect capacity?

Series cells increase voltage. They do not increase amp hour capacity. A 4S pack has four times the cell voltage, but the same Ah as one parallel group.

How do parallel cells affect capacity?

Parallel cells increase amp hour capacity. Three 3000 mAh cells in parallel give 9000 mAh. The pack voltage stays based on the series count.

Why is usable energy lower than gross energy?

Usable energy is reduced by discharge limits, efficiency, aging, temperature, and reserve margin. This gives a more realistic runtime estimate than the label rating alone.

What does C-rate mean?

C-rate compares load current with pack capacity. A 10 Ah pack at 1C can deliver about 10 A. Always check the cell datasheet and BMS limit.

Can I use this for an e-bike pack?

Yes, it can estimate voltage, capacity, energy, and runtime. Still, final e-bike packs need correct BMS selection, fusing, wiring, enclosure design, and thermal testing.

Why add reserve capacity?

Reserve capacity helps avoid deep discharge and unexpected shutdown. It also improves planning when loads change, temperatures drop, or the pack ages over time.

Is this a safety certification tool?

No. It is an estimate for planning. Lithium ion packs require datasheet checks, protection circuits, safe charging, thermal review, short circuit protection, and local code compliance.