Pack Voltage
Nominal pack V = series count × nominal cell V
With four 3.6V cells in series, nominal pack voltage is 14.4V. Full-charge reference voltage is calculated separately from the full-charge cell value you enter.
Lithium Ion Battery Calculator
Use this Lithium Ion Battery Calculator to turn individual cell specifications into pack voltage, total capacity, watt-hours, estimated battery current, and runtime. It is built for common cylindrical Li-ion pack planning such as 18650 and 21700 NMC/NCA arrangements, with editable voltage values so your own datasheet stays in control.
Enter cell capacity, series and parallel counts, nominal voltage, full-charge voltage, load power, usable-energy allowance, and delivery efficiency. The tool shows the pack math step by step instead of hiding assumptions inside one runtime number.
CC0 image via Wikimedia Commons
Lithium Ion Battery Calculator Pack Sizing Tool
Calculate a series/parallel lithium-ion pack using the ratings from the exact cells you intend to use. The default 3.6V nominal and 4.2V full-charge values are examples, not universal lithium-ion limits.
Cell-to-Pack Voltage, Capacity, Energy & Runtime
Start with one cell, define the S/P layout, then apply realistic usable-energy and efficiency assumptions.
Planning estimate only: this Lithium Ion Battery Calculator does not certify a pack design or choose a safe charger, BMS, fuse, conductor, busbar, enclosure, thermal strategy, or cell-matching method. For those decisions, review the site's battery BMS guide and lithium battery safety guide, then follow the exact manufacturer documentation for your cells and equipment.
How the Lithium Ion Battery Calculator Works
The tool separates voltage, capacity, energy, current, and runtime so you can see which part of a series/parallel layout changes each result.
A lithium-ion pack is usually described by its series and parallel layout. Series connections add cell voltage, while parallel connections add cell capacity. A 4S2P pack therefore has four cell groups in series and two cells in each parallel group. With 3.6V nominal, 3000mAh cells, that becomes 14.4V nominal and 6000mAh, or 6Ah, before any energy or runtime calculations are applied.
The Lithium Ion Battery Calculator keeps nominal cell voltage editable because “lithium-ion” is a family rather than one universal electrical specification. Molicel's INR-18650-P30B, for example, lists 3.6V nominal and 4.2V charge, while Panasonic's NCR18650BF documentation also lists 3.6V nominal with a 4.20V charging condition. Those are useful examples, not permission to assume the same values for every chemistry or product. Check the lithium battery types guide when comparing chemistries.
Pack Capacity
Pack mAh = cell mAh × parallel count
Two 3000mAh cells in parallel provide 6000mAh at the pack level. Series count does not multiply the Ah rating of one parallel group.
Energy & Runtime
Runtime h = delivered Wh ÷ average load W
Nominal Wh is pack Ah × nominal pack voltage. The calculator then applies usable-energy and delivery-efficiency percentages before dividing by load watts.
Lithium Ion Battery Calculator Worked Examples
These examples use transparent arithmetic so you can reproduce the result by hand and see why series and parallel changes affect different pack ratings.
Start with the default 4S2P example. Each cell is 3000mAh, or 3Ah, at 3.6V nominal. Four groups in series produce 14.4V nominal. Two cells in parallel produce 6Ah. Multiplying 14.4V × 6Ah gives 86.4Wh nominal energy. If the planning model assumes 90% usable energy and 90% delivery efficiency, the modeled load receives 69.984Wh. A steady 25W load therefore runs for about 2.80 hours.
You can cross-check the energy conversion with the site's Watt Hour Calculator or Battery Ah to Wh Calculator. If your load is expressed in current instead of watts, remember that current depends on pack voltage and the efficiency of the delivery path.
| Layout | Nominal voltage | Capacity | Nominal energy | Example load | Estimated runtime* |
|---|---|---|---|---|---|
| 1S1P | 3.6V | 3Ah | 10.8Wh | 5W | 1.85 h |
| 4S2P | 14.4V | 6Ah | 86.4Wh | 25W | 2.80 h |
| 10S3P | 36V | 9Ah | 324Wh | 150W | 1.79 h |
| 13S4P | 46.8V | 12Ah | 561.6Wh | 500W | 0.86 h |
*Runtime values use the preset usable-energy and delivery-efficiency assumptions shown in the calculator. They are not guaranteed field runtimes.
Example: 10S3P 18650 Pack
A 10S3P arrangement using 3000mAh, 3.6V nominal cells contains 30 cells. Nominal pack voltage is 36V and capacity is 9Ah. Energy is 324Wh. With 90% usable energy and 92% delivery efficiency, delivered energy is about 268.27Wh. At a constant 150W load, estimated runtime is about 1.79 hours.
Battery-side current at nominal voltage is roughly 4.53A in this simplified model, or about 1.51A per parallel cell if current shares evenly. That does not replace a cell-current or C-rate check. Use the Battery C Rating Calculator when current capability is central to the design.
Example: 13S4P 48V-Class Pack
Thirteen 3.6V cells in series produce 46.8V nominal, and thirteen 4.2V charge references produce 54.6V at the entered full-charge value. Four 3000mAh cells in parallel produce 12Ah. Nominal energy is 561.6Wh. With 85% usable energy and 90% efficiency, modeled delivered energy is about 429.6Wh.
At a 500W average load, runtime is about 0.86 hours. For a broader system-level comparison that starts from a complete battery rather than individual cells, use the Lithium Battery Calculator.
5 Essential Checks After Using the Lithium Ion Battery Calculator
A useful pack estimate is the beginning of a design review, not the end. The five checks below help turn the Lithium Ion Battery Calculator result into a more realistic engineering conversation. They deliberately separate energy arithmetic from the limits that must come from cell, BMS, charger, conductor, connector, and application documentation.
1. Verify nominal, maximum, and cutoff voltages from the exact cell
Do not identify a cell only by its form factor. Two 18650 cells can have different nominal capacity, current capability, charge limits, discharge limits, temperature ranges, and internal resistance. The calculator can multiply whatever voltage you type, but it cannot decide which value belongs to your product. Confirm the nominal voltage used for energy math and the manufacturer's charging data before you choose a series count. A pack that appears to be in the right “12V,” “36V,” or “48V” class can still be incompatible with the intended charger or electronics if the actual voltage window is wrong. The site's lithium battery voltage chart is useful background, but the individual cell datasheet remains the controlling source.
2. Separate average load from peak and startup current
Runtime is an energy problem, so average watts are the right input for elapsed-time planning. Hardware survival is a current problem, and that can be dominated by brief peaks. Motors, pumps, compressors, power tools, inverters, radios, and digital systems can draw more current during startup or bursts than during steady operation. A pack that has enough watt-hours may still trip a BMS or exceed a cell's safe current limit. Use the calculator's battery-current result as a scale estimate only. Then compare continuous and peak demands against the cell datasheet, BMS specification, connector ratings, and the Battery C Rating Calculator.
3. Check state of charge, reserve, and usable-energy assumptions
The usable-energy percentage is intentionally editable because real systems rarely deliver every nominal watt-hour to the load. Some energy remains below a device or BMS cutoff, some may be left as an operating reserve, and capacity can change with temperature and age. If you enter 100%, the Lithium Ion Battery Calculator shows a mathematical upper-bound comparison, not a recommendation to drain every pack to the lowest possible voltage. If you need to interpret a measured pack voltage or track remaining energy, compare your planning result with the Battery State of Charge Calculator and the exact voltage-versus-state-of-charge information for your chemistry.
4. Model conversion losses and idle consumption separately
The delivery-efficiency input reduces usable battery energy before runtime is calculated. That is convenient for modeling an inverter or DC-DC converter, but one fixed percentage cannot reproduce efficiency across every load level. Some equipment also consumes power even when the useful load is small. An inverter that draws standby watts for many hours can materially shorten runtime, especially with a modest battery. Add meaningful idle power to the average load rather than hiding it inside an unrealistically low efficiency percentage. If you are comparing measured input and output energy, the Lithium Battery Efficiency Calculator provides a focused efficiency check.
5. Confirm BMS, wiring, fusing, thermal design, and pack construction independently
Correct voltage and watt-hour math does not specify a safe pack. A real build needs suitable cell matching, interconnects, mechanical support, insulation, spacing, sensing, balancing, fault protection, thermal management, and charging control. Conductor and fuse choices also depend on fault current, peak load, cable length, voltage drop, insulation temperature rating, terminals, and installation conditions. The Lithium Ion Battery Calculator intentionally stops before those design decisions because they require more information than energy arithmetic provides. Review the site's BMS and safety guides, follow manufacturer instructions, and use qualified engineering review for high-energy or safety-critical systems.
Limitations, Accuracy & Safety Notes
The calculator is intentionally simple enough to audit. That transparency is useful, but it also means several real-world effects remain outside the model.
The Lithium Ion Battery Calculator uses nominal voltage, rated capacity, fixed usable-energy percentage, fixed delivery efficiency, and a steady average load. Real packs operate across a changing voltage curve. Internal resistance, temperature, state of charge, aging, cell matching, interconnect resistance, and BMS cutoffs can all change available energy and current sharing.
Series/parallel arithmetic can also be correct while the physical pack layout is poor. Long current paths, unequal conductor resistance, weak welds, mixed cell groups, or inadequate thermal spacing can produce uneven behavior that no simple energy model can detect. If you are still deciding how many cells belong in series and parallel, compare the result with the Series / Parallel Battery Calculator and then verify that the chosen cells are approved for the intended configuration and current.
Do not use the calculated full-charge pack voltage by itself to configure a charger. Multiplying the entered cell charge voltage by the series count is only arithmetic. Charging also requires the correct method, current limit, termination behavior, monitoring, and temperature rules for the exact cell. Manufacturer documents illustrate why this matters: the Molicel P30B datasheet pairs its 4.2V charge value with current and temperature limits, and the Panasonic NCR18650BF specification lists a charging method and current condition alongside voltage.
Voltage Sag & Cutoff
High load can pull terminal voltage down. A BMS or device may reach its low-voltage cutoff before a nominal-energy calculation suggests the battery is empty.
Temperature & Aging
Available capacity and internal resistance change with temperature, cycle history, calendar aging, and storage. New-cell room-temperature ratings are not lifetime guarantees.
Current Sharing
Parallel cells do not always share current perfectly. Matching, connection resistance, layout, temperature, and cell condition influence branch current.
Application boundary: use this page for planning and comparison, not for approving a medical, aviation, traction, emergency, or other critical-energy system. If you are working with pouch-style high-discharge packs, the LiPo Battery Calculator may be more relevant. For LiFePO4-specific planning, use the LiFePO4 Battery Calculator rather than carrying conventional NMC/NCA voltage assumptions across chemistries.
Lithium Ion Battery Calculator FAQ
Short answers to common questions about lithium-ion pack voltage, capacity, watt-hours, runtime, and series/parallel layout.
It calculates nominal pack voltage, entered full-charge pack voltage, total mAh/Ah, nominal Wh, modeled delivered Wh, total cell count, estimated runtime, approximate battery current, and approximate current per parallel cell. It does not determine whether a pack is safe to build.
Enter the nominal voltage printed in the exact manufacturer datasheet. Many cylindrical NMC/NCA cells use 3.6V, while other products may use 3.7V or another nominal value. The calculator keeps this field editable so the math follows your source data.
No. 4.2V is common for many conventional lithium-ion cells, but chemistries and high-voltage variants differ. Use the cell manufacturer's specified charging data and never increase charging voltage just because a calculator accepts a higher input.
Series wiring adds cell voltage while the Ah capacity of one parallel group stays the same. Four 3Ah cells in pure series remain 3Ah as a pack, while nominal pack voltage becomes four times the single-cell nominal voltage.
Parallel wiring adds mAh/Ah capacity while nominal voltage stays at the cell voltage. Two identical 3000mAh cells in parallel are modeled as 6000mAh. Real packs still require compatible cells, suitable interconnects, and safe current sharing.
The calculator applies your usable-energy percentage and delivery-efficiency percentage after calculating nominal Wh. That makes reserve and conversion losses visible instead of pretending every rated watt-hour reaches the load.
No. Average current is only one input. BMS, fuse, wire, busbar, connector, and protection decisions also depend on surge current, short-circuit behavior, voltage, cell limits, temperature, cable length, installation method, and applicable standards.
The basic series/parallel energy arithmetic works if you replace every voltage and usage assumption with the correct LiFePO4 values, but this page is written around conventional Li-ion NMC/NCA examples. A chemistry-specific calculator is the safer starting point.
Build the Energy Model Before You Build the Pack
Use the Lithium Ion Battery Calculator with the exact cell datasheet, compare more than one series/parallel layout, and test realistic load, reserve, and efficiency assumptions. Then verify charging, BMS current, temperature, wiring, and protection requirements before selecting hardware.
Recalculate a Battery Pack