Battery Capacity Calculator

First-pass runtime arithmetic. Not a discharge model, and not a battery safety assessment. Capacity divided by average current, adjusted for a regulator and a reserve fraction. Real packs sag under load, age, derate with temperature, and fail in ways nothing on this page models, including thermally. Estimates only, used at your own risk. Open for the full scope limits.

What this tool actually does

Runtime estimation and first-pass power budgeting only. It produces estimates from the values you enter and a small set of simplifying assumptions. It is not a replacement for discharge testing, regulator thermal review, or oscilloscope and current-probe validation of transient behaviour.

Usable fraction is a design reserve you choose, not a chemistry model. Nothing here knows where your cell's knee is.
Fixed-efficiency regulator mode is a simplification: a real converter's efficiency moves with input voltage, load current and temperature, and is worst exactly where a low battery leaves it.
LDO mode treats input current as roughly equal to load current, which is the point of the mode and also its whole error term.
Peak current and current limits are checks against the average-current estimate, not a transient analysis.

Not modelled at all

Detailed discharge curves and voltage sag under load.
Cell aging, cycle life and capacity fade.
Temperature derating and self-discharge.
Regulator transient response and inrush behaviour.
Parasitic load, charger losses and BMS overhead.
Cell-to-cell variation, manufacturing tolerance and ambient conditions.
The safety, fire or thermal-runaway behaviour of any chemistry.

Battery and regulator datasheets, cell-to-cell variation, real load profiles and ambient conditions can shift actual runtime substantially in either direction. A runtime that comes out comfortable here can still be short in the field, and a pack that runs long enough can still be unsafe.

Never use this for

Battery safety, thermal runaway or fire risk assessment of any kind.
Cell or pack certification and transport qualification, including UN 38.3, IEC 62133, UL 2054 and UL 1642.
Flight endurance, range or reserve calculations for any aircraft or unmanned vehicle.
Medical device battery life or end-of-service indication under IEC 60601.
Emergency lighting, alarm or life-safety backup runtime under NFPA 111, NFPA 72 or an equivalent code.
Production sign-off, warranty terms, or any contractual or advertised runtime claim.

Validate before you build

Confirm every design with measured discharge data on the actual cell, calibrated current probes on the actual load, thermal testing at the temperature extremes you expect, and the chemistry-specific safety guidance from the cell vendor, before committing to a build, a flight envelope, a deployment or a production run. For lithium chemistries in a sealed enclosure or anywhere near people, involve an engineer who does battery packs for a living, and design the protection around the failure rather than the runtime.

No warranty. No liability. You assume all risk. This page and its estimates are provided as-is and as-available, without warranty of any kind, express or implied, including accuracy, completeness, timeliness, reliability, fitness for a particular purpose, merchantability, engineering or professional suitability, regulatory or certification acceptability, or non-infringement. To the maximum extent permitted by law, the author accepts no liability for any loss, injury, death, damage, cost or claim arising from use of or reliance on this page.

Independent verification required. Verify every figure against vendor datasheets, measured behaviour and qualified engineering judgement before acting on it. The datasheet, the governing standard and professional judgement control; where this page disagrees with them, this page is wrong.

Consequential damages excluded. To the maximum extent permitted by law, in no event will the author or maintainers be liable for any direct, indirect, incidental, special, exemplary, punitive or consequential damages, including fire, equipment failure, mission failure, downtime, regulatory exposure, certification outcome, business decision, or loss of use, data, profits or business interruption, however caused and on any theory of liability, whether in contract, strict liability or tort including negligence, arising out of or in connection with this page, even if advised of the possibility of such damage.

Indemnity. By using this page you agree to indemnify and hold harmless the author and maintainers against any claim, demand, loss or expense arising out of your use of it or of anything derived from it.

Severability. If any part of this notice is held unenforceable, the remainder continues to apply, and the unenforceable part is limited to the least extent necessary rather than struck out.

Battery And Load Estimator

Estimate runtime from nominal pack assumptions, load behavior, regulator losses, and optional burst-current limits. This is an engineering sizing tool, not a chemistry-resolved discharge simulator.

Battery Model

Battery model mode i Use direct pack entry when the pack rating is already known. Use cell pack derivation when you want the tool to build pack voltage and capacity from cells in series and parallel. Chemistry preset i This seeds a nominal cell voltage and a conservative usable fraction. It is a design starting point, not a full chemistry model.

Chemistry presets seed typical cell voltage and a conservative usable fraction. Treat them as design defaults, not hard battery guarantees.

Pack capacity

Pack nominal voltage

Usable battery fraction (%) i This is the share of nominal battery energy you expect to use in practice after leaving reserve for cutoff voltage, aging, burst sag, and conservative design margin. Load/system voltage i Use the voltage seen by the device after regulation. Leave it blank when the load runs directly from the battery pack voltage.

Regulator And Peak Limits

Regulator topology i Direct feed means no converter stage. Fixed means use the entered efficiency like a buck or boost approximation. LDO treats battery current as roughly equal to load current plus quiescent current. Regulator efficiency (%) i This is the fraction of input power that reaches the load. A 90% efficient regulator wastes about 10% of battery-side power as heat and conversion loss. Peak regulator efficiency (%) i Optional. Converters often run lower efficiency during burst load than at their rated average efficiency. When provided, this value is used for peak battery current only. Leave blank to reuse the average efficiency. Regulator quiescent current i This is the current the regulator itself consumes just to stay alive, even when the load is very small or asleep. It can dominate long-sleep battery life in low-power embedded systems.

Peak load current (optional) i Use this for startup spikes, radio bursts, motor inrush, or other short events that average current hides. It is used for feasibility checks, not average runtime.

Regulator output current limit i Enter the maximum current the regulator can safely supply to the load. This helps catch designs where runtime looks fine but burst current may still brown out the system.

Battery-side current limit i Use this when the cell, pack, holder, protection board, or wiring has a known current ceiling. This is checked against the estimated battery-side peak current.

Direct feed assumes no converter stage. Fixed-efficiency regulators use the entered efficiency. LDO mode treats battery current as approximately load current plus quiescent current.

Load Estimate Mode

Embedded preset i Presets load a realistic starting profile for common embedded workloads. They are meant to save setup time, not replace measured current data.

Preset actions

Preset mode seeds a realistic starting point for embedded workloads without locking the fields.

Mode i Use constant current when a measured average already exists, multi-state profile when the device cycles through wake and sleep states, and average power when the load is specified in watts.

Constant Current Load

Average load current

Use this when average current is already measured or known from the device data sheet or field logging.

Scenario Library

Scenario name Saved scenarios

Saved scenarios stay in local storage on this browser. Use a saved scenario as the baseline to compare a current design against a past case.

Estimate Results

Estimates only, used at your own risk. These figures come from nominal pack values and average currents. Voltage sag, cell aging, temperature derating, self-discharge, inrush and BMS overhead are not modelled, and nothing here assesses fire or thermal-runaway risk. Confirm with measured discharge data and a calibrated current probe before you commit to a build, a flight envelope or a deployment.

Estimated Runtime

Runtime appears here after calculation.

Usable Battery Energy

Derived from pack voltage, capacity, and usable fraction.

Average Load Current

Device-side average current at the load voltage.

Average Load Power

Average device-side power used for runtime conversion.

Battery-Side Avg Current

Equivalent battery current after conversion losses and quiescent draw.

Peak Battery Current

Peak battery-side current used for feasibility checks.

Feasibility Check

Burst-current warnings and entered limits appear here.

Battery Model Summary

Pack model and regulator assumptions appear here.

Baseline Compare

No baseline

Capture a baseline, then change the design to compare runtime and peak-current impact.

Runtime Drivers

Contribution breakdown appears here after calculation.

How it works

Usable battery energy = pack capacity x nominal voltage x usable fraction. Runtime = usable battery energy / battery-side average power.

This estimate assumes nominal voltage and average load behavior. It does not model detailed discharge curves, transient voltage sag, temperature effects, or cell aging.

How To Use This

Choose the battery model: use direct pack entry when the pack is already known, or cell pack derivation when you are still sizing series and parallel count.
Set the conversion path: choose direct feed for battery-to-load systems, fixed for buck or boost style regulators, and LDO when the load current roughly matches input current.
Pick the load mode: constant current for measured averages, multi-state profile for wake/sleep devices, and average power when the load is specified in watts.
Add limits if bursts matter: peak load current and current limits help catch regulator or battery sizing issues that average-current estimates miss.

Direct pack entry: Use when you already know pack capacity and nominal voltage.
Cell pack derivation: Use when sizing a pack from cell voltage, capacity, and series/parallel count.
Multi-state profile: Use for embedded systems with wake, transmit, sample, idle, and sleep states.
Embedded presets: Use them as realistic starting points, then replace the defaults with measured current data whenever possible.

Important Limits

Usable fraction: This is a design reserve, not a chemistry model.
Regulator topology: Fixed-efficiency mode is a simplification. LDO mode treats input current as roughly equal to load current.
Burst warnings: Enter peak current and current limits if startup, radio, or transmit spikes matter.

Scope

This tool is intended for runtime estimation and first-pass power budgeting. It is not a replacement for discharge testing, regulator thermal review, or oscilloscope/current-probe validation of transient behavior.