Builder tools

Battery pack tools that are easier to read, choose, and trust.

Start with the pack builder, then check conductors, heat, runtime, and charging. The tools are grouped by job so a beginner can find the right calculator, while advanced builders still get the full numbers.

Battery pack workbench with high-current wiring and measurement probes

1 Design the pack Choose cell, S/P layout, pack current, capacity, weight, sag, heat. 2 Size the cables Check AWG, voltage drop, cable heat, and realistic battery-wire limits. 3 Check strip and busbars Compare copper, pure nickel, plated steel, stainless, and parallel paths. 4 Plan real use Estimate runtime, range, charge time, and I2R heat before testing.

Important limitation

There is no universal ampacity number for nickel or copper strip.

Real current capacity changes with strip length, weld quality, airflow, insulation, cell temperature, contact resistance, series/parallel layout, duty cycle, and allowable temperature rise. This page calculates resistance and heat from geometry, then estimates current from a heat target you choose.

Recommended order Pack builder AWG cable Strip estimator Heat check

Tool 1

AWG cable voltage drop

Cable

Source voltageBattery or pack voltage at the start of the cable. Use nominal voltage for planning, or live voltage for a real condition. Load current ACurrent flowing through the cable. Use the peak or continuous amps you expect the pack to supply. One-way lengthLength of one cable from battery to load. Round-trip is handled by the Path option below. Length unitChoose whether the one-way cable length is in feet or meters. Wire sizeAWG gauge of each conductor. Smaller AWG number means larger cable and lower resistance. Conductor materialMaterial inside the cable. Copper is the common baseline; aluminum needs more area for the same drop. Battery cable profileApplies practical temperature assumptions for battery leads, enclosed harnesses, or burst-only wiring. Parallel conductors per sideNumber of identical positive cables and identical negative cables sharing current. Example: dual positives and dual negatives = 2. PathUse round trip for normal battery wiring. Single conductor is for checking just one cable segment. Conductor temperature CEstimated metal temperature. Hotter metal has higher resistance, so voltage drop increases. Insulation rating CMaximum temperature rating printed for the cable insulation, not a target operating temperature. Ambient / pack temp CAir or enclosure temperature around the cable. Hot packs leave less cooling headroom. Heat target W/m/conductorHow much heat each physical conductor may dissipate per meter. Lower is more conservative. Target max drop %Maximum voltage loss you are willing to accept. 2-3% is a common planning range.

Resistance--total path

Voltage drop----

Heat loss--I2R in cable

Nearest target size----

Heat per meter--per physical conductor

Thermal current est.--from W/m target

Drop current--at target drop

Temp headroom--battery cable mode

Waiting for inputs

Tool 2

18650 / 21700 strip and busbar estimator

Strip

Preset sizeCommon strip dimensions. Choose a preset or use Manual and enter width/thickness yourself. MaterialStrip or busbar material. Copper carries much more current than nickel-plated steel at the same size. Width mmFlat strip width across the cell top, measured in millimeters. Thickness mmMetal thickness. Doubling thickness roughly doubles cross-section and lowers resistance. Current path length mmApproximate distance current travels through the strip between welds or connection points. Current ATotal current through this strip path. If several strips share current, enter total current and set parallels. Parallel stripsNumber of identical strip paths sharing the current. More parallels lower heat and voltage drop. Heat target W/cmAllowed heat per centimeter of strip. Use gentle/moderate for conservative builds.

Area--cross section

Resistance--effective

Voltage drop--at entered current

Heat----

Est. current--at heat target

Parallels needed--for heat target

Waiting for inputs

Tool 3

Pack builder and C-rate calculator

Database pack

Series and parallel battery pack builder visual

Cell from BatteryRealData database Choose a public BRD cell to auto-fill datasheet/test values, or select Manual to enter your own cell.

Manual cell Choose a tested/datasheet cell or enter your own values below.

Series cellsCells in series. This raises voltage. Example: 13S lithium-ion is about 48V nominal. Parallel cellsCells in parallel per series group. This raises Ah capacity and current sharing. Pack load current ATotal current demanded from the full pack by the controller/load. Optional load power WEnter watts if you know power instead of amps. Leave 0 to use Pack load current. Pulse duration sLength of a high-current burst. Used for pulse energy and pulse-limit comparisons. BMS / controller limit ACurrent limit from your BMS or controller. Leave 0 if you want the cell limit only. Cell capacity mAhRated capacity of one cell. Pack Ah = cell Ah multiplied by parallel count. Nominal V per cellAverage voltage used for energy estimates. Most Li-ion cells are around 3.6-3.7V nominal. Full charge V per cellVoltage at 100% charge for one cell. Standard Li-ion is usually 4.20V unless datasheet says otherwise. Cutoff V per cellLowest cell voltage you plan to allow under load. This affects usable voltage range. Max continuous A / cellDatasheet continuous discharge rating for one cell, or your chosen conservative limit. Max pulse A / cellShort-duration current rating for one cell. Leave 0 if the datasheet does not give a pulse value. Cell IR mOhmInternal resistance of one cell in milliohms. Higher IR means more voltage sag and heat. Cell weight gWeight of one cell. Pack weight shown is cells only, not nickel, busbars, case, or BMS. Max charge A / cellMaximum recommended charge current per cell. Leave 0 if unknown. BRD test referenceWarns when your pack plan asks for more current than BatteryRealData has tested for that cell.

Configuration----

Voltage----

Capacity----

Energy--nominal estimate

Continuous limit--datasheet/manual

Pulse limit--datasheet/manual

Per-cell load--parallel shared

C rating--load / cell capacity

Runtime--at entered load

Voltage sag--from cell IR

Cell heat--I2R estimate

Charge--max charge estimate

Weight--cells only

Pulse charge moved--during entered pulse

BRD tested up to--public catalog

Build note--fusing, busbars, BMS, cooling not included

Waiting for inputs

Tool 4

Runtime, range, and required battery size

Runtime

Pack voltage VNominal pack voltage used for watt-hour estimates. Pack capacity AhTotal pack amp-hours. For a pack, this is one cell Ah times parallel count. Usable capacity %Percent of pack capacity you plan to actually use before stopping. System efficiency %Efficiency after controller, motor, wiring, drivetrain, and other losses. Average load WAverage power draw during use, not peak power. Vehicle Wh / mileEnergy needed to travel one mile. Lower means better range. Target distance milesDesired range. The tool calculates the battery size needed for this distance. Reserve %Capacity you want left unused for safety margin, voltage sag, aging, or bad weather.

Nominal energy--V x Ah

Usable energy--after usable %, efficiency, reserve

Runtime--at average load

Range--based on Wh/mile

Required energy--for target distance

Required Ah--at entered pack voltage

Waiting for inputs

Tool 5

Battery charge-time planner

Charge

Pack capacity AhTotal pack capacity in amp-hours. Start SOC %Approximate state of charge before charging starts. Target SOC %Charge level you want to reach. Use less than 100% if you normally partial-charge. Charger current AOutput current from the charger into the pack. Charge efficiency %Allowance for charging losses. 90-95% is a common planning range. CV taper extra %Extra time for the constant-voltage taper near full charge. Series countNumber of cell groups in series. Used to estimate full charger voltage and power. Charge V per cellFull-charge voltage for one cell, usually 4.20V for standard Li-ion.

Ah returned--battery side

Bulk time--before taper allowance

Estimated total--with efficiency + CV taper

Charger power--near full voltage

Waiting for inputs

Tool 6

I2R heat and sag helper

Loss

Resistance mOhmResistance of the part or path being checked, in milliohms. Current ACurrent flowing through that resistance. Duration sHow long the current pulse lasts. Used to calculate total heat energy. Parts in parallelNumber of identical parts sharing current. Effective resistance drops as parallels increase.

Effective R--parallel adjusted

Voltage sag--I x R

Heat power--I2R

Heat energy--over duration

Waiting for inputs

Quick reference

Common 18650 / 21700 strip sizes by material.

The table estimates current from the selected heat target and path length. It is not a fuse rating or a pack approval.

MaterialMaterial used for the quick-reference table. Changing this updates every strip estimate. Path length mmDistance current travels through the strip in the table assumptions. Heat target W/cmTarget heat level used to estimate current for each common strip size.

Size	Area	Resistance	Est. A @ heat target	A @ 50 mV drop	Notes

How to read the strip estimator

Heat per centimeter is the main comparison number.

Two strips can carry the same current but waste very different power. Higher resistance materials turn more current into heat, especially at narrow/thin dimensions. Use more parallel strip, shorter paths, or better material when heat per centimeter climbs.

Reference basis

Calculations use published resistivity values and geometry.

Copper values are based on NIST copper wire data at 20 C. AWG geometry follows the standard logarithmic AWG diameter formula. Stainless 304 and other material values are typical planning values and can vary by alloy, plating, work hardening, and supplier. The cable tool is for short battery leads and pack wiring. It does not use residential or commercial building-code ampacity tables.

NIST copper wire tables NBS 304 stainless report AWG formula reference