Planning battery runtime and spare packs

Scope. This method estimates energy and time from entered data. It does not decide whether a battery, adaptor, charger or tool combination is permitted by the manufacturer.

Start with watt-hours, not the badge alone

Amp-hours (Ah) describe charge capacity, not energy by themselves. For one declared pack/configuration, nominal energy is Wh = V × Ah. [ARITH] A 5 Ah pack at 18 V is therefore 90 Wh nominal; a 5 Ah figure at a different voltage is not the same energy.

Voltage and Ah must describe the same configuration. Where a tool platform uses two packs in series or parallel, do not add values until the manufacturer’s system arrangement is understood. The label, manual and compatibility documentation govern.

The contract’s Makita watt-hour and runtime pages could not be verified through trusted TLS in the research environment on 24 July 2026. Their manufacturer-specific claims are therefore withheld here rather than presented from search snippets. The elementary V × Ah arithmetic does not depend on that unresolved source. See the transparent gap in the source record.

Separate nominal from usable energy

A pack’s label is not a promise that every nominal watt-hour reaches the tool as useful work. Protection cut-off, voltage sag, application load, temperature, pack age/condition and conversion losses can reduce usable energy.

The Job Lab asks for a usable-energy percentage rather than hiding one. usable Wh = nominal Wh × usable % ÷ 100. The percentage is an entered planning assumption, not law, a manufacturer guarantee or a measured state-of-health result. [POLICY]

Use a measured or manufacturer-supported basis when available and state it. If none exists, run more than one explicit scenario rather than treating 80% as a universal truth. Keep actual observed runtime alongside the estimate so future planning can use evidence from the same tool, task and conditions.

Scenario ranges are more honest than extra decimals. For example, compare 60%, 70% and 80% usable energy while holding the declared load constant. The spread shows which assumption drives pack planning and what needs measuring.

Separate trigger time from elapsed time

On-load power is the electrical watts drawn while the tool performs the modelled operation. Mechanical output and a marketing power comparison are not substitutes. If watts vary materially through the task, a single value may be too crude and should be replaced with measured energy or a bounded range.

Trigger-on minutes are usable Wh ÷ tool W × 60. If the tool is on-load for only part of elapsed job time, average power in this simplified model is tool W × duty %, and elapsed minutes are usable Wh ÷ average W × 60. Trigger minutes equal elapsed minutes × duty percentage.

This assumes zero off-cycle draw. A fan, electronics, light, vacuum, heater or standby system that continues drawing between trigger events needs a separate energy row or a measured average. Duty cycle is about time, not a derating factor for the tool’s on-load watts.

A duty estimate should come from a representative work cycle: observe trigger-on and total elapsed time across several cuts or holes, then state the sample. Do not copy a percentage from a different operator, material or accessory without marking that limitation.

Worked 18 V, 5 Ah example

One declared pack is 18 V and 5 Ah: 18 × 5 = 90 Wh nominal. An entered usable factor of 80% gives 90 × .8 = 72 Wh. The tool is entered at 360 W on load.

The result says 12 minutes of trigger time spread across an estimated 30 elapsed minutes. It does not promise the pack will deliver that time, and it does not say how many packs to buy or hire.

Plan packs and charging without false precision

Turn runtime into a shift plan only with the actual sequence. Record required trigger minutes, breaks, pack-change time, charger turnaround, number of charger bays, available supply, cooling delays and whether charging overlaps work. A spare protects against interruption only if it is charged and permitted for the tool.

A simple manual pack scenario is required trigger minutes ÷ estimated trigger minutes per pack, rounded up, but the Job Lab does not automate that recommendation because charging overlap, deterioration and task variability can dominate. Test low/central/high energy assumptions and preserve all three rather than publishing one precise-looking quantity.

Where several tools share packs, schedule energy demand rather than counting batteries twice. A pack charging while another tool runs may increase site-power demand, while a hot pack waiting to charge may break the intended overlap.

Write the assumed charging overlap explicitly.

Record whether charging is allowed in the proposed location and who monitors damaged, hot or rejected packs.

Charging load may matter on a limited 110 V supply or generator. Include chargers in the concurrent load schedule and use the site-power method with charger electrical input and verified supply data.

Compatibility and condition remain separate

Manufacturer instructions govern battery/tool/charger compatibility, permitted adaptors, transport, inspection, charging environment and damaged-pack handling. Electrical Safety First provides general battery-tool precautions, but general guidance cannot approve a platform combination. [HSE]

Stop the estimate if voltage/Ah do not describe the same configuration, usable percentage or on-load watts are absent, duty is zero/unknown, or off-cycle draw is material but omitted. Stop using a pack and follow manufacturer procedures where there is damage, swelling, unusual heat, odour, leakage or impact.

See the battery source group and Methodology for source availability, precision and fail-closed rules.

This guide provides an energy estimate. Actual runtime and compatibility depend on the manufacturer’s system, application, temperature, pack condition and protection behaviour.