Engineering guide

Battery-powered product power budget workflow

Battery life is the visible result, but the engineering workflow starts earlier: average load current, conversion losses, distribution loss, thermal limits, and rail behaviour all change the runtime estimate.

Reviewed 4 July 2026

Quick answer

How should you build a first-pass battery-powered product power budget?

Start from the load-current budget and usable battery capacity, then compare regulator architectures, include conversion and distribution losses, check linear-regulator thermal margin where relevant, and treat the final runtime as an estimate rather than a guaranteed field result.

Model summary

Core equations

First-pass model summary

Use these equations, assumptions, and variables as the shared model behind the guide before moving to the worked example and linked calculators.

Linear regulator loss

P_{loss} = (V_{in} - V_{out}) \times I_{load}

Power wasted as heat in a linear regulator.

Battery input current (switching regulator)

I_{batt} = \frac{P_{out}}{V_{batt} \times η}

Battery current drawn by a switching regulator at efficiency η.

Runtime estimate

t_{run} \approx \frac{C_{batt}}{I_{avg}}

First-pass runtime from usable capacity and average battery current.

Variables and natural units

These symbols match the guide equations and use the same engineering-unit conventions as the linked calculators.

`P`_loss: Regulator loss

Unit: W

Power dissipated in a linear regulator as heat.

`P`_out: Output power

Unit: W

Power delivered to the load.

`V`_batt: Battery voltage

Unit: V

Nominal battery terminal voltage at the operating point.

`η`: Regulator efficiency

Unit: -

Switching regulator efficiency (0-1); reduces battery current demand.

`I`_batt: Battery current

Unit: A

Average current drawn from the battery, including regulator losses.

`C`_batt: Usable battery capacity

Unit: Ah

Practical capacity of the battery between full charge and cutoff voltage.

`t`_run: Runtime

Unit: h

Estimated operating time from usable capacity and average current.

Model boundary

Battery runtime is driven by usable capacity and average input current, not only nominal battery mAh.
Linear-regulator loss is (Vin − Vout) × Iload, while buck conversion shifts the trade-off toward efficiency, ripple, and design complexity.
Wiring and PCB resistance create voltage drop that can reduce usable battery range before the nominal capacity is exhausted.
PDN target impedance is an adjacent rail-stability check, not a battery-runtime guarantee.

Worked example

2200 mAh pack powering a 5 V, 300 mA load

This example estimates battery input current through a 90% efficient buck converter and compares runtime against a linear regulator path.

Inputs

Battery: 2200 mAh at 7.4 V nominal
V_out: 5 V
I_out: 300 mA average
η: 90% buck converter efficiency

Equation and substitution

I_{batt} = \frac{V_{out} \times I_{out}}{η \times V_{batt}} = \frac{5 V \times 0.3 A}{0.9 \times 7.4 V} \approx 225 mA

t_{run} = \frac{Capacity}{I_{batt}} = \frac{2200 mAh}{225 mA} \approx 9.8 h

Battery input current

I_batt ≈ 225 mA

First-pass runtime

≈9.8 h (buck path)

Next check

Verify pulse loads, cutoff voltage, and converter efficiency curve do not materially reduce usable runtime.

Calculator workflow

Work through these calculators in order for a complete first-pass check.

Choose the architecture before trusting the runtime

A runtime number is only meaningful after the regulator architecture is plausible. A buck stage can extend battery life and reduce heat, while a linear stage may be acceptable only when the voltage drop and load current are both modest enough that dissipation remains controlled.

PDN and hold-up are adjacent checks

Battery life does not describe short transient behaviour or shutdown timing. Use the PDN target-impedance wording for early rail-droop planning, and use stored-energy or discharge tools where hold-up or controlled shutdown matters.

Common mistakes

Using nominal battery capacity without accounting for usable depth, temperature, ageing, or cutoff voltage.
Comparing buck and linear options by voltage only instead of by loss and thermal behaviour.
Quoting the runtime estimate as a guaranteed field life.

When the model breaks down

Battery chemistry behaviour, pulse loading, converter efficiency curves, and low-temperature performance can move the real runtime far from the first-pass estimate.
PDN and hold-up checks support the architecture review but do not prove stability, startup sequencing, or EMC behaviour.

Further checks and references

Use battery datasheet discharge curves, low-temperature behaviour, protection cutoff, and ageing assumptions for a more realistic runtime model.
Check regulator quiescent current, efficiency over load, dropout, startup surge, and thermal conditions before freezing the architecture.
Include wiring, connector, PCB, and sense-path resistance if voltage drop can reduce usable battery range.

Stylised battery module feeding a measured load path with runtime arc

Next check

Battery life calculator

Start with usable capacity and average current assumptions.

Stylised linear regulator thermal path showing junction temperature and heat flow

Next check

Linear regulator thermal calculator

Check whether a linear option is thermally realistic.

Hub

Power and energy calculators

Return to the power hub for related battery, regulator, and energy checks.

FAQ

How do I estimate battery life?

Divide usable battery capacity (in mAh) by the average current draw from the battery. Account for all operating modes weighted by their duty cycle, regulator conversion losses, and the minimum battery voltage before the system resets or shuts down.

What is quiescent current and why does it matter?

Quiescent current is the regulator or IC current consumed when no active load is being driven. In sleep-dominant products it can be the dominant drain on the battery and significantly shortens estimated life when overlooked in the power budget.

Should I include regulator efficiency in the power budget?

Yes. Losses at a switching regulator appear as extra current drawn from the battery even when the downstream circuit is efficient. A buck stage at 90 per cent efficiency means the battery must supply roughly 1.11 times the output power.

Battery-powered product power budget workflow

How should you build a first-pass battery-powered product power budget?