# [Battery Runtime Calculator](https://blog.hirnschall.net/tools/battery-runtime/) author: [Sebastian Hirnschall](https://blog.hirnschall.net/about/) meta description: Calculate battery runtime from capacity and current draw. Supports power-based loads, converter efficiency, and depth of discharge. Solves for runtime, capacity, or current. meta title: Battery Runtime Calculator — mAh, Current, and Runtime date published: 11.05.2026 (DD.MM.YYYY format) date last modified: 11.05.2026 (DD.MM.YYYY format) --- Calculator ---------- Provide any two of capacity, current, and runtime — the third is solved. Battery voltage enables Wh output and power-based input. Efficiency and depth of discharge are optional corrections. * Battery capacity: * mAh Ah * Load current: * µA mA A * Runtime: * h min days --- * Battery voltage — optional: * V mV * Load power — optional (replaces current): * mW W * Converter efficiency — optional: * % * Depth of discharge — optional: * % Provide any two of capacity, current, and runtime to calculate. Calculate Battery Runtime — Explanation ----------------------------- The runtime of a battery-powered device follows directly from the relationship between stored charge and current consumption. A battery rated at capacity \( C \) (in mAh or Ah) can deliver an average current \( I \) for a time: \[ t = \frac{C}{I} \] A 2000 mAh battery powering a 100 mA load will last 20 hours in the ideal case. The formula is the same regardless of chemistry — the mAh rating is the fundamental quantity that defines how much charge the battery holds. When the load is specified as a power draw rather than a current, we first convert using the battery voltage \( V \): \[ I = \frac{P}{V}, \qquad t = \frac{C \cdot V}{P} \] This is useful when the datasheet of a module gives a power figure rather than a current figure, or when the load operates from a regulated voltage that differs from the battery voltage. Converter Efficiency -------------------- Most battery-powered systems include a voltage regulator or DC-DC converter between the battery and the load. These converters are not lossless: a boost converter running at 85% efficiency requires the battery to supply 100/85 ≈ 1.18 times the power the load actually consumes. The calculator accounts for this by dividing the load current by the efficiency factor before computing the runtime: \[ I\_{\text{battery}} = \frac{I\_{\text{load}}}{\eta} \] Ignoring converter losses is one of the most common sources of error when estimating battery life. A system that looks like it should run for 10 hours at full load may actually run for 8.5 hours once a typical 85% efficient boost converter is factored in. Depth of Discharge ------------------ Most battery chemistries should not be discharged to zero. Doing so shortens cycle life significantly. The depth of discharge (DoD) sets what fraction of the nominal capacity is actually used: * **Li-Ion / LiPo** — typically 80–90% DoD to preserve cycle life; 100% is possible but accelerates aging. * **Lead-acid** — typically 50% DoD; deeper discharges can permanently sulfate the plates. * **LiFePO\(\_4\)** — tolerates deeper cycling than standard Li-Ion; 80–90% is common. * **NiMH** — relatively tolerant; 80–100% DoD is workable, though memory effect applies to older cells. When DoD is entered, the calculator reduces the effective capacity to \( C \cdot \text{DoD} \) before computing runtime or required capacity. For sizing a battery pack with longevity in mind, enter the DoD along with the required runtime to find the nominal capacity needed. Purpose of the Calculator ------------------------- Any two of capacity, current, and runtime determine the third. The rearranged formulas are: * To solve for runtime (how long a known battery lasts at a given load): \[ t = \frac{C}{I} \] * To solve for capacity (sizing a battery pack for a required runtime): \[ C = I \cdot t \] * To solve for current (finding the maximum load a battery can sustain for a given runtime): \[ I = \frac{C}{t} \] When efficiency and DoD are provided, both are applied before solving, and the result reflects the real-world figure rather than the idealized one. Practical Caveats ----------------- The calculation assumes a constant average current draw. Real devices rarely have a flat profile: a microcontroller sleeping most of the time with periodic radio bursts has a very different average than its peak consumption. For duty-cycled loads, compute the time-weighted average current and use that as the input. Battery capacity also degrades with temperature. Most lithium cells deliver their rated capacity only at around 20–25 °C. At 0 °C available capacity can drop to 70–80% of the nominal value; at -20 °C it can fall below 50%. For outdoor or automotive applications, it is worth applying an additional temperature derating factor on top of the DoD. For lead-acid batteries, the Peukert effect is relevant: capacity decreases at higher discharge rates, so the simple formula overestimates runtime under heavy loads. For lithium chemistries the Peukert effect is minor and can usually be ignored in first-order estimates. Related Tools ------------- * [Voltage Divider Calculator](https://blog.hirnschall.net/tools/voltage-divider/) — design the resistive divider for battery voltage monitoring at a microcontroller ADC input. More calculators: [blog.hirnschall.net/tools/](https://blog.hirnschall.net/tools/).