Battery Life Calculator
Calculate how long a battery will last given its capacity (mAh or Wh) and the device's power draw. Includes presets for common batteries and devices.
About the Battery Life Calculator
A battery life calculator estimates how long a battery will power a device by dividing the battery's energy capacity by the device's power consumption. In simple terms: Battery Life (hours) = Capacity (mAh) ÷ Current Draw (mA) × Efficiency Factor. The efficiency factor accounts for real-world losses from internal resistance, temperature, and power conversion — typically 80–90% for most batteries.
This tool is useful for a wide range of applications: sizing a battery bank for a camping trip, estimating how long a UPS will power your server, calculating runtime for IoT sensor nodes, checking how long a calculator's batteries will last, or planning battery requirements for off-grid solar systems. The mAh/mA mode works for DC circuits; the Wh/Watts mode is better for devices that specify power draw in watts, such as laptops and AC-powered devices.
Battery capacity ratings (mAh) are measured under ideal laboratory conditions — room temperature, low discharge rate, new battery. In practice, several factors reduce real-world runtime: high discharge rates reduce capacity by 10–30% (Peukert's effect), cold temperatures reduce capacity significantly (lithium batteries lose up to 50% at freezing), and aged batteries hold less charge. The efficiency slider in our calculator lets you account for these real-world conditions.
How to Check Battery Life on a Calculator (TI-84, Casio, and Others)
Graphing calculators like the TI-84 Plus and Casio fx-9750GIII do not have a software battery life indicator like smartphones do. The TI-84 Plus CE runs on a rechargeable lithium battery rated at approximately 1200 mAh; typical active usage draws around 300–500 mA, giving roughly 2.5–4 hours of active screen-on time per charge. The classic TI-84 Plus (non-CE) uses 4 AAA batteries (about 1200 mAh each) and draws roughly 100–150 mA, giving 30–50 hours of use.
To check battery life on a TI-84: for the CE model, press [2nd] [MEM] (above the '+' key) and the battery indicator appears in the top-right of the Memory Management screen. For classic models, the battery level indicator appears in the top-right corner when you first power on and on the home screen. On Casio calculators, a low-battery icon appears on the display; there is no percentage readout. If your calculator is running slowly or the screen is dim, the batteries are likely running low.
Battery Life of Common Batteries
AA alkaline batteries hold approximately 2,400–3,000 mAh at low discharge rates, but this drops significantly at higher currents. AAA batteries hold about 1,000–1,200 mAh. Coin cell batteries like the CR2032 hold only about 220 mAh but have very low self-discharge (shelf life of 10+ years), making them ideal for low-power devices like key fobs and calculators. Lithium 18650 cells (used in laptops and electric vehicles) hold 2,500–3,600 mAh at a nominal 3.7V.
Car batteries are typically rated in Cold Cranking Amps (CCA) for starting, but also have an Amp-Hour (Ah) rating — usually 45–75 Ah for standard vehicles, up to 100+ Ah for trucks. A 60 Ah car battery can theoretically power a 10A device for 6 hours, but automotive batteries are designed for shallow discharge and should not be discharged below 50% to avoid damage. Deep cycle batteries (marine, golf cart, solar) are designed for this kind of use.