Battery Life Calculator

Battery capacity in milliamp-hours is a charge rating, not an energy rating: 5000 mAh means the cell can deliver 5000 mA for one hour, or 250 mA for twenty hours. Dividing capacity by average current gives runtime directly, which is why the core expression here is capacity divided by effective current. Energy follows from multiplying charge by nominal voltage, so a 5000 mAh cell at 3.7 V stores 18.5 Wh. Duty cycling dominates the result in embedded designs because sleep current is typically one to three orders of magnitude below active current: a node drawing 150 mA active and 3 mA asleep at a 10 percent duty cycle averages 17.7 mA, roughly seven times better than always-on operation. Typical reference points are 2000 to 5000 mAh for phone cells, 220 to 235 mAh for a CR2032 coin cell at 3 V, and 2500 to 3000 mAh for an alkaline AA at 1.5 V nominal. The efficiency term absorbs everything the ideal division ignores. Internal resistance dissipates part of the stored charge as heat, regulators convert with 85 to 95 percent efficiency for switching designs and far worse for linear ones, and devices cut off around 3.0 V per lithium cell rather than draining to zero. Peukert first described in 1897 the fact that available capacity falls as discharge rate rises, an effect that is severe in lead-acid and modest but real in lithium chemistries. Temperature is the other large error term, with 10 to 20 percent capacity loss near freezing, and cells commonly lose about 20 percent of rated capacity after 500 full cycles. Values of 80 to 90 percent are realistic for a healthy lithium-ion pack at room temperature; use 70 to 80 percent for a conservative field estimate.

The measurement this calculator rests on dates to the first practical storage cells. Alessandro Volta built the voltaic pile in 1800, Gaston Plante demonstrated the rechargeable lead-acid cell in 1859, and Waldemar Jungner patented nickel-cadmium in 1899. The amp-hour became the trade rating for these cells because charge, unlike energy, could be measured straightforwardly by timing a constant-current discharge to a defined cutoff voltage, and manufacturers still publish capacity that way with the discharge rate stated alongside it. Wilhelm Peukert quantified in 1897 that the rating depends on how fast you draw it, which is the reason datasheets specify a C-rate with every capacity figure. Consumer electronics reshaped the numbers rather than the arithmetic. Lewis Urry developed the alkaline cell at Union Carbide in the 1950s, giving the AA form factor its familiar 2500 mAh class rating. Lithium-ion, built on research by Stanley Whittingham, John Goodenough and Akira Yoshino who shared the 2019 Nobel Prize in Chemistry, was commercialized by Sony in 1991 and brought the 3.7 V nominal, 4.2 V full, 3.0 V cutoff curve that most modern runtime estimates assume. As microcontrollers gained deep sleep modes in the 1990s and 2000s, the always-on assumption stopped being useful and duty-cycle-weighted average current became the standard planning method for battery-powered sensors, supported by coulomb-counting fuel gauges and by bench instruments such as current profilers that capture microsecond-scale wake events.