Free Battery Life Calculator – Estimate Runtime from mAh

Predicting How Long a Battery Will Last

This calculator estimates how many hours a battery can power a device before it runs flat, using a simple ratio: Hours = Battery Capacity (mAh) ÷ Device Current Draw (mA). Because both figures share the same milliamp base, the milliamps cancel and you are left with a runtime in hours. It is the number every maker wants when designing a battery-powered gadget, and the one every consumer wonders about when reaching for a power bank. An optional efficiency factor lets you turn the ideal figure into a more honest real-world estimate. The variables are easy to source. Battery capacity, in milliamp-hours (mAh), is printed on most cells and power banks and represents how much charge the battery holds. Current draw, in milliamps (mA), is how much current your device pulls on average while running. Dividing capacity by draw gives the theoretical runtime; multiplying that by an efficiency percentage (as a decimal) trims it down to account for losses. In plain terms, a bigger battery or a hungrier device pushes the runtime up or down in direct proportion, which makes trade-offs easy to reason about. Here is a worked example. Take a 3,000 mAh battery driving a 150 mA load. The ideal runtime is 3,000 ÷ 150 = 20 hours. Real circuits are not perfectly efficient, so apply a typical 85 percent factor: 20 × 0.85 = 17 hours, which is closer to what you would actually see. If the same battery instead powered a 300 mA device, the ideal runtime would halve to 10 hours, and about 8.5 hours after the efficiency adjustment — a clear reminder that current draw is the lever that matters most. A low-power sensor node makes the contrast obvious: a 2,000 mAh cell feeding an 8 mA average load lasts 2,000 ÷ 8 = 250 hours, a little over ten days on one charge. Hobbyists and product designers lean on this when planning an Arduino sensor node, a Bluetooth beacon, an LED strip, a drone, or any portable device. It answers whether a chosen cell is big enough, how often recharging will be needed, and whether a larger battery justifies its extra weight and cost. Four situations show up repeatedly. Someone building a soil-moisture sensor for a terrace garden checks that a 2,000 mAh cell at 8 mA average will run 250 hours between charges, which sets a weekly recharge routine. A traveller works out that a 10,000 mAh power bank charging a phone at 500 mA gives 20 hours in theory but nearer 13 once the 65 percent conversion loss between a 3.7 V cell and 5 V USB is counted. A drone pilot divides a 1,200 mAh pack by a 15,000 mA flight current and sees under five minutes of hover, which explains why racing packs are swapped so often. A shop fitting a battery-backed LED sign estimates nightly runtime before choosing the pack size. Treat the result as an estimate, not a promise. Usable capacity shrinks in cold weather and as cells age, high discharge rates cut the effective capacity through the Peukert effect, and most devices draw more current when active than when idle. Voltage conversion in regulators and boost circuits burns energy too, which is exactly why the efficiency factor exists — for phone charging from a 3.7 V power bank to 5 V USB, drop it to around 60 to 70 percent. Use the average current over a realistic duty cycle for the best figure. The mistake beginners make most often is comparing capacities across different voltages: a 10,000 mAh power bank does not fully charge a 5,000 mAh phone twice, because the phone cell and the bank cell sit at different voltages and the boost converter burns the difference. Compare watt-hours (mAh × volts ÷ 1000) when the voltages differ. Also avoid draining lithium cells flat — most protection circuits cut off well above zero, so usable capacity is less than the printed rating. If your pack is wired into mains-powered charging or vehicle wiring, follow local electrical code and get a qualified electrician or technician to sign off. All the math runs locally in your browser, so nothing is uploaded.

Battery Life Calculator Formula & Method

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