Battery Life Calculator

Battery runtime: how to estimate it

At its simplest, runtime_hours = capacity_mAh / load_mA, and if you prefer energy units, runtime = Wh / W. The C-rate tells you how fast a pack discharges. At 1C it empties in an hour, at 0.5C it lasts two. Take a 4,000 mAh phone pulling 200 mA on average: the math says 20 hours, but you'll never see that, because the screen, GPS and CPU spikes draw way past the average. Phones today carry 3,500-5,500 mAh. A 10,000 mAh power bank only hands you about 6,500-8,000 mAh of real charge once you account for the 3.7 V → 5 V conversion loss, which runs at 70-85% efficiency. EVs are rated in kWh (a Tesla Model 3 packs around 60 kWh, good for roughly 400 km), and laptops land at 50-90 Wh, enough for 8-15 hours of light work. Lithium cells shed about 20% of their capacity after 500-1,000 full cycles.

Applications

Planning a trip, working out how long a drone stays up (weight against propulsion draw), running camping and off-grid kit, gauging how long an IoT sensor lasts, sizing a UPS, or picking a power bank that'll survive a long journey.

FAQ

Why does my phone die before the calculation predicts? The average load is misleading. With the screen on, 5G running, GPS active and the CPU spiking, instant draw can jump 5-10× over the idle baseline.

Does a 10,000 mAh power bank fully recharge a 5,000 mAh phone twice? No. The 3.7 V to 5 V conversion eats 15-30%, so in practice you get around 1.3-1.6 full charges.

How do I convert mAh to Wh? Multiply by the nominal voltage with Wh = mAh × V / 1000. So a 3.7 V, 4,000 mAh cell holds 14.8 Wh.