Batteries store electric charge, but the way we measure that storage varies wildly — milliamp-hours for phones, watt-hours for laptops, and kilowatt-hours for electric vehicles. Understanding these units reveals how to compare batteries and estimate real-world performance.
mAh vs Wh: The Key Difference
mAh (milliamp-hours) measures charge — how many milliamps for how many hours. But charge alone doesn't tell you energy. A 5,000 mAh battery at 3.7V stores 18.5 Wh. A 5,000 mAh battery at 1.2V stores only 6 Wh.
Wh (watt-hours) measures energy — the true capacity. Wh = mAh × V ÷ 1,000. Always compare batteries using Wh (or kWh), not mAh, especially when voltages differ.
Phone and Laptop Batteries
Typical phone: 3,000–5,000 mAh at 3.7–3.85V = 11–19 Wh. If your phone uses 2W average, a 15 Wh battery lasts 7.5 hours.
Laptop: 40–100 Wh. FAA limits carry-on batteries to 100 Wh (about 27,000 mAh at 3.7V). A 60 Wh laptop running at 15W lasts 4 hours.
EV Batteries and Range
EV batteries: 40–100 kWh. A Tesla Model 3 Long Range has a 75 kWh battery. Energy consumption: about 15 kWh/100 km (4 mi/kWh). Range: 75 ÷ 15 × 100 = 500 km (310 miles).
Factors affecting range: speed (higher = more drag), temperature (cold reduces battery efficiency by 20–40%), terrain, HVAC usage, and driving style. Regenerative braking recovers 10–30% of energy in city driving.
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