Battery Run Time Calculator: How Long Will It Last?

This battery run time calculator estimates how long a battery will power a device before it needs recharging or replacement. You enter the battery capacity (in amp-hours or milliamp-hours), the system voltage, and the average current draw (in amps or milliamps), and it returns the run time in hours and minutes. It uses the standard formula: run time = battery capacity ÷ load current.

Formula

The core formula is simple:

Run Time (hours) = Battery Capacity (mAh) ÷ Load Current (mA)

Where:

• Battery Capacity (t) – the total charge stored in the battery, measured in milliamp-hours (mAh) or amp-hours (Ah). 1 Ah = 1000 mAh.
• Load Current (c) – the average current the device draws, measured in milliamps (mA) or amps (A). 1 A = 1000 mA.
• Run Time (rt) – the result, in hours (decimal) or minutes.

If you have capacity in amp-hours and current in amps, the same formula works directly: hours = Ah ÷ A. The calculator above uses mAh and mA to match common consumer battery ratings.

Worked Example

Using the default values from the calculator:

• Battery capacity = 3000 mAh
• Voltage = 3.7 V (typical lithium-ion cell)
• Load current = 500 mA

Step 1: Write the formula: Run Time = 3000 ÷ 500

Step 2: Divide: 3000 ÷ 500 = 6

Step 3: The result is 6.00 hours (or 360 minutes).

That means a 3000 mAh battery powering a device that draws 500 mA will last about 6 hours under ideal conditions.

How to Read the Result

The result is a theoretical maximum under constant, steady load. Real-world run time is almost always shorter. The calculator gives you a baseline: if the device draws more current in practice (e.g., screen brightness, Wi-Fi, motor surges), the run time drops proportionally. If the load is intermittent (e.g., a flashlight used in bursts), the average current over time is what matters, not the peak.

For example, a phone with a 4000 mAh battery and an average drain of 800 mA will last 5 hours. But if a game pushes the drain to 1200 mA, the same battery lasts only 3.3 hours.

Common Mistakes and Misconceptions

• Mixing units: Using mAh with amps (or Ah with milliamps) gives an answer off by 1000×. Always convert to the same unit pair.
• Ignoring voltage: The formula above does not use voltage because capacity and current are both charge-based. However, if you know power (watts) instead of current, you must convert: Current (A) = Power (W) ÷ Voltage (V).
• Forgetting efficiency losses: Batteries are not 100% efficient. Heat, internal resistance, and voltage sag can reduce usable capacity by 10–30%, especially at high discharge rates.
• Assuming linear discharge: Some devices stop working before the battery is fully drained. A device with a cutoff voltage may leave 5–10% of capacity unused.

When to Use This Calculator

Use it whenever you need a quick estimate for:

• Choosing a battery for an Arduino or Raspberry Pi project
• Estimating how long a power bank will charge your phone
• Planning a camping trip with battery-powered lights or fans
• Comparing batteries with different capacities for the same device
• Checking if a solar battery bank can run a load overnight

For devices with variable loads (like laptops or drones), calculate the average current draw over a typical cycle. You can measure it with a USB power meter or multimeter, or use the manufacturer’s rated current.

Limits of the Formula

The simple formula assumes a constant current draw and ideal battery behavior. In reality:

• Discharge rate affects capacity: A battery rated at 3000 mAh at 0.2C (slow discharge) may only deliver 2500 mAh at 2C (fast discharge). This is known as Peukert’s law, mostly relevant for lead-acid batteries.
• Temperature matters: Cold batteries lose capacity; hot batteries may degrade faster.
• Age and cycles: A battery that has been recharged hundreds of times holds less charge than a new one.
• Voltage drop: As the battery drains, voltage falls. Some devices draw more current to maintain power, shortening run time further.

For lead-acid batteries, a common rule is to use only 50% of the rated capacity to avoid damage. For lithium-ion, 80–90% usable is typical. Multiply the rated capacity by the usable fraction before dividing by current for a more realistic estimate.

Alternatives and Related Calculations

If you know power in watts instead of current, use this two-step method:

1. Convert watts to amps: Amps = Watts ÷ Volts
2. Then: Run Time (hours) = Battery Capacity (Ah) ÷ Amps

Alternatively, you can compute energy directly: Battery Energy (Wh) = Capacity (Ah) × Voltage (V). Then Run Time (hours) = Energy (Wh) ÷ Power (W). This is often easier because device power is usually listed in watts.

For example, a 12V battery rated 20Ah stores 240 Wh. If your device draws 60W, run time = 240 ÷ 60 = 4 hours.

Frequently Asked Questions

Does voltage affect battery run time?

Not directly in the formula above, because capacity (mAh) already accounts for charge. However, voltage matters when you convert from watts. A higher voltage battery with the same mAh rating stores more energy (Wh = Ah × V), so it will last longer for the same wattage load.

Why does my actual battery run time differ from the calculation?

Real devices rarely draw a constant current. Screens, processors, radios, and motors all spike. Also, battery capacity is rated at a specific discharge rate and temperature; using it faster or colder reduces usable capacity. Efficiency losses in voltage converters also eat into run time.

How do I measure the current draw of my device?

Use a multimeter in series with the positive lead, or a USB power meter for USB devices. For intermittent loads, measure over a few minutes and average the readings. Many devices list typical current in their datasheet or label.

What is the difference between mAh and Wh?

mAh measures charge (how many electrons), while Wh measures energy (charge × voltage). A 3000 mAh battery at 3.7V stores 11.1 Wh. Two batteries with the same mAh but different voltages store different amounts of energy, so Wh is a better comparison across different voltages.

Can I use this calculator for car batteries?

Yes, but be careful with lead-acid batteries. Use the 50% rule: only use half the rated Ah to avoid damage. Also, car batteries are not designed for deep discharge; a deep-cycle battery is better for sustained loads.

How do I calculate run time for a battery pack made of multiple cells?

If cells are in series (voltage adds, capacity stays the same), use the same mAh. If in parallel (capacity adds, voltage stays the same), add the mAh values. For example, two 2000 mAh cells in parallel give 4000 mAh at the same voltage.

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in t | Battery capacity | mAh | 3000 | 0
in v | Voltage | V | 3.7 | 0
in c | Load current | mA | 500 | 0
out rt | Run time | hours | t / c | 2
out rt_min | Run time | minutes | (t / c) * 60 | 0