Battery Life Calculator
Runtime from capacity and current
Result
| Battery life | — |
|---|---|
| Battery capacity | — |
| Device current draw | — |
How to Use This Battery Life Calculator
The label on your pack says 4500 mAh, your meter reads 850 mA, and you still have to guess whether the project dies at lunch or lasts the whole field day. In my experience on bench builds and travel kits, that mental math is where mistakes show up—you either over-buy heavy cells or under-size a logger that quits mid-shoot. Think of amp-hours like fuel in a tank and current draw like the flow rate out of the nozzle: same tank, wider open the valve, faster it runs dry.
To get started, pick what you already know under Find, then fill in the other fields. Results refresh as you type—no separate calculate button.
- Choose Find. Leave Battery life (runtime) selected when you know capacity and draw. Switch to Battery capacity when you need a target mAh for a fixed runtime. Choose Device current draw when you know how long the device must run and how big the pack is.
- Enter Battery capacity. Use the mAh or Ah figure printed on the cell, power bank, or datasheet. Phone and drone packs are usually labeled in mAh; deep-cycle lead-acid specs often list Ah directly.
- Set Discharge safety. Type the percent of capacity you refuse to use—20% is a common LiPo reserve so you do not hit BMS cutoff or puff the pack. Raise it when your flight controller or ESC already enforces a hard floor.
- Enter Device current draw. Log average amperage with a USB meter, bench supply readout, or datasheet continuous figure—not brief peak splash screens. If load pulses, average over several minutes of real use.
- Read Result and Calculation. The table shows life, capacity, and draw in consistent units. The calculation block walks through usable capacity and the final hour estimate when inputs are valid.
Moving onto common jobs: a filmmaker sizing a V-mount block for a mirrorless rig, a maker checking whether a 18650 holder feeds an ESP32 overnight, or an FPV pilot sanity-checking flight time before a meet all use the same ratio—usable amp-hours divided by average amps. For voltage, resistance, and power at the load, continue with the Ohm’s Law Calculator. For wall-power cost from wattage, use the Electricity Calculator. For thrust, mass, and LiPo placement on quads, read our RC drone torque and mass guide.
Battery Life Calculator Formulas and Practical Applications
Continuous draw until the usable slice of the pack is gone follows the standard approximation:
Battery life (h) = Capacity (Ah) ÷ Current (A) × (1 − Discharge safety)
Example walk-through: 4500 mAh equals 4.5 Ah. At 1000 mA (1 A) average draw and 20% discharge safety, usable capacity is 4.5 × 0.8 = 3.6 Ah, so runtime is 3.6 ÷ 1 = 3.6 hours.
Solve for required capacity
When you know how many hours you need and what the load draws, flip the relationship:
Capacity (Ah) = Battery life (h) × Current (A) ÷ (1 − Discharge safety)
In practical environments, we use this before ordering cells for a GPS tracker that must report for 72 hours at 15 mA average—with 25% reserve, the math tells you the minimum Ah rating to spec on the purchase order.
Solve for maximum current draw
When runtime and pack size are fixed—say a product enclosure that only fits one 3000 mAh pouch—solve for the average current budget:
Current (A) = Capacity (Ah) × (1 − Discharge safety) ÷ Battery life (h)
Firmware teams use that ceiling to cap radio duty cycles or dim displays so the hardware stays inside the allotted milliamp average.
Where estimates drift from real life
Cold weather thickens electrolyte and sags voltage early. Old cells hold less than nameplate mAh. Buck converters waste a few percent as heat. None of that appears in a straight division problem—treat the output as a planning number, then bench-test with your actual board and battery age. For high continuous amp draws, verify the cell CDR separately; our Ohm’s Law amp-draw safety guide walks through current-limit thinking on rebuildable loads, and the same discipline applies to any lithium pack driving a heavy motor or coil.
Standard Units and Conversion Tables
Keep capacity and current in parallel scales—mixing mAh with amperes without converting is the usual spreadsheet error we see in kit reviews.
| Measurement | Common label | Convert to base |
|---|---|---|
| Capacity | milliamp-hours (mAh) | Ah = mAh ÷ 1000 |
| Capacity | amp-hours (Ah) | Use directly in formulas |
| Current draw | milliamps (mA) | A = mA ÷ 1000 |
| Current draw | amps (A) | Use directly in formulas |
| Runtime output | hours / days | Days = Hours ÷ 24 |
Quick reference draws (always verify on your hardware): a phone on standby often sits between 50 and 150 mA equivalent average; a 5 V Arduino with Wi-Fi association bursts can land near 80–250 mA average depending on firmware; a compact FPV quad at hover may pull 15–25 A from a 4S pack—far above what a phone cell could safely deliver, which is why capacity alone never tells the whole story without current.
Frequently Asked Questions
What is the battery life formula?
Battery life in hours equals capacity in amp-hours divided by average current in amperes, multiplied by one minus the discharge safety fraction. Enter mAh and mA in the tool and it converts internally—the algebra is the same as doing Ah ÷ A by hand.
What does discharge safety mean?
It is the slice of rated capacity you deliberately leave unused—often about 20% on LiPo packs so you stop above empty and protect cycle life. A 20% reserve means only 80% of the printed mAh counts toward the runtime line in the result table.
How long will a 4500 mAh battery last?
At 1000 mA average draw and 20% safety, usable capacity is 3600 mAh (3.6 Ah), which yields about 3.6 hours. Lower average current stretches that time linearly; a 500 mA load roughly doubles it if the reserve percentage stays the same.
How do I find average current draw from wattage?
Divide watts by volts at the battery or regulator: I = P ÷ V. A 5 W load at 3.7 V is about 1.35 A at that instant—use the Ohm’s Law Calculator for related V/I/R checks, and prefer a meter when the device sleeps or spikes between idle and transmit.
Battery life vs battery lifespan?
Battery life is hours per charge. Battery lifespan is how many charge cycles the cell survives before replacement. This page estimates single-charge runtime only—not calendar aging, C-rate heating, or warranty cycle counts.