🔋 Battery Life Calculator

A battery life calculator answers the question every maker eventually asks: “how long will this thing actually run?” The answer is a single division. You take the energy stored in the battery — its capacity — and divide it by the rate at which your device drains it. The result is the runtime in hours. It is the same basic bookkeeping you use when sizing the dropper resistor for an LED or checking the idle draw behind a voltage divider, just expressed as time instead of resistance.

Capacity is measured in milliampere-hours (mAh) or ampere-hours (Ah). Despite the name, it is a measure of charge, not power: a 2000 mAh cell can deliver 2000 mA for one hour, or 100 mA for twenty hours, or any other combination whose product equals 2000. The current your device pulls is measured in milliamperes (mA). Divide the two and the “milliampere” units cancel, leaving hours.

For a 2000 mAh battery, the table shows how the runtime shrinks as the load grows:

The core formula is just a ratio of charge to current:

Runtime (h) = Capacity (mAh) ÷ Load (mA)

If several identical devices share one battery, multiply the load by the device count first. The calculator does this for you: a 2000 mAh pack feeding four 100 mA gadgets draws 400 mA in total, so it lasts 2000 ÷ 400 = 5 hours instead of 20. Watch the denominator — doubling the current halves the runtime, every time. The same "add them up" logic shows up when you combine cells in parallel, where capacity behaves like the branch currents in a parallel resistance network.

As a derived check, the calculator also reports the C-rate, which is the load current divided by the capacity expressed in the same units: C = I(mA) ÷ Capacity(mAh). A 0.05 C draw (100 mA from 2000 mAh) is a gentle, longevity-friendly trickle; a 1 C draw means the battery empties in exactly one hour; anything above 2 C is a hard, heat-generating discharge that shortens cell life. The C-rate is the number battery datasheets actually quote, so it is the honest way to compare a tiny coin cell with a chunky 18650.

Real batteries hide two wrinkles. First, capacity is normally rated at a modest C-rate (often 0.2 C); pull harder and you get less than the label claims — that falling-off is Peukert's effect, most visible in lead-acid and old NiMH cells. Second, the “cut-off” voltage matters: a device that dies at 3.0 V squeezes more run time out of a Li-ion than one that quits at 3.4 V. The calculator gives the ideal figure; treat it as a confident upper bound.

Building a pack is a separate choice from the runtime maths. Cells stacked in series add their voltages but keep the same capacity, so runtime is unchanged; cells placed in parallel add capacity (and current capability) while keeping the voltage, so runtime grows in proportion. A “3S2P” Li-ion pack therefore runs roughly twice as long as a single series string and at three times the voltage. The capacity you type into the calculator should be that parallel-added total, not the rating of one cell.

1. Enter the battery capacity and pick its unit (mAh for small cells, Ah for big packs).
2. Enter the current your device draws and pick its unit (mA, A, or uA).
3. Enter how many identical devices share the battery (leave at 1 if there is only one).
4. Press Calculate to see the runtime in hours and days, the total current, and the C-rate.
5. Compare the C-rate against your cell's datasheet and keep it well under 1 C for long life.

Why does my device die sooner than the calculator predicts?

The figure is ideal. Real cells deliver less at high drain (Peukert's effect), the cut-off voltage cuts the run short, and self-discharge nibbles away while idle. Cold weather also shrinks usable capacity. Expect 80-90% of the number in practice.

What is a safe C-rate?

Below 0.5 C is gentle and longevity-friendly for most chemistries. Around 1 C is normal for phones and power tools. Sustained draws above 2 C generate heat and wear the cell; only purpose-built cells tolerate that.

Can I use this for a USB power bank?

Yes, but read the bank's rated mAh at the 3.7 V cell level, not the 5 V output figure printed on the box — the 5 V number already includes conversion losses, so it understates what the cells actually hold.

Does a bigger battery always last proportionally longer?

In the ideal model, yes: double the mAh, double the hours. In reality a larger cell also has lower internal resistance and stays cooler, so very high drains recover some of the lost capacity. The relationship is close to linear for modest loads.

How do I account for a device that sleeps most of the time?

Average the current over a full cycle: if it draws 5 mA for 9 seconds and 100 mA for 1 second, the mean is about 14.5 mA. Feed that average into the calculator for a realistic estimate.

For low-power wireless nodes, aim the average draw under 0.05 C — a 2000 mAh Li-ion sipping 20 mA stretches past four days, which pairs naturally with the idle-current thinking behind a voltage divider built from large resistors.

When you size a supply, remember the calculator''s number is a ceiling. If your product must survive a cold week on one charge, design for roughly 70% of the ideal runtime and you will not be caught out.

Mixing old and new cells in series is a false economy: the weakest cell hits cut-off first and the pack dies, even though the others still hold charge. Replace the whole series string together.

If runtime falls short, raise the capacity or lower the draw — and check passive leaks like a status LED that never turns off; a single always-on 5 mA indicator can quietly eat a third of a small battery''s life.

For radio nodes the duty cycle ties two worlds together: the average current you enter here decides the runtime, while the carrier you broadcast is set by the wavelength maths in a frequency to wavelength tool. Trim the transmit time and both the battery and the antenna budget improve.

Temperature quietly steals capacity: a Li-ion at 0 °C delivers perhaps 80% of its room-temperature mAh, and below freezing the figure drops further while charging becomes unsafe. If your product lives outdoors in winter, design for that reduced capacity rather than the lab number, and the calculator''s ceiling will still steer you right.