How long will my battery last?

The math is simple: capacity divided by current. The tricky part is that the number on the battery is optimistic. This calculator takes that into account.

mAh, explained once

Battery capacity is in milliamp-hours, mAh. A 2000 mAh battery could, in theory, give 2000 mA for one hour, or 20 mA for 100 hours, or 1 mA for 2000 hours.

Runtime in hours = capacity (mAh) ÷ current (mA). Then take off a safety margin.

Why 80% and not 100%

• Your circuit stops before the battery is empty. Most circuits quit when the voltage drops below some level. The rest of the charge is still in there, you just can't use it.
• High current wastes capacity. An alkaline AA can give around 2000 mAh at a few mA, but much less if you pull hundreds of mA from it.
• Cold makes it worse. Batteries lose a lot of capacity in the cold.

80% is a fair default for small loads. For motors or anything that pulls a lot, slide it down to 60% or lower.

Typical capacities

• AA alkaline: around 2000 mAh at low drain
• AAA alkaline: around 1000 mAh
• 9 V alkaline: around 500 mAh
• CR2032 coin cell: around 220 mAh, made for tiny currents
• 18650 Li-ion: around 2500 to 3500 mAh, rechargeable

Two examples

An LED on two AA batteries

A red LED at 10 mA on two AA in series (3 V). 2000 × 0.8 ÷ 10 = 160 hours. Almost a week of being on all the time.

An Arduino on a 9 V battery

An Arduino Uno draws around 50 mA just sitting there. On a 9 V battery: 500 × 0.8 ÷ 50 = 8 hours. That's why a 9 V battery is a bad choice for it. Six AA in a holder last much longer.

Series and parallel batteries

• In series the voltages add, the capacity stays the same. Two 1.5 V AA give 3 V and still about 2000 mAh.
• In parallel the capacity adds, the voltage stays the same. Only do this with identical batteries of the same age and charge.

Measure, don't guess

The best input to this calculator is a measured current. Set your multimeter to the mA range and put it in series with the battery, so all the current goes through the meter. Measure in every mode your circuit has: idle, busy, LEDs on.

Don't know where the current goes? An LED with its resistor is usually 5 to 20 mA (see the LED resistor guide). A small motor can easily be 100 mA or more. Ohm's law gives you the rest.