calculator
Battery life calculator
How many hours — or days — will your pack last? Pick a battery, type the current your circuit draws, and this shows both the datasheet number and the honest one you'll see on the bench.
Expected runtime
3.9 days
About 75% of the datasheet number — alkaline cells sag under load and your circuit stops working before the pack is truly flat.
- Capacity ÷ current: 2500 mAh ÷ 20 mA = 125 hours — ideal
- Trim 25% for real-world sag: 125 × 0.75 = 93.8 hours — practical
- In days (24 hours each): 3.9 days
simulating…
How this works
A battery is sized by milliamp-hours (mAh): how many milliamps it can deliver for one hour before it stops. A fresh single AA alkaline holds about 2500 mAh. Pull 10 mA out of it and it'll last roughly 250 hours; pull 100 mA and it'll last roughly 25. The equation is simple: hours = mAh ÷ mA.
That's the ideal. Real-world numbers are smaller because the voltage sags as the battery runs down. An LED stops lighting below about 2 V. A 555 timer gives up around 3 V. The battery still has charge inside — your circuit just doesn't care anymore. So the honest answer, printed above, cuts the ideal by about 25%.
Why 9 V batteries are a bad deal
A 9 V battery looks big and important, but inside it is six AAAA-sized cells stacked in series. More voltage, far less capacity — about 550 mAh instead of 2500. For a desk clock project that blinks an LED for a week, a 9 V dies in three days while a 3×AA pack keeps going for twelve. Reach for AAs unless your circuit genuinely needs the extra voltage (a buzzer that only sounds right above 6 V, a motor, a 7805 regulator feeding a 5 V line).
Coin cells are for whispers only
A CR2032 coin cell holds about 225 mAh — a tenth of an AA. It's rated for drains measured in microamps, not milliamps. Push 20 mA through one and the voltage collapses so hard that the quoted capacity is a fiction; you'd be lucky to get an hour. Use them for low-power sensors, remotes, memory backup — never for an LED on a breadboard.
Measuring your real circuit
The number you plug in matters more than the pack. Put a multimeter in current (mA) mode in series with the battery and read the number. If the circuit blinks, the display will too — pick the average, or the peak and then times by whatever fraction of the time the LED is actually on. A circuit that is on 50% of the time at 20 mA averages 10 mA, so the pack lasts twice as long.
Rough numbers for TinyOhms projects
- First Light (one LED at 11 mA on 3×AA): about 230 hours ideal, roughly 7 days real-world of continuous use.
- Traffic Light (three LEDs in parallel, ~33 mA on 3×AA): about 75 hours ideal, roughly 2 days real-world.
- Doorbell (buzzer only sounds when pressed, resting at 0 mA): the pack lasts shelf life — alkaline keeps 90% of its charge for about five years sitting still.
Common questions
Why cut the ideal number by 25%? The mAh on a datasheet is measured at a slow, steady drain down to a 0.8 V-per-cell cutoff. Real circuits give up earlier: an LED stops below 2 V, a 555 below 3 V. The pack is still half-full at that point, but your project calls it dead. The 25% figure is a conservative first approximation — a thrifty circuit might go longer; a hungry one much less.
A 9 V runs out so fast — why? It is six tiny cells in a single shell: more voltage per fresh cell, far less total capacity. For runtime, choose AAs.
What about rechargeables? A NiMH AA holds roughly 2000 mAh at 1.2 V per cell — a bit less capacity, slightly lower pack voltage. Use the AA option here and trim another 20%. LiPo cells print their own mAh on the side.
My circuit draws different amounts at different moments. Use the average. Blinking half the time at 20 mA averages 10 mA. A sleeping sensor that wakes briefly might average well under 1 mA.
See the exact circuit this calculator runs by opening First Light, or walk through Ohm's law — the equation behind every current reading — on the Ohm's law calculator.
Capacities verified against: SparkFun — Battery Technologies · Wikipedia — AA battery · Wikipedia — CR2032