How Long Will My Battery Last? A Practical Guide to Battery Runtime

You've built a circuit. It works on the bench. Now you want to run it on batteries — at a market stall, at a remote sensor station, inside a product, wherever. The question that immediately follows is always the same: how long will it actually last?

The basic maths is simple. The tricky part is understanding why real-world battery life almost always comes in shorter than the calculation suggests — and how to account for that properly.

The basic formula

Battery runtime (hours) = Battery capacity (mAh) / Current draw (mA)

A 2000mAh battery powering a circuit that draws 100mA:

2000 / 100 = 20 hours

That's the ideal figure. In practice, you should apply an efficiency factor of around 0.7–0.85 to account for real-world losses.

Actual runtime ≈ (Capacity × Efficiency) / Current draw

2000 × 0.8 / 100 = 16 hours

For most projects, 0.8 is a reasonable efficiency factor. Use 0.7 if you're in cold environments, using older batteries, or running the battery close to full depth of discharge. Use 0.85 for fresh lithium cells at room temperature with shallow discharge cycles.

Why the basic formula lies to you

Battery capacity ratings are measured under ideal conditions — usually at a slow discharge rate (C/20, meaning the battery is drained over 20 hours) and at room temperature. Your circuit almost certainly doesn't match those conditions.

The Peukert effect: batteries deliver less total capacity at higher discharge rates. A 2000mAh battery discharged in 2 hours gives you less than 2000mAh of usable energy. This matters most for high-current loads.

Temperature: cold kills battery performance hard. A lithium cell rated 2000mAh at 25°C might deliver 1400mAh at 0°C. This is critical for outdoor projects in winter.

Cut-off voltage: your circuit has a minimum operating voltage. The battery still has charge left when the voltage drops below that threshold — you can't use it. For a 3.7V lithium cell, a circuit that stops working at 3.2V leaves usable capacity behind.

Self-discharge: batteries lose charge sitting idle. For long deployments with sleep modes, factor in self-discharge on top of the active current draw.

Measuring your actual current draw

Don't estimate — measure. Put a multimeter in series with your circuit and measure actual current in the operating state you care about.

For circuits with varying current draw (microcontrollers with sleep modes, circuits that transmit data periodically), calculate the average:

Average current = (I_active × t_active + I_sleep × t_sleep) / (t_active + t_sleep)

Example: an ESP32 that transmits for 100ms every 10 seconds, drawing 250mA when active and 10µA when sleeping.

Average = (250mA × 0.1s + 0.01mA × 9.9s) / 10s

= (25 + 0.099) / 10

= 2.51mA average

A 2000mAh battery at 2.51mA average = roughly 795 hours (33 days), not the 8 hours you'd calculate from peak current.

Common battery types and what to expect

Practical tips for extending runtime

Use sleep modes aggressively. A microcontroller that sleeps 99% of the time uses a fraction of the power of one running continuously — even if the sleep current is only 10µA versus 50mA active, the numbers are transformative.

Match your battery voltage to your circuit voltage. Running a 3.3V circuit from a 9V battery through a linear regulator wastes most of the battery's energy as heat. Use a buck converter or choose a battery voltage close to your operating voltage.

Use lithium over alkaline for cold environments. Alkaline batteries lose capacity rapidly in cold weather. Lithium primary cells (not rechargeable) maintain their capacity down to -40°C.

Oversizing the battery is cheap insurance. If your calculation says 20 hours and you need 24, don't go for the smallest battery that technically works — the next size up costs little and provides a genuine margin.