Battery Life Calculator
Enter the battery capacity and what your device draws to see how many hours or days it will run, including sleep and wake cycles for IoT devices and losses in real batteries.
Battery pack builder (series & parallel)
How to use the Battery Life Calculator
- Pick a battery preset or enter the capacity and nominal voltage yourself.
- Describe the load: a constant current, a power in watts, or a duty cycle with sleep current and active states.
- Set real-world losses: efficiency, usable depth of discharge and, for lead-acid, Peukert's law.
- Read the estimated battery life in hours and days with the formula shown.
- Build a pack from cells in series and parallel, then press Use this pack above to calculate its runtime.
Battery life formulas
- Current load: hours = capacity (mAh) × efficiency × depth of discharge ÷ load current (mA)
- Power load: hours = energy (Wh) × efficiency × depth of discharge ÷ power (W), where Wh = mAh × V ÷ 1,000
- Duty cycle: average current = sleep current × (1 − active fraction) + Σ (active current × its fraction of time)
- Peukert (lead-acid): t = H × (C ÷ (I × H))k, with C in Ah, I in A, H the rated hours and k the Peukert exponent
Example: a 12 V 100 Ah lead-acid battery running a 10 A load. Without Peukert it would last 100 ÷ 10 = 10 h. With k = 1.2 and a 20-hour rating, t = 20 × (100 ÷ (10 × 20))1.2 = 20 × 0.51.2 = 8.71 h to fully discharged, and only about 4.4 h if you stop at 50% depth of discharge to protect the battery (before efficiency losses).
Typical battery capacities
| Battery | Nominal voltage | Typical capacity | Energy |
|---|---|---|---|
| AA alkaline | 1.5 V | 2,000–3,000 mAh (at low drain) | ≈ 3–4 Wh |
| AAA alkaline | 1.5 V | ≈ 1,000–1,200 mAh | ≈ 1.5 Wh |
| AA NiMH | 1.2 V | 1,900–2,500 mAh | ≈ 2.4–3 Wh |
| CR2032 coin cell | 3 V | ≈ 225 mAh (at very low drain) | ≈ 0.7 Wh |
| 9 V alkaline | 9 V | ≈ 400–600 mAh | ≈ 4.5 Wh |
| 18650 Li-ion | 3.6–3.7 V | 2,500–3,500 mAh | ≈ 9–12.6 Wh |
| Power bank "10,000 mAh" | 3.7 V (cells) | 10,000 mAh | 37 Wh |
| 12 V 100 Ah lead-acid | 12 V | 100 Ah (20-hour rate) | 1,200 Wh |
Alkaline capacity falls sharply at high current: an AA that delivers about 2,500 mAh at 25 mA may give well under half that at 1 A. Coin cells are rated at microamp loads and lose capacity with pulse currents above a few milliamps.
Why a power bank doesn't charge your phone as often as you'd expect
Power banks quote the capacity of their 3.7 V cells. Charging a phone involves boosting to 5 V (or higher for fast charging) and then charging the phone's own cell, losing roughly 20–35% along the way. A 10,000 mAh (37 Wh) bank typically delivers about 25–28 Wh to the phone, which is roughly 1.5–1.8 charges of a 4,500 mAh (17.3 Wh) phone battery.
Tips for longer battery life
- For battery-powered electronics, the sleep current usually dominates. Cutting it from 100 µA to 5 µA can extend life many times over.
- Lead-acid batteries last far longer if regularly discharged only to about 50%; LiFePO4 tolerates 80–100% depth of discharge.
- Cold reduces available capacity, especially for alkaline and lead-acid chemistries; allow extra margin in winter.
- Self-discharge matters for multi-year designs: alkaline and lithium primary cells lose only a few percent a year, NiMH much more unless it is a low-self-discharge type.
Frequently asked questions
How do I calculate battery life from mAh?
Divide the capacity by the load current: battery life (hours) = capacity (mAh) ÷ current (mA). A 3,000 mAh battery powering a 250 mA load lasts about 12 hours in theory, or about 10.2 hours with an 85% derating factor.
Why use an efficiency or derating factor?
Real batteries rarely deliver their full rated capacity: capacity drops with high current, cold temperatures and age, and voltage regulators waste some energy. Using about 70–90% of the rating gives a more realistic estimate.
How do I convert mAh to Wh?
Multiply by the nominal voltage and divide by 1,000: Wh = mAh × V ÷ 1,000. A 10,000 mAh power bank at 3.7 V holds 37 Wh. Use Wh when comparing batteries with different voltages.
How do I estimate battery life for an IoT sensor?
Work out the average current: sleep current for the time asleep plus each active current multiplied by the fraction of time it runs. A device sleeping at 10 µA that transmits at 120 mA for 0.1% of the time averages about 0.13 mA before other loads, so a CR2032 can last months.
What is Peukert's law?
Lead-acid batteries deliver less capacity at higher currents. Peukert's law, t = H × (C ÷ (I × H))^k, adjusts the runtime using the rated discharge time H (often 20 h) and an exponent k of about 1.1–1.3 for lead-acid. Lithium batteries are close to k = 1.
Do batteries in series or parallel last longer?
Cells in series add voltage at the same mAh; cells in parallel add mAh at the same voltage. Either way the energy (Wh) is the sum of all cells, so runtime depends on how much power the device draws.