Battery Life Calculator
Estimate how long a battery lasts from its capacity, the load it drives, efficiency, and an optional Peukert correction.
Enter battery capacity, load, nominal voltage, and an efficiency factor. Toggle the advanced option to apply a Peukert exponent for lead-acid or high-drain work.
- t(h) = Crated × η / Iload
- E(Wh) = Crated × Vnom
How It Works
- 1
Enter battery capacity and load
Pick mAh, Ah, or Wh for capacity and mA, A, or W for load. If you use Wh or W, the calculator uses the nominal voltage to convert to amperes.
- 2
Set nominal voltage and efficiency
Default 3.7 V covers most lithium-ion cells. Use 1.5 V for alkaline, 1.2 V for NiMH, or 12 V for lead-acid. Efficiency 0.85 is a reasonable general-purpose derating; drop to 0.5 if you're holding a lead-acid bank above 50% state of charge.
- 3
Optional: add Peukert correction
Toggle the advanced option and enter a Peukert exponent above 1. Use 1.05 for lithium-ion, 1.15 for AGM, 1.25 for flooded lead-acid. The calculator shows runtime in hours, days, and H:MM clock form plus the effective capacity at the discharge rate.
How long a battery really lasts
Alessandro Volta built the first battery in 1800: a stack of copper and zinc disks separated by brine-soaked cloth. Two centuries later, the math for estimating runtime is still almost as simple — capacity divided by load current — but with two honest corrections. The first is depth of discharge and temperature derating, rolled into an efficiency factor η. The second is Peukert's law, published by Wilhelm Peukert in 1897, which captures the fact that a battery gives up less usable capacity at high discharge currents than at low ones. The formula C_eff = C_rated × (I_ref / I_load)^(k_p − 1) makes this explicit: k_p = 1 is the ideal battery, 1.1–1.3 is lead-acid, and 1.02–1.05 is modern lithium-ion. mAh and Ah measure charge, not energy — multiplying by the nominal cell voltage gives Wh, which is the number airlines care about (100 Wh carry-on limit, 160 Wh with approval). A 3000 mAh lithium cell at 3.7 V holds 11.1 Wh; the same mAh rating on a 1.2 V NiMH holds only 3.6 Wh. The common misconception that a phone with a 5000 mAh battery should last twice as long as one with 2500 mAh ignores screen size, radio power, and software — real-world runtime at the same load typically scales linearly, but loads are rarely the same. For back-of-the-envelope numbers the simple model is fine; for high-drain applications like drones, e-bikes, or off-grid solar banks the Peukert correction changes the answer materially.
Common pitfalls
Confusing mAh with Wh. Airlines, shippers, and IEC 62133 all regulate in watt-hours, not milliamp-hours. FAA carry-on limit is 100 Wh per cell without approval; a 27 000 mAh '27 Ah' power bank at 3.7 V is 99.9 Wh (allowed), but the same cell count in a 12 V pack is 324 Wh (forbidden).
Ignoring Peukert's effect at high discharge. A 100 Ah lead-acid rated at the 20 hour rate (C/20 = 5 A) delivers only about 55 Ah at C/1 (100 A). Drones, e-bikes, and winches land deep in this regime; the simple C / I_load model over-predicts runtime by 40% or more.
Forgetting that cold batteries lose capacity. A lithium-ion pack at 0 °C holds roughly 85% of rated capacity; at -20 °C closer to 60%. Lead-acid halves at -18 °C. The temperature derating in the efficiency factor is not optional for outdoor or automotive use.
Discharging too deep. Lead-acid lifespan drops sharply below 50% DoD; LiFePO4 tolerates 80-90% DoD for thousands of cycles. Rated mAh assumes a full discharge you should almost never actually perform on a Li-ion pack (full 0% to 100% repeatedly will halve cycle life).
Mixing old and new cells. A 12 V lead-acid bank where one cell is older looks fine on a static voltmeter but fails under load; the weak cell gets reverse-charged at deep discharge, off-gassing and accelerating failure. Replace whole banks, not individual cells.
Frequently Asked Questions
What does Peukert's law actually say?
Wilhelm Peukert published the empirical relation in 1897: the usable capacity of a lead-acid battery falls as discharge current rises. In modern form, C_eff = C_rated × (I_ref / I)^(k − 1), where k is the Peukert exponent and I_ref is the current at which the rated capacity was measured (often the C/20 rate). A perfect battery has k = 1 and loses no capacity at high load. Real lead-acid batteries sit between 1.1 and 1.3. Lithium-ion cells barely notice: typical k is 1.02–1.05. A k value of 1.0 collapses the formula to the simple C/I model used for back-of-the-envelope estimates.
Why does my phone never last as long as the mAh rating suggests?
Three reasons. First, the rating is measured at a slow discharge rate — roughly C/20 — not the bursty load of a modern smartphone. Second, cells age: after 500 full cycles a lithium-ion battery has usually lost 15–20% of its original capacity. Third, screens, radios, and processors rarely draw a steady current, and the peak load during video playback or a 5G upload is several times the average. The calculator's efficiency factor is a blanket derating for these combined effects; 0.85 is a reasonable default for a healthy device.
What's the difference between mAh and Wh?
mAh is charge; Wh is energy. Two batteries with the same mAh can hold very different energy depending on voltage: a 3000 mAh cell at 3.7 V stores 11.1 Wh, while a 3000 mAh cell at 1.2 V stores 3.6 Wh. Wh is the honest comparison between chemistries and the figure airlines use for carry-on limits (100 Wh without approval, 160 Wh with). If the datasheet only lists mAh, multiply by the nominal voltage to get Wh.
How does temperature affect battery life?
Cold batteries deliver less usable capacity because internal resistance rises. Lithium-ion typically loses 20% of capacity at 0 °C and 50% or more at −20 °C. Heat hurts long-term cycle life instead of short-term runtime: a lithium-ion cell stored at 40 °C ages roughly twice as fast as one at 20 °C. Lead-acid is the opposite — cold storage is fine, heat is deadly for plate corrosion. The efficiency field is the right place to derate when running outside 15–25 °C.
What does depth of discharge mean for runtime?
Depth of discharge (DoD) is the fraction of capacity you actually use on each cycle. Running a lead-acid battery to 100% DoD cuts its cycle life by an order of magnitude versus stopping at 50%. Lithium-ion handles deeper discharges better but still prefers to stay between 20% and 80% for long life. Set the efficiency field to your target DoD if you want the calculator to give a realistic daily runtime rather than an absolute floor — for example, 0.5 for a solar lead-acid bank held above 50% SoC.
Why is my battery worse after a year in the drawer?
Calendar aging. Even unused, lithium-ion cells lose 2–4% of capacity per year at room temperature and much more if stored fully charged or fully empty. Best-practice long-term storage is 40–60% state of charge at 15 °C or below. Lead-acid self-discharges faster — around 5% per month — and must be topped up periodically or the plates sulfate.
Can I just divide mAh by mA to get hours?
For quick estimates, yes: 2500 mAh / 250 mA ≈ 10 hours. The answer is optimistic because it ignores Peukert effects, efficiency, and voltage sag near the cutoff. Use the simple model when the load is well below C/10 (one tenth of the numerical Ah rating) and the battery is warm. For loads above C/5 — power tools, drones, e-bikes — the Peukert correction materially changes the answer.
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