Battery Life Calculator
Type your battery's capacity and how much your device draws. You get how long it lasts straight away, in any mix of mAh, Ah, Wh, mA, A or W.
Series adds up the voltage.
Parallel adds up the capacity.
On the label or in the specs: mAh (milliamp-hours), Ah or Wh (watt-hours).
Typical: 3.7 V phone or Li-ion cell, 1.5 V AA, 12 V car or leisure battery.
Current in mA or A, or power in watts (from the device label or a USB meter).
E.g. awake 1 second in every 100 = 1%.
Also called depth of discharge. Typical figures only; use your battery maker's advice.
Energy lost as heat between battery and device. Typical figures; check the spec sheet.
1.2 is typical for flooded lead-acid; AGM and gel are often 1.05–1.15, lithium close to 1.0.
Lead-acid capacity is usually quoted at the 20-hour rate (C20).
Charging time
Watts are turned into amps at the battery voltage above.
About 85% is typical; lead-acid can be lower.
Power bank: how many phone charges
Power bank mAh is nearly always quoted at 3.7 V (or 3.6 V).
60–80% is typical once the step-up to 5 V and the phone's charging losses are counted.
How your battery life is worked out
Capacity, the part you can use, the load, then divide.
- 1
Your battery
2,000 mAhat 3.7 V= 7.4 Wh
- 2
The part you can use
2,000 mAh× 100% × 100%= 2,000 mAh
- 3
Your load
100 mA× 3.7 V= 0.37 W
- 4
How long it lasts
2,000 ÷ 100= 20 h
Read more: the formula, a worked example, Peukert, charging and power banks
A battery's capacity tells you how much charge it holds. Divide it by how fast your device uses that charge and you get how long it lasts. Capacity in milliamp-hours (mAh) goes with a load in milliamps (mA); capacity in watt-hours (Wh) goes with a load in watts (W). The voltage links the two.
The formula
hours = usable capacity ÷ load
usable capacity = capacity × usable % × efficiency %
watt-hours = mAh × volts ÷ 1,000
average load (sleeps) = awake current × awake % + sleep current × (100% − awake %)
Worked example
A 2,000 mAh lithium-ion battery at 3.7 V runs a device that draws 100 mA. The battery holds 2,000 × 3.7 ÷ 1,000 = 7.4 Wh. With all of it usable (100%) and no converter (100%), the usable capacity is 2,000 × 1 × 1 = 2,000 mAh. The device uses 100 mA × 3.7 V = 0.1 A × 3.7 V = 0.37 W. It lasts 2,000 ÷ 100 = 20 hours; in watt-hours, 7.4 ÷ 0.37 = 20 hours too.
With a typical allowance for lithium-ion of 90% usable and a 90% efficient converter, it would be 2,000 × 0.9 × 0.9 = 1,620 mAh ÷ 100 mA = 16.2 hours (16 h 12 min).
Devices that sleep
A sensor that draws 20 mA while awake for 1% of the time and 50 µA (0.05 mA) asleep the rest averages 20 × 0.01 + 0.05 × 0.99 = 0.2 + 0.0495 = 0.2495 mA. A 2,000 mAh battery then lasts 2,000 ÷ 0.2495 = 8,016 hours, about 334 days. Over months, the battery's own self-discharge starts to matter, so real life is often shorter.
Cells in series and parallel
pack voltage = cell voltage × cells in series
pack capacity = cell capacity × cells in parallel
A 2S3P pack of 3.7 V, 3,000 mAh cells is 3.7 × 2 = 7.4 V and 3,000 × 3 = 9,000 mAh (9 Ah), which is 7.4 × 9 = 66.6 Wh from 6 cells.
Peukert's law (lead-acid)
hours = H × (C ÷ (I × H))k
C is the rated capacity in Ah, H the hours it is rated over (often 20), I the current in amps and k the Peukert exponent (about 1.2 is typical for flooded lead-acid; 1 means no effect). A 100 Ah battery rated at 20 hours on a 10 A load: 20 × (100 ÷ (10 × 20))1.2 = 20 × 0.51.2 = 20 × 0.4353 = 8.71 hours, not the 10 hours a plain 100 ÷ 10 gives. Using only 50% of it, as is common for lead-acid, gives 4.35 hours.
Charging time
hours = capacity × (to % − from %) ÷ charge current ÷ charge efficiency
2,000 mAh from empty with a 1 A (1,000 mA) charger at 85%: 2,000 ÷ 1,000 ÷ 0.85 = 2.35 hours (2 h 21 min). From 20% to 80%: 1,200 ÷ 1,000 ÷ 0.85 = 1.41 hours. The charge rate is 1 A ÷ 2 Ah = 0.5C. Lithium chargers slow down near full, so the last part can take longer.
Power bank charges
charges = (bank mAh × bank V ÷ 1,000) × efficiency ÷ (phone mAh × phone V ÷ 1,000)
10,000 mAh × 3.7 V ÷ 1,000 = 37 Wh in the bank. At 70% efficiency, 37 × 0.7 = 25.9 Wh reaches the phone. A 4,000 mAh phone battery at 3.85 V holds 4,000 × 3.85 ÷ 1,000 = 15.4 Wh, so you get 25.9 ÷ 15.4 = 1.68 full charges.
These are estimates. Real runtime also depends on temperature (cold cuts capacity), battery age, how the load changes over time and the cut-off voltage of your device. Typical percentages on this page are rough guides you can change, not rules; check your battery's datasheet.
Common questions
How do you work out battery life?
Divide the battery capacity by the current the device draws: hours = mAh ÷ mA. A 2,000 mAh battery running a 100 mA device lasts 2,000 ÷ 100 = 20 hours. With watt-hours and watts it is the same sum: a 50 Wh battery running a 10 W load lasts 5 hours.
How do I change mAh to Wh?
Multiply by the voltage and divide by 1,000: Wh = mAh × V ÷ 1,000. A 2,000 mAh lithium-ion cell at 3.7 V holds 2,000 × 3.7 ÷ 1,000 = 7.4 Wh. To go back, mAh = Wh × 1,000 ÷ V.
Why does my battery not last as long as the sum says?
Real batteries give less than the label: you should not drain most batteries to zero, converters and inverters lose some energy as heat, cold weather cuts capacity, and lead-acid batteries give less at high currents (Peukert's law). Advanced mode lets you allow for each of these.
What is depth of discharge?
It is how much of the capacity you let the battery use. Lead-acid batteries are often only run down to about half (50%) to keep them healthy, while lithium batteries are often used to 80–90%. These are typical figures; check your battery maker's advice and put your own figure in the Usable capacity box.
How many times can a power bank charge my phone?
Turn both into watt-hours and allow for losses. A 10,000 mAh power bank at 3.7 V holds 37 Wh; at a typical 70% efficiency about 25.9 Wh reaches the phone. A 4,000 mAh phone battery at 3.85 V holds 15.4 Wh, so you get about 25.9 ÷ 15.4 = 1.7 full charges.
How long does a battery take to charge?
Divide the capacity you need to put back by the charging current, then allow for charging losses: 2,000 mAh at 1 A (1,000 mA) with 85% charging efficiency takes 2,000 ÷ 1,000 ÷ 0.85 = 2.35 hours. Lithium batteries charge more slowly for the last 10–20%, so a full charge can take a little longer.
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