Battery Life Calculator

How this is calculated

⚠️ mAh IS NOT A CAPACITY UNTIL YOU KNOW THE VOLTAGE. Milliamp-hours measure charge, not energy, so comparing two batteries by mAh alone only works if they run at the same voltage - and power banks and phones do not. The unit that actually compares is the WATT-HOUR: capacity in amp-hours multiplied by voltage. Every honest comparison on this page happens in watt-hours and is converted back at the end. ⚠️ THIS IS WHY A 10,000 mAh POWER BANK DOES NOT HOLD 10,000 mAh OF PHONE CHARGING. The cells inside run at about 3.7 V, so the pack holds around 37 Wh. The USB port delivers at 5 V, and the conversion between the two is not free. After a typical 90% efficiency you have about 33 Wh to give away, which at 5 V is roughly 6,660 mAh - about two thirds of the number on the box. Nothing is faulty and nobody is lying; the figure on the label is measured at the cell voltage, which is the convention, and the port runs at a different one. RUNTIME IS USABLE ENERGY DIVIDED BY DRAW, and both halves need care. Usable energy is the rated capacity reduced by how deep you are willing to discharge and by conversion losses. Draw is rarely constant - a phone screen, a radio transmitting, a motor starting all spike well above the average, and it is the AVERAGE over your actual use that this calculation needs. DEPTH OF DISCHARGE MATTERS ENORMOUSLY OUTSIDE LITHIUM. A lithium pack can use nearly all its rated capacity, and its own protection circuit stops it going too far. A lead-acid battery should not routinely go below about half, because regularly draining it further shortens its life dramatically - so a 100 Ah lead-acid battery is a 50 Ah battery for planning purposes. Getting this wrong is the single biggest error in sizing an off-grid or backup system. EFFICIENCY IS NOT ONE NUMBER, IT IS EVERY STAGE MULTIPLIED. A boost converter, an inverter, the charging circuit at the far end and the cable all take a share. If you are charging another battery, expect losses at BOTH ends - the pack converting up and the device converting back down - so the round trip can easily be 75-80% rather than the 90% of a single stage. ⚠️ COLD DESTROYS CAPACITY TEMPORARILY AND THIS TOOL DOES NOT MODEL IT. A lithium cell near freezing can deliver a fraction of its rated energy, and it recovers when warm - so a device that dies in an hour outdoors is not necessarily faulty. Charging a lithium battery below freezing, on the other hand, does permanent damage, which is a genuine hazard rather than an inconvenience. BATTERIES ALSO AGE. A pack a few years old may hold 80% of its original rating or less, and the label never changes. If you are sizing something that matters, measure the real capacity rather than trusting the print.

Common questions

Why does my 10,000 mAh power bank not charge my 3,000 mAh phone three times?

Because milliamp-hours are only comparable at the same voltage, and these two are not. The cells in the pack run at about 3.7 V, so 10,000 mAh is roughly 37 Wh of energy. The USB port hands that out at 5 V, and the conversion costs around ten percent - leaving about 33 Wh, which at 5 V is roughly 6,660 mAh. Your phone then loses a little more converting it back down to charge its own cells. Two and a half charges is a realistic result and nothing is faulty; the label is measured at the cell voltage, which is the industry convention, and the port simply runs at a different one.

What unit should I compare batteries in?

Watt-hours, every time. Milliamp-hours measure charge rather than energy, so they only compare fairly between batteries at identical voltage - which is why a 5,000 mAh phone battery and a 5,000 mAh 12 V pack are nothing like the same thing. Amp-hours multiplied by volts gives watt-hours, and that figure is directly comparable across any battery, any chemistry and any voltage. It is also the unit airlines use for their carry-on limits, which is a useful hint about which one actually means something.

What should I use for efficiency?

About 90% for a single conversion stage, and considerably less for a round trip. If you are charging one battery from another, energy is lost at both ends - the pack boosting up to USB voltage and the device converting back down to its cell voltage - so 75 to 80 percent is a fairer figure for the whole journey. An inverter running mains equipment from a 12 V battery sits in a similar range. Cables and connectors take a small share too, which matters more at high current than most people expect.

Why does depth of discharge matter?

Because for some chemistries the rated capacity is not the usable capacity. A lithium pack can use nearly all of what it holds, and its protection circuit stops it before any harm is done. A lead-acid battery is different: routinely taking it below about half its capacity shortens its life dramatically, so a 100 Ah lead-acid battery should be planned as a 50 Ah battery. That single factor is the most common reason an off-grid or backup system is sized at half what it needed to be, and it is why the setting exists rather than being assumed.

My device dies much sooner than this predicts. Why?

Most often because the average draw is higher than assumed. Screens, radios transmitting, and motors starting all pull far more than a datasheet's idle figure, and a device that spends a few seconds a minute doing something expensive has a much higher average than it looks. Cold is the other big one: a lithium cell near freezing delivers a fraction of its rated energy and recovers when it warms up, so poor outdoor runtime is often temperature rather than a fault. Age is the third - a pack a few years old may hold 80 percent of its original rating while the label still says otherwise.

Does this work for a car or leisure battery?

Yes, with the voltage set to 12 and the discharge depth set honestly. A 100 Ah 12 V battery is 100,000 mAh at 12 V, or 1,200 Wh - and at a sensible 50 percent depth for lead-acid you are planning around 600 Wh before inverter losses. Running mains equipment from it costs another ten to fifteen percent through the inverter. The arithmetic is identical to a phone power bank; only the numbers change, which is rather the point of working in watt-hours.

Is it safe to charge a battery in the cold?

Discharging a lithium battery in the cold is fine and the lost capacity comes back when it warms. CHARGING one below freezing is not - it causes permanent damage to the cell, and repeated attempts can create a genuine safety hazard rather than just reduced life. Many devices refuse to charge when they detect low temperatures for exactly this reason, which is a feature rather than a fault. Warm the battery to room temperature before charging it, and treat a device that will not charge in the cold as behaving correctly.

Take it further with AI

Copy this into ChatGPT or Claude with your own numbers filled in. It hands over the figures this calculator worked out, so the answer is built on real arithmetic instead of a guess.

I used the Battery Life Calculator at https://www.bfcbrilliance.com/tools/battery-life-calculator.

What I entered:
- Battery capacity (mAh): ___
- Battery voltage (V): ___
- Average power the device draws (W): ___
- Conversion efficiency (%): ___
- How deep you will discharge it (%): ___

What it calculated:
- How long it runs: ___
- That runtime in hours and minutes: ___
- Energy the battery holds: ___
- Energy you actually get out: ___
- What that is worth at USB 5 V: ___

Use those figures as given — they are already worked out, so please don't recalculate or estimate your own. Help me turn them into a plan: what to buy or do, in what order, roughly what it should cost, and the mistakes people most often make with this job.

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