Why Your Power Bank Holds Less Than It Says

mAh is not a capacity

Milliamp-hours measure charge, not energy. They only compare fairly between batteries running at the same voltage — and a power bank and the phone it's charging do not.

The unit that actually compares anything is the watt-hour: amp-hours × volts.

The power bank arithmetic

A 10,000 mAh pack runs its cells at about 3.7 V. So it holds:

10 Ah × 3.7 V = 37 Wh

The USB port hands that out at 5 V, and the conversion isn't free. At a typical 90% efficiency you have 33.3 Wh to give away — which at 5 V is about 6,660 mAh.

Two-thirds of the number on the box.

Nothing is faulty and nobody is lying. The label is measured at the cell voltage, which is the industry convention, and the port simply runs at a different one.

So how many phone charges?

Now add the loss at the other end — your phone converting 5 V back down to charge its own cells. Call the round trip 81% (0.9 × 0.9):

• Energy reaching the phone's battery: ~30 Wh
• A 3,000 mAh phone battery at 3.85 V: 11.55 Wh
• Charges: about 2.6

The naive 10,000 ÷ 3,000 says 3.3. The gap is entirely voltage and conversion.

Runtime, and what the label overstates

Same tool, different question. A device drawing 2.5 W average from that pack:

	Runtime
Ignoring losses	14.8 h
With 90% conversion	13.3 h

The label overstates by 11.1% — modest, because lithium can use nearly all its capacity and there's only one conversion stage.

Change either of those and it stops being modest.

Where it gets serious: lead-acid

A 100 Ah 12 V leisure battery is 1,200 Wh on paper. But routinely discharging lead-acid below about 50% shortens its life dramatically, so half of it isn't really yours.

	Energy
Rated	1,200 Wh
Usable at 50% depth, 90% inverter	540 Wh

The rated figure overstates by 122% — it's more than double the usable energy. Running a 100 W load gives 5.4 hours, not 12.

This is the single most common reason an off-grid or backup system ends up sized at half what it needed. Lithium behaves completely differently, which is exactly why the tool asks rather than assuming.

Efficiency is every stage multiplied

One conversion is about 90%. But losses stack:

• Battery to battery — both ends convert, so 75–80% for the round trip
• Inverter running mains kit from 12 V — a similar range
• Cables and connectors — small, but larger at high current than people expect

At 78% round trip, that same power bank drops to 28.9 Wh usable and the label overstates by 28.2%.

Three things that aren't in the arithmetic

Average draw isn't idle draw. Screens, radios transmitting and motors starting pull far more than a datasheet's resting figure. A device spending a few seconds a minute doing something expensive has a much higher average than it looks — and average is what runtime needs.

Cold. A lithium cell near freezing delivers a fraction of its rated energy, and recovers when it warms. Poor outdoor runtime is often temperature rather than a fault.

⚠️ But charging a lithium battery below freezing causes permanent damage, and repeated attempts are a genuine safety hazard rather than just reduced life. Many devices refuse to charge when cold — that's the device behaving correctly, not failing.

Age. A pack a few years old may hold 80% of its original rating while the label still says otherwise. If you're sizing something that matters, measure the real capacity rather than trusting the print.

The habit worth keeping

Compare in watt-hours, always.

A 5,000 mAh phone battery and a 5,000 mAh 12 V pack are nothing like the same thing, and mAh will never tell you that. It's also the unit airlines use for carry-on limits — a decent hint about which number actually means something.