The Truth About Battery Capacity: Why Your 5000mAh Power Bank Feels Smaller

I recently had to buy yet another power bank. I already own a 20,000mAh one that can even charge a laptop, but it's way too heavy to carry in a pocket when all you need is a phone top-up. So, I decided to get something smaller, lighter, and more compact. Since my phone’s battery is around 4000mAh (or at least that’s what I recalled), I figured a 5000mAh battery should charge my phone one to two times. Even the packaging on these power banks usually promises something like that.

From past experience, I knew these claims might be a bit optimistic. Years ago, I bought a couple of USB meters for a personal project, not to measure power banks but to monitor phone power draw. This time, they came in handy.

I had a small budget and went for something cheap — super cheap. Sure, there were better-known brands like Verbatim offering extras like wireless charging, but at 3x the price, I wasn't ready to commit. All I really needed was USB-C and a regular USB port to support older devices. Eventually, the thought crept in: maybe I should’ve invested in something higher quality. But I brushed it off — if this one dies in a year, I’ll recycle it like I do with all my dead batteries and move on.

Out of curiosity, I decided to test this new power bank. I charged it to 100%, let my phone drop to 20%, hooked up the USB meter, and began measuring. The meter showed readings in mWh, V, A, and W. To calculate capacity in mAh, I divided mWh by voltage:

13760 mWh / 5V = 2752 mAh

That’s... way less than 5000mAh. Like, nearly half. I wondered — maybe capacity is based on how much energy it takes in during charging? That would make more sense, though still a bit shady. So I measured that:

19724 mWh / 5V = 3944 mAh

Better, but still not close to 5000. After some digging, here’s what I found:

Lithium-ion batteries have a nominal voltage of 3.7V (discharged around 3.0V, fully charged around 4.2V)
Lithium-polymer variants often average around 3.8V
Ni-MH batteries typically operate at 1.2V
Capacities listed in mAh are based on the nominal voltage, which depends on the battery chemistry

So the industry-standard way to report capacity is:

mWh: shows the real usable energy
mAh: shows electric charge but only makes sense if voltage is known

Example:

5000mAh @ 3.7V = 18500mWh
5000mAh @ 5V = 25000mWh

Same mAh, different energy levels depending on voltage.

Conclusion: If a battery lists its capacity in mAh, it must also state the voltage. Otherwise, it’s misleading. And most power banks say they charge and discharge at 5V — not 3.7V — which creates confusion.

Why does this matter?

Because inside your power bank is a 3.7V battery. But USB is 5V. That means:

When charging the power bank: 5V is stepped down to ~4.2V, losing energy in conversion
When discharging to charge your phone: 3.7V is boosted up to 5V, again losing energy (~10–15%)

This is true for standard USB-A and many USB-C ports — but modern power banks and phones with USB Power Delivery (PD) can negotiate higher voltages like 9V, 12V, 15V, or even 20V, depending on the device. This allows for faster charging, but conversion still happens, and energy loss is still present.

Also, fast charging isn't free — higher power means more heat, especially in the receiving device. Heat is wasted energy and accelerates battery wear, meaning that fast charging is often less efficient overall, even if it’s convenient.

So in both directions — in and out — energy is lost. That’s why your 5000mAh power bank might only give you 2700–3200mAh at 5V output.

Real-world test results

Before we get to the numbers, it's worth mentioning how the batteries were calibrated for accuracy. Calibration was done by fully discharging each battery to 0%, then charging it to 100% — and repeating this process 2–3 times. This helps the internal battery controller (BMS) better track actual capacity and improves consistency across tests.

I tested two cheap 5000mAh power banks and one mid-range 20,000mAh power bank. All three were calibrated in advance — meaning I fully discharged them, recharged to 100%, then repeated the process 2–3 times to help reset their internal battery statistics and improve measurement accuracy. Two of the batteries were brand new, and the 20,000mAh one had fewer than 30 cycles. The test included 3 full charge and 3 full discharge cycles, and I used the average values normalized to 3.7V to make a fair comparison.

Cheap Battery A (5000mAh claimed)

Charge: 5538mAh → +6.62% over claimed
Discharge: 3795mAh → -24.09% under claimed
Loss: ~40.45% between charge input and discharge output

This suggests poor components or inflated specs.

Better Battery B (5000mAh claimed)

Charge: 5538mAh → +10.76% over
Discharge: 4160mAh → -16.8% under
Loss: ~33.12%

Much better performance, more honest capacity.

Mid-range Battery C (20000mAh claimed)

Charge: 22399mAh → +11.99%
Discharge: 16726mAh → -19.57%
Loss: ~33.91%

So what's the real usable capacity?

From these tests, a typical "real" output from a power bank is about 80% of the advertised mAh, assuming honest labeling. But then you also lose 10–20% more when your phone charges from that power bank.

So even a 6000mAh battery only gives you ~4800mAh output at best, and maybe only 3800–4000mAh after your phone takes its share.

BUT WAIT — THERE’S MORE!

Most of the time, you're charging other devices that also use lithium-ion batteries — just like the power bank itself. And just like we talked about the “output tax” (energy lost when converting 3.7V to 5V on the way out), there’s also an “input tax” on the receiving device.

That means: even after the power leaves your power bank, your phone or tablet also loses another 10–20% converting 5V USB back into the internal 3.7V–4.2V battery it charges.

In short:

🔋 Your power bank already loses 15–20% on the way out
📱 Then the receiving device loses another 10–20% on the way in

The actual energy your phone gets can be 30–40% less than what your power bank claims on the box.

Let that sink in!

Which means — 2 full phone charges? Maybe, if you have a tiny phone. Most modern phones have 4000–5000mAh batteries themselves.

Tips When Choosing a Power Bank

One more thing to keep in mind — the input port and cable type matter, especially for high-capacity power banks (e.g. 10,000mAh and up). Charging a large battery through a basic micro-USB port will be much slower and less efficient.

Micro-USB inputs are limited in power delivery and are not ideal for modern, high-capacity banks.
USB-A to USB-C cables work, but they still use legacy USB power standards — usually limited to 5V at lower currents.
For faster charging, look for power banks with USB-C input supporting USB Power Delivery (PD) and pair them with PD-certified chargers and cables. This can drastically cut down charging time and improve efficiency.

⚠️ Don’t Underestimate the Cable

Even with a good charger and power bank, a cheap or thin cable can ruin everything. I tested several cables, and in some cases, the output was up to 4× lower compared to a quality cable — especially under fast charging.

A low-quality cable adds resistance, causing voltage drop and heat, and slowing down charging drastically.

Even if your charger supports 18W, 30W, or more, the phone may only receive 5–7W with a poor cable.

Always use short, thick, high-quality cables, preferably USB-IF or PD-certified, to get the full performance you're paying for.

In other words: a fast charger + a slow cable = a slow charger.

Pro tip from experience: flashy, colorful cables that look fun often perform terribly. They're more fashion than function.

Final Thoughts

For me, a good 5000mAh power bank is still worth it. I’m often just going from home to office and need a top-up. I’d rather carry something light than a brick. And honestly, the phrase “More power is always better” isn’t always true — sometimes, just enough is perfect.

Want to avoid marketing traps? Look for: