How Long Will Your Power Bank Last? The Honest mAh Loss Guide

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Amon37210

April 13, 2026

On April 13, 2026

Gadgeterians.com · Power Backup Series – Part 4

Your 10,000mAh power bank does NOT give you 10,000mAh. Here is the exact science behind why – and how to calculate what you actually get.

Updated: April 2026
9 min read
Gadgeterians Team

The Short Answer

A 10,000mAh power bank delivers roughly 6,000–7,000mAh to your phone. A 20,000mAh one gives about 12,000–14,000mAh. You always lose 30–40% to physics – not fraud. This guide explains why, shows the exact formula, and tells you how to pick the right size for your actual needs in Bangladesh.

You buy a 10,000mAh power bank. You expect to charge your 5,000mAh phone twice. But in real life, you barely get one and a half charges. You think the product is fake. Your friend thinks yours is defective. You both go back to Daraz to write a 1-star review.

Here is the truth: no power bank on earth gives you its full rated mAh – not Baseus, not Anker, not Xiaomi, not Samsung. It is not a scam. It is physics. And once you understand it, you will never be confused again.

This guide breaks down exactly where your mAh goes, gives you a simple formula to calculate the real output of any power bank, and helps you pick the right capacity for daily load-shedding in Bangladesh. This is Part 4 of our complete load-shedding survival guide for Bangladesh – which covers all backup gadgets together.

What This Guide Covers

Why your power bank can never deliver its rated mAh
The 3 losses that eat your capacity – explained simply
The real output formula – step by step
Real output table for every common power bank size
How many times can you charge popular Bangladesh phones?
How quality brands vs fake brands compare in real tests
Heat, cables, and cold – extra losses you never think about
How to pick the right mAh for your situation
FAQ

1. Why Your Power Bank Can Never Deliver Its Full Rated mAh

The number printed on your power bank – 10,000mAh, 20,000mAh, 30,000mAh – is the capacity of the internal battery cells. These cells store energy at 3.6–3.7 volts.

Your phone charges at 5 volts through the USB port. Your phone’s own battery then stores that charge at 3.7–4.2 volts internally.

Every time voltage is converted from one level to another, energy is lost as heat. This is not a manufacturing defect. It is a law of physics called energy conservation. You cannot convert voltage without losing some energy – just like you cannot pump water uphill without some spilling.

The Energy Journey From Power Bank Cell to Your Phone Battery

3.7V

Power bank cell

→

Loss 1: voltage boost

USB output

→

Loss 2: cable heat

Phone charging port

→

Loss 3: phone circuit

3.7–4.2V

Phone battery fills

Every conversion step wastes energy as heat. Three conversions = 30–40% total loss.

This is why a 10,000mAh power bank with perfectly good cells and a quality circuit still only delivers around 6,000–7,400mAh to your device. The missing capacity was not stolen from you. It became hot in the converter circuit and your charging cable.

2. The 3 Losses That Eat Your mAh – Explained Simply

Your mAh disappears at three specific points. Understanding each one tells you exactly what you can and cannot control.

Voltage Conversion Loss – The Biggest One

The power bank cell holds energy at 3.7V. USB charges at 5V. A DC-DC boost converter inside the power bank steps up the voltage. This step alone causes a 26% theoretical loss – regardless of brand or quality – because energy (Wh) is conserved, but the mAh count drops when voltage rises.

Theoretical 5V output = (3.7 × rated mAh) ÷ 5 = 74% of rated capacity

A 10,000mAh bank can theoretically deliver max 7,400mAh at 5V – even with zero losses

Circuit Efficiency Loss – This Is Where Quality Matters

The boost converter circuit is not perfect. Some energy converts to heat inside the circuit itself. This is where brand quality makes the biggest difference. Premium brands (Baseus, Anker, Xiaomi) achieve 85–93% circuit efficiency. Cheap no-brand units can drop to 60-70%, which is why budget power banks feel warm quickly and die faster.

Brand Tier	Circuit Efficiency	Examples in Bangladesh
Premium	88–93%	Anker, Baseus (Bipow series), UGREEN
Mid-range	82–88%	Xiaomi, Oraimo, Remax, Joyroom
Budget	70–82%	Generic brands, some Walton entry models
Fake / No-brand	40–65% (or cells are not the rated size)	Facebook page sellers, ৳400-600 “50,000mAh” units

Phone Charging Circuit Loss – The One Nobody Talks About

Your phone’s own charging circuit also converts voltage from 5V (or 9V with fast charge) down to the 3.7-4.2V that its internal lithium battery needs. This conversion also loses energy as heat – that is why your phone gets warm while charging. This adds another 5-10% loss.

This is why two phones with the same battery size (e.g., both 5,000mAh) can get different amounts of charge from the same power bank. A Samsung with efficient charging circuits gets more than a cheap phone with poor circuitry.

3. The Real Output Formula – Calculate Any Power Bank in 3 Steps

You can calculate any power bank’s real usable output with this simple formula. It works for every brand and every size.

The 3-Step mAh Reality Check

Step 1

Find the stored energy in Watt-hours (Wh)

Wh = (Rated mAh × 3.7) ÷ 1000

Example: 10,000mAh × 3.7 ÷ 1000 = 37 Wh

Step 2

Apply the total efficiency (circuit + phone charging losses)

Usable Wh = Wh × 0.60 to 0.75

Use 0.75 for premium brands (Baseus, Anker) · 0.65 for mid-range · 0.60 for budget

Example (Baseus): 37 Wh × 0.75 = 27.75 Wh usable

Step 3

Convert back to mAh at 5V output

Usable mAh = (Usable Wh ÷ 5) × 1000

Example: (27.75 Wh ÷ 5) × 1000 = 5,550 mAh

Combined Single Formula

Usable mAh = Rated mAh × 3.7 ÷ 5 × Efficiency

Efficiency = 0.75 (premium) · 0.65 (mid-range) · 0.60 (budget)

Quick Sanity Check

If your power bank delivers more than (Rated mAh × 3.7 ÷ 5) at 5V – it is lying about its capacity. No power bank can output more energy than its cells store. This number is the hard physical ceiling. Anything claiming to exceed it is fake.

4. Real Output Table – Every Common Power Bank Size

Use this table to instantly look up what any power bank actually delivers. We show three columns – for premium, mid-range, and budget brands – so you can compare what you are really buying at each price point.

Rated mAh	Max Possible (physics ceiling)	Premium Brand 75% efficiency	Mid-Range Brand 65% efficiency	Budget Brand 60% efficiency
5,000 mAh	3,700 mAh	2,775 mAh	2,405 mAh	2,220 mAh
10,000 mAh	7,400 mAh	5,550 mAh	4,810 mAh	4,440 mAh
20,000 mAh ★	14,800 mAh	11,100 mAh	9,620 mAh	8,880 mAh
25,000 mAh	18,500 mAh	13,875 mAh	12,025 mAh	11,100 mAh
30,000 mAh	22,200 mAh	16,650 mAh	14,430 mAh	13,320 mAh
“50,000 mAh” ৳500	37,000 mAh IMPOSSIBLE	Fake cells – 1,500–3,000 mAh actual	SCAM	SCAM

★ 20,000mAh is the recommended sweet spot for most Bangladeshi households dealing with 1–4 hour daily outages.

5. How Many Times Can You Charge Your Phone? Bangladesh Edition

Now, let us make this practical. Here are the phones most commonly used in Bangladesh, paired with the most popular power bank sizes, showing real charge counts – not the misleading marketing math.

Phone (Battery Size)	10,000mAh PB (premium, ~5,550mAh usable)	20,000mAh PB (premium, ~11,100mAh usable)	10,000mAh PB (budget, ~4,440mAh usable)
Symphony Z50 / Z55 (5,000mAh)	~1.1 charges	~2.2 charges	~0.9 charges
Redmi Note 13 (5,000mAh)	~1.1 charges	~2.2 charges	~0.9 charges
Samsung Galaxy A15 (5,000mAh)	~1.1 charges	~2.2 charges	~0.9 charges
Samsung Galaxy A55 (5,000mAh)	~1.1 charges	~2.2 charges	~0.9 charges
iPhone 15 (3,349mAh)	~1.6 charges	~3.3 charges	~1.3 charges
Walton Primo RX8 Mini (4,000mAh)	~1.4 charges	~2.8 charges	~1.1 charges

The Key Insight

A 10,000mAh premium power bank gives about 1.1 full charges for a 5,000mAh phone – not 2 as the marketing math suggests. This is the single most important number to understand before buying. If you need 2 full charges during load-shedding, buy 20,000mAh, not 10,000mAh.

6. Quality Brands vs Fake Brands – The Real Gap in Bangladesh

This is where buying from a trusted source matters. The mAh loss between a genuine 10,000mAh Baseus and a fake “10,000mAh” Facebook page product is not 30% – it is often 70–80%.

Genuine 10,000mAh (Baseus Bipow)

Rated: 10,000mAh at 3.7V

Cell quality: Grade A lithium-ion

Circuit efficiency: 88–91%

Real output: 5,200 – 5,800 mAh

Result: 1.0 – 1.15 full charges of a 5,000mAh phone. Consistent across all cycles. Stays close to this number for 300+ charge cycles.

Fake “10,000mAh” (Facebook ৳500)

AVOID

Claimed: 10,000mAh (printed on label)

Actual cells: 1,500 – 3,000mAh recycled or B-grade

Circuit efficiency: 55–65%

Real output: 700 – 1,500 mAh

Result: Barely charges your phone from 20% to 60%. Gets hot fast. Capacity drops to near-zero within 30–50 cycles. Fire risk with degraded cells.

Why Fake Power Banks Are Dangerous, Not Just Disappointing

Recycled or B-grade lithium cells can swell, leak, and in extreme cases catch fire – especially when left charging overnight or in a hot room during Bangladesh’s summer months. A cheap power bank left plugged in at 40°C ambient temperature is a genuine hazard. This is not an exaggeration. It is the reason quality control and brand accountability matter.

7. Heat, Cables, and Temperature – The Hidden Losses You Never Think About

Even with a premium power bank, you can lose additional capacity if you charge in bad conditions. These losses stack on top of the voltage conversion losses already calculated above.

Condition	Extra Loss	Bangladesh Context
Using a thin micro-USB or old cable	3 – 8% extra loss	Thin wires have high resistance. Use the original cable or a thick USB-C cable.
Ambient temperature above 35°C	5 – 12% extra loss	Bangladesh’s summer averages 36–40°C. Charging during a hot load-shedding night loses more capacity than charging in the morning.
Charging and discharging simultaneously	10 – 20% extra loss	Charging your power bank from the wall while also charging your phone from it. Most power banks are inefficient in this mode.
Power bank left unused for 2+ weeks	5 – 15% self-discharge	Lithium cells self-discharge slowly. A fully charged power bank sitting untouched for 3 weeks loses ~8–12% capacity before you even start using it.
Battery age (after 300+ full cycles)	15 – 30% capacity fade	A 2-year-old 10,000mAh power bank used daily may only store 7,000–8,000mAh of usable energy internally – even before conversion losses.

Pro Tip for Bangladesh Summers

Always charge your power bank during the early morning – when temperatures are lower. Charging at 7am vs 3pm in summer can give you 8–12% more stored energy. Also, keep your power bank in a cool, shaded spot. Never leave it on a metal surface in direct sunlight – internal temperatures can reach 55°C+, which accelerates cell degradation.

8. Now You Know the Math – How to Pick the Right Size

Armed with the real numbers, here is how to choose correctly. The formula to find the right capacity is simple:

Formula to Find Your Ideal Power Bank Size

Required mAh = (Phone mAh × Number of charges) ÷ 0.65

0.65 = average real-world efficiency for mid-range brands in Bangladesh

Example: You have a 5,000mAh phone and want 2 full charges during load-shedding. Required mAh = (5,000 × 2) ÷ 0.65 = 15,385mAh. So you need at least a 20,000mAh power bank from a mid-range brand.

Your Goal	Phone Battery	Minimum Capacity Needed	Best Pick
Emergency backup, stay at 30%+	5,000 mAh	7,700 mAh	10,000mAh is fine
1 full charge (0% to 100%)	5,000 mAh	7,692 mAh	10,000mAh (premium brand)
2 full charges (survive a full day)	5,000 mAh	15,385 mAh	20,000mAh minimum
Charge 2 family phones once each	2 × 5,000 mAh	15,385 mAh	20,000mAh minimum
Freelancer – phone + earbuds + 1 top-up	5,000 + 500 mAh	17,000 mAh	20,000mAh minimum

This Article Is Part of a Bigger Series

Now that you understand real power bank capacity, make sure the rest of your load-shedding setup is also correct. Our full power backup guide for Bangladesh covers rechargeable fans, Mini UPS for WiFi routers, IPS systems, and a regional breakdown by outage hours.

Already know your capacity needs? Browse our tested and verified power bank range directly at Gadgeterians power banks page – every product is checked for real capacity before listing.

Frequently Asked Questions

Is it normal for my power bank to give less than the rated mAh?

Yes, it is completely normal for every power bank on the market. A premium 10,000mAh power bank realistically delivers 5,200–5,800mAh to your phone. This is because the internal cells store energy at 3.7V, but your phone charges at 5V – the conversion process loses 25–30% before the circuit, and cable losses are added. It is physics, not fraud.

How do I calculate how many times my power bank can charge my phone?

Use this formula: Charges = (Rated mAh × 3.7 ÷ 5 × Efficiency) ÷ Phone battery mAh. For a 10,000mAh Baseus (75% efficiency) charging a 5,000mAh phone: (10,000 × 3.7 ÷ 5 × 0.75) ÷ 5,000 = 5,550 ÷ 5,000 = 1.11 charges. So you get about one full charge plus a bit extra.

Why does my power bank get warm when charging?

Warmth is normal – it is the energy being lost as heat during voltage conversion. A quality power bank gets slightly warm. A fake or cheap power bank gets noticeably hot because its circuits are less efficient and waste more energy as heat. If your power bank becomes hot to the touch, stop using it and do not leave it charging unattended.

What is the difference between mAh and Wh on a power bank?

mAh (milliampere-hours) is the charge the internal cells store at 3.7V. Wh (watt-hours) is the actual energy stored, accounting for voltage. Wh = mAh × 3.7 ÷ 1000. A 10,000mAh power bank stores 37Wh of energy. Wh is the more honest number – it is voltage-independent and is what airlines use to determine if a battery is allowed on flights. When comparing power banks, Wh is the better metric.

Does fast charging reduce the amount of charge delivered?

Not significantly for the total charge delivered, but it does increase heat – which slightly reduces efficiency per session. Fast charging (22.5W, 33W, 65W) is still worth using because it minimises the time your phone spends plugged in during a load-shedding window. The trade-off of slightly higher heat loss is worth the speed advantage in Bangladesh’s unpredictable outage durations.

How many charge cycles does a power bank last in Bangladesh?

A quality power bank from Baseus, Anker, or Xiaomi lasts 300–500 full charge cycles before noticeable capacity loss. In Bangladesh, where many households charge their power bank once every 1–2 days during the load-shedding season, that means 1.5–3 years of regular use. Cheap power banks often fail after 50–100 cycles, making the cost-per-charge much higher over time.

Is a “50,000mAh” power bank for ৳500 from Facebook worth buying?

No. It is physically impossible. A genuine 50,000mAh power bank at 3.7V stores 185 Wh of energy. The lithium cells alone to build this legitimately cost more than ৳3,000–4,000 at wholesale. A ৳500 product claiming 50,000mAh either contains recycled B-grade cells with 1,500–3,000mAh actual capacity, or the label simply has a number printed on it with nothing real behind it. These products also carry fire and explosion risks. Never buy them.

Real Capacity. Real Warranty. Delivered Across Bangladesh.

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Written by

Gadgeterians Team

The Gadgeterians Team is made up of gadget enthusiasts, tech writers, and product testers based in Dhaka, Bangladesh. We test every product we sell and every claim we write about – from charging a phone 10 times to measure real mAh output, to running rechargeable fans for 12-hour sessions in summer heat. Our goal is simple: give Bangladeshis the most honest, practical gadget advice available.

Product Testing
Power Backup
Bangladesh Gadgets
Load-Shedding Solutions

Published on gadgeterians.com · Part of the Power Backup Series

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