A 4,000 mAh power bank delivers about 0.7–1.1 full phone charges, based on device size and real-world efficiency.
Here’s the short version: a 4,000 mAh pack holds enough energy for roughly one full top-up on a small or mid-size phone, or a strong partial top-up on a large phone. The exact count depends on your phone’s battery capacity, converter losses, cable quality, and how you use the device while it’s plugged in.
How The Math Works (And Why Your Result Varies)
Power banks list capacity in milliamp-hours (mAh), but energy is better compared in watt-hours (Wh). Converting lets you match the pack to your device fairly. The industry guidance expresses Wh as capacity (in Ah) multiplied by nominal voltage (V). In plain terms: Wh = (mAh ÷ 1000) × V. The air-transport handbook explains the same Wh rating method used across the battery world (IATA lithium battery guidance). Most small power banks use 3.7 V lithium-ion cells. So a 4,000 mAh unit stores about 4.0 Ah × 3.7 V ≈ 14.8 Wh.
Not all of that energy reaches your phone. DC-DC conversion, heat, and cable losses trim the delivered energy. Battery-testing references describe these losses through coulombic and energy efficiency: the charge voltage is higher than the discharge voltage, which wastes some energy as heat (Battery University on efficiency). In the real world, a compact pack often delivers about 70–85% of its stored Wh to the device.
Quick Estimate Table For A 4,000 mAh Pack
Use this as a fast range check. Pick the phone battery size closest to yours and scan the two efficiency columns. This table lands within the first third of the page so you can act fast.
| Phone Battery (mAh) | Full Charges (85% Eff.) | Full Charges (70% Eff.) |
|---|---|---|
| 3,000 | ~1.3 | ~1.1 |
| 3,500 | ~1.1 | ~0.9 |
| 4,000 | ~1.0 | ~0.8 |
| 4,500 | ~0.9 | ~0.7 |
| 5,000 | ~0.8 | ~0.6 |
| 5,500 | ~0.7 | ~0.6 |
Step-By-Step: Estimate Your Own Charge Count
This method takes a minute and gives a realistic number tailored to your phone and usage.
1) Convert The Pack To Watt-Hours
Multiply the power bank’s capacity (Ah) by 3.7 V. For a 4,000 mAh unit: 4.0 Ah × 3.7 V ≈ 14.8 Wh. The Wh formula is standard across battery handling references, including airline safety guides that cite international test methods (IATA lithium battery guidance).
2) Apply A Realistic Efficiency
Pick 0.85 for a quality pack with modest draw, or 0.70 for a basic unit, fast charging, or a warm day. Battery engineering primers explain why charging uses a higher voltage than discharge, and why that gap shows up as heat, not useful energy (Battery University on efficiency).
3) Convert Your Phone To Watt-Hours
Phone batteries are also rated at roughly 3.7–3.85 V nominal. A 5,000 mAh phone is roughly 5.0 Ah × 3.85 V ≈ 19.3 Wh. If you only know mAh, just use 3.85 V as a solid default for modern phones.
4) Divide Delivered Wh By Phone Wh
Delivered Wh = 14.8 × efficiency. At 85% that’s ~12.6 Wh; at 70% that’s ~10.4 Wh. Full charge count = Delivered Wh ÷ Phone Wh. For a 5,000 mAh phone (~19.3 Wh), you land near 0.65–0.54 charges. For a 4,000 mAh phone (~15.4 Wh), you land near 0.82–0.68 charges. For a 3,500 mAh phone (~13.5 Wh), you land near 0.93–0.77 charges.
Close Variant: How Many Phone Charges From 4000 mAh — Realistic Range
Here’s a clear breakdown of the main factors that push the count up or down.
Pack Quality And Heat
Higher quality converters waste less energy as heat. Heat rises during fast charging and during heavy phone use while charging. When the pack or device feels warm, efficiency drops and the count slides.
Charging Speed And Protocol
Fast protocols raise voltage and current to move energy quicker. That speed can add conversion loss. Standards like USB Power Delivery describe the voltage steps and programmable power supply modes used by many phones; higher settings move charge fast, but the wall-to-battery path sheds more energy as heat (USB Power Delivery overview).
Cable And Connector Losses
Long, thin, or damaged cables add resistance. That resistance lowers the voltage at the phone end, which forces the converter to work harder and spill a bit more energy as heat.
Background Use While Charging
Screen time, gaming, LTE hotspot mode, or video recording draw power while the pack is filling the phone. Energy spent running the phone cannot go into the battery, so the “number of full charges” score drops.
Worked Examples You Can Copy
Small Or Mid-Size Phone (~3,300–3,800 mAh)
Phone Wh ~12.7–14.6. Delivered Wh from the pack: ~10.4–12.6 (70–85%). Charge count ≈ 0.7–1.0. With the screen off during charging and a short cable, you’ll be closer to the high end.
Large Phone (~4,500–5,000 mAh)
Phone Wh ~17.3–19.3. Delivered Wh: ~10.4–12.6. Charge count ≈ 0.5–0.7. Enough for a long evening, not a weekend away.
Compact Accessories
Earbuds cases and watches sip energy. A small pack can top them many times, even when it can’t fill a large phone once.
Table Of Common Accessories And Rough Charge Counts
This second table sits in the latter half of the page to give you a handy planner once you’ve read the method.
| Accessory Type | Typical Battery (Wh) | Approx Charges From 4,000 mAh Pack |
|---|---|---|
| Wireless Earbuds Case | 1.3–2.0 | ~5–9 |
| Smartwatch | 0.3–0.6 | ~17–42 |
| Fitness Band | 0.2–0.4 | ~26–63 |
| Compact Camera | 6–9 | ~1.1–2.1 |
| Handheld Console | 13–20 | ~0.5–1.0 |
Tips To Squeeze More From A Small Pack
Top Up At Higher Battery Percentages
Charging from 30–80% is usually gentler than pushing from near-empty to 100%. The pack runs cooler and wastes less energy.
Use Short, Good-Gauge Cables
Short cables with thick conductors cut voltage drop. Less drop means less heat and better end-to-end delivery.
Keep It Out Of Hot Pockets And Sun
Both the pack and the phone run more efficiently when they’re not baking. Shade and airflow help.
Pause Screen-Heavy Tasks While Charging
If you need every watt-hour, dim the screen and pause gaming or video capture until you reach the target level.
Why Many Charts Disagree
You’ll see guides claim tidy “x charges” numbers that don’t match your experience. Here’s why numbers drift:
- Different Nominal Voltages: Phone cells often rate near 3.85 V; packs may be 3.6 or 3.7 V cells. Small shifts change Wh on both sides.
- Efficiency Guesswork: Some charts assume 90%+ with perfect cabling and slow charge. Others assume 60% with fast protocols and warm hands. Real life sits between those.
- Device Overhead: Active radios, bright screens, GPS, and background compute shave the delivered energy before it reaches the battery.
A Deeper Look At Efficiency (Plain English)
Think of the converter inside the power bank like a translator passing energy to your phone at a voltage the phone expects. The translator needs headroom, so it raises voltage above the battery’s level during charging. That gap turns into heat in the converter, cable, and phone. Battery-testing primers describe this as coulombic and energy efficiency; the takeaway is simple: some energy is lost on the way in, and faster charge modes lose a bit more (Battery University on efficiency).
When A 4,000 mAh Pack Makes Sense
City Days And Commutes
Need a light backup for a train ride, a long workday, or a night out? A small pack is pocket-friendly and covers maps, messages, and a ride-share home.
Travel With A Laptop Bag
If you already carry a bag, weight matters less. A larger pack (10,000–20,000 mAh class) keeps phones, earbuds, and a compact console going through a layover. That larger size does add bulk at security checkpoints, but the trade-off suits long travel days.
Kids And Shared Charging
For family outings, a small pack serves as an emergency buffer. If multiple phones need juice, step up in size so the day doesn’t end with a red bar.
FAQ-Style Clarity Without A FAQ Block
Does Fast Charging Change The Count?
It can. Higher voltages and currents mean more stress on converters. You’ll still get a solid bump in battery level, but the “number of full charges” score trends lower than slow-charge runs of the same pack. USB PD modes raise voltage to move energy faster; that’s great for speed, with a small trade-off in delivered Wh (USB Power Delivery overview).
Why Use Watt-Hours Instead Of Only mAh?
mAh alone hides voltage. Wh compares energy apples-to-apples, which is the clean way to estimate charge count. The airline handbook lays out the Wh method that airlines and shippers use globally (IATA lithium battery guidance).
Copy-And-Keep Formula
Estimated full charges = (Power-bank mAh × 3.7 V ÷ 1000) × efficiency ÷ (Phone mAh × 3.85 V ÷ 1000)
Use 0.85 for a good cable and calm use. Use 0.70 for fast-charge runs, heat, or heavy screen time.
Practical Scenarios
Concert Night
You shoot short clips and navigate after. A 4,000 mAh pack bumps a 4,500 mAh phone from 20% to roughly 70–80% before the headliner ends. That’s a comfortable ride back.
Weekend Hike With Light Use
A 3,500 mAh phone that spends most of the day in airplane mode gets close to a full top-up from a small pack. Add a watch and earbuds and you’ll still have enough for photos at the overlook.
Workday Tethering
Hotspotting drains the phone quickly. Expect about half to two-thirds of a full refill on a large device, which is still enough to finish a long call and upload a doc.
Buy Or Skip?
Get a 4,000 mAh pack if you want pocketable peace of mind and your phone lives in the 3,300–4,000 mAh range. Step up in size if your phone’s a 5,000 mAh model, you rely on fast charging, or you often charge while streaming or gaming. Match your pick to the days you live, not a spec sheet.