A 4,000 mAh power bank usually delivers about 1–1.5 phone recharges, depending on device size and efficiency.
A compact 4,000 mAh bank carries a modest amount of energy. The headline question boils down to three things: usable energy after conversion losses, the battery size of the device you’re topping up, and the way you charge. This guide runs the quick math, then shows how to squeeze the most from a small pack without guesswork.
Charges From A 4,000-mAh Bank: The Short Math
Phone and power-bank cells store energy around 3.6–3.7 V internally. USB outputs deliver 5 V (or higher with fast-charge modes), so a boost converter does the voltage step-up. Some energy turns into heat during that step and during the phone’s own charging step. A good real-world range for total efficiency sits near 70–85% based on major battery makers’ explanations and field tests.
Back-Of-Envelope Formula
Estimated charges ≈ (Bank mAh × 3.7 × efficiency) ÷ (Device mAh × 3.85)
Why those numbers? Most Li-ion packs are rated around 3.7 V internally, and many phones use cells specified near 3.85 V. The ratio lands you in the right ballpark for “full-to-dead” counts.
Quick Answer Table For Common Device Sizes
The table below uses an 80% efficiency estimate to keep it simple. If your bank or cable is less efficient, your count slides down a bit; if everything runs cool and tidy, it slides up.
| Device Battery (mAh) | Estimated Full Charges* | Notes |
|---|---|---|
| 2,000 | ~1.5–1.7× | Small phones, earbuds cases, cameras |
| 2,500 | ~1.2–1.4× | Compact phones |
| 3,000 | ~1.0–1.2× | Older or slim models |
| 3,500 | ~0.9–1.0× | Mid-range phones |
| 4,000 | ~0.8–0.9× | One near-full top-up |
| 4,500 | ~0.7–0.8× | Most modern Androids |
| 5,000 | ~0.6–0.7× | Large-battery phones |
*Range accounts for temperature, cable quality, and charging speed.
What Drives The Real Count
Two identical banks can yield different results because the charging chain has several stages. Each stage trims a little energy. Here’s what shifts the outcome the most.
Conversion Losses In The Bank
A boost converter raises the cell’s native ~3.7 V to the 5 V expected over USB (or 9–20 V in fast-charge modes). High-quality designs keep losses low, but they aren’t zero. Anker, a top power-bank brand, pegs practical efficiency figures around the low-80s in its capacity explanations, which matches hands-on measurements many users see on USB testers. See the brand’s technical note on conversion efficiency and “rated capacity” math for a clear breakdown.
Phone-Side Charging Overhead
Inside the phone, a buck converter lowers the incoming voltage to the cell’s charging level. That step and the phone’s battery management add small overheads. When devices run hot or when fast-charge modes push high power, losses rise further.
Cell Voltage Reality
Manufacturers list milliamp-hours, but energy is milliamp-hours times voltage. Most single Li-ion cells are specified near 3.6–3.7 V; that’s the number you need for watt-hours, which is the real currency of stored energy. Battery University’s reference on voltage terminology aligns with this industry practice and explains why nominal values vary slightly across data sheets; see the primer on nominal Li-ion voltages.
Battery Size Of Your Device
Modern phones often list 4,000–5,000 mAh typical capacities on their spec pages. A bank with 4,000 mAh simply holds less total energy than a phone with 5,000 mAh, even before losses. That’s why a small bank usually buys you a single safety top-up, not a weekend of power.
Worked Examples You Can Copy
Use these as templates. Swap your device size and adjust the efficiency number if you know your setup runs cooler or hotter than average.
Example A: 3,000 mAh Phone
Bank: 4,000 mAh at 3.7 V → 14.8 Wh stored. At 80% efficiency → 11.84 Wh usable at the port.
Phone: 3,000 mAh at ~3.85 V → ~11.55 Wh per full charge.
Result: 11.84 Wh ÷ 11.55 Wh ≈ ~1.0× full charge.
Example B: 4,500 mAh Phone
Bank: same as above → ~11.84 Wh usable.
Phone: 4,500 mAh at ~3.85 V → ~17.33 Wh.
Result: 11.84 Wh ÷ 17.33 Wh ≈ ~0.7× charge.
Example C: Earbuds Case (500–600 mAh)
Bank usable: ~11.84 Wh.
Case energy: 600 mAh × 3.85 V ≈ 2.31 Wh.
Result: ~11.84 Wh ÷ 2.31 Wh ≈ ~5× case recharges.
Dialing In Your Own Number
You can get surprisingly close with three quick checks. No meters needed.
Step 1: Find Your Device’s Battery Size
Look on the phone maker’s tech-spec page or the retail box. Many current Android models sit near 4,500–5,000 mAh, while compact models land a bit lower. Some makers publish “typical” and “rated” values; either works for rough math.
Step 2: Use 0.8 As A Practical Efficiency
Unless you’re slow-charging with a top-tier cable in a cool room, 80% is a fair middle ground. Hot conditions, long or thin cables, and constant fast-charge bursts slide real-world efficiency toward the low-70s. Cooler sessions and moderate currents nudge it toward the mid-80s.
Step 3: Plug The Numbers Into The Formula
Example for a 4,000 mAh bank feeding a 4,000 mAh phone: (4000 × 3.7 × 0.8) ÷ (4000 × 3.85) ≈ 0.77× — close to one solid top-up from 20–30% back to full.
Why Your Count Can Change Day To Day
Some days you’ll squeeze out a little more. Some days a little less. These are the levers.
Cables And Adapters
Short, thicker USB-C leads keep voltage sag in check. Old, frayed, or ultra-long cables waste energy as heat. Swapping to a better cable often adds a few percentage points of delivered energy.
Fast-Charge Modes
Quick Charge and USB Power Delivery raise voltage for speed. That can shrink losses in the cable itself, but the higher power raises heat and converter losses. If you only need a small top-up, a slower, cooler session can end up more efficient overall.
Screen-On Charging
Streaming, navigation, and gaming during a charge siphon energy that never reaches the battery. Airplane mode or a screen-off break turns more of the bank’s watt-hours into stored charge.
Temperature
Hot dashboards and sunny patios aren’t friendly to efficiency. Keep both bank and phone shaded, and you’ll see steadier results.
Reality Check Against Specs You See Online
Brands list milliamp-hours because it’s simple, yet watt-hours tell the full story. A quick mental conversion helps sanity-check claims anywhere on the web:
- Power bank energy: 4,000 mAh × 3.7 V ≈ 14.8 Wh stored.
- Usable at the port: ~70–85% of that → about 10–12.6 Wh.
- Phone energy: device mAh × ~3.85 V.
If a spec sheet or ad claims multiple full phone charges from a small bank, check the math with those three bullets. It takes seconds and keeps expectations realistic.
Mid-Range Phones: How A Small Bank Performs
Here’s a second table with example phones by battery class and what a 4,000 mAh pack tends to deliver. These are estimates, not lab reports, and assume moderate efficiency.
| Battery Class | Phone Examples* | Typical Result |
|---|---|---|
| ~3,000–3,300 mAh | Compact iOS/older flagships | ~1.0–1.2× full charges |
| ~4,000–4,500 mAh | Many mid-range Androids | ~0.7–1.0× depending on usage |
| ~5,000 mAh | Large-battery models | ~0.6–0.7×; a healthy top-up |
*Check your exact model’s spec page for the stated “typical” capacity.
How To Stretch A Small Pack Further
Traveling with a pocket-size bank is still worth it. A few easy tweaks boost delivered energy and speed.
Pick The Right Cable
- Use a short, well-made USB-C cable with solid strain relief.
- Avoid overly thin or mystery-brand leads that droop voltage under load.
Charge Smarter, Not Hotter
- Fast-charge when you’re in a rush; default to a gentler session when time allows.
- Keep both devices off hot surfaces, out of direct sun, and away from vents.
Trim Background Drains
- Flip on battery saver or airplane mode while topping up.
- Close power-hungry apps; let the phone rest screen-off for part of the session.
When To Size Up
Pick capacity by use case, not by a round number. If you rely on maps, tethering, or a camera all day, a small pack may leave you short by afternoon. Stepping from 4,000 mAh to 10,000 mAh roughly triples stored energy and feels far more relaxed for a long day out. For overnight camping or multi-device kits, 20,000 mAh and up starts to make sense.
Specs And Claims: Reading Between The Lines
Two more tips help decode marketing pages:
- Rated vs. typical capacity: Some makers list both for phones. “Typical” reflects average production; “rated” is a minimum. Differences are normal and small.
- Efficiency claims: If a page quotes only output milliamp-hours, look for the footnote. Credible brands often explain that internal cell capacity at ~3.7 V isn’t equal to output at 5 V, and they show the math so buyers won’t be misled.
Why This Math Matches How Batteries Work
Energy, not milliamp-hours alone, tells you the number of charges. Li-ion basics place internal voltage near 3.6–3.7 V, USB runs at 5 V or more, and converters bring you from one to the other with some loss. That’s the entire story in a paragraph and the reason your pocket bank seldom equals its label in real-world output.
Reference Notes
For readers who want source material: a major power-bank maker’s article breaks down why output capacity is lower than the number printed on the shell and shows sample math for conversion losses (Anker rated-capacity explainer). For background on cell voltages and why 3.6–3.7 V appears across spec sheets, Battery University’s overview remains a handy technical primer (Li-ion nominal voltage).
Bottom Line For A 4,000-mAh Pack
Expect roughly one phone refill on a mid-size handset, a heavier top-up on a large-battery model, or several rounds for smaller gadgets like earbuds cases and action cameras. Pack a better cable, charge a bit cooler, and the odds tilt in your favor.