Charge count equals power bank watt-hours × real-world efficiency, divided by your phone’s watt-hours.
Here’s a straight answer you can use today. A phone refuel count isn’t about raw milliamp-hours alone. The right way is to compare energy (watt-hours), then factor in losses from voltage step-up, cable resistance, and your phone’s charging electronics. Once you run that math, you’ll get a realistic number of full charges your power bank can deliver for your phone model.
Power Bank Charges For Your Phone: The Formula
Step 1: Convert capacities to watt-hours (Wh). Use the basic energy equation: Wh = (mAh × V) ÷ 1000. Power banks are rated at the cell’s nominal voltage (usually 3.6–3.7 V). Phones use lithium cells in the same ballpark. You can reference a manufacturer calculator or explainer for the same formula (see mAh → Wh formula).
Step 2: Apply usable efficiency. Some energy is lost when the bank boosts from ~3.7 V up to 5 V (or a PD voltage), and then your phone steps it down again. A practical efficiency range for quality gear lands near 60%–80%. Pick a number in that band that matches your gear quality and cable condition.
Step 3: Divide by your phone’s energy. Final charges ≈ (Power bank Wh × Efficiency) ÷ Phone Wh.
One Fast Example (With Real Numbers)
Say you have a 10,000 mAh bank at 3.7 V. That’s 10,000 × 3.7 ÷ 1000 = 37 Wh. Your phone battery is 4,000 mAh at ~3.85 V, so 4,000 × 3.85 ÷ 1000 ≈ 15.4 Wh. With 75% usable efficiency, the bank’s deliverable energy is 37 × 0.75 = 27.75 Wh. Estimated full charges: 27.75 ÷ 15.4 ≈ 1.8 (about 1–2 full charges before the bank itself needs a refill).
Know Your Phone’s Battery Size
Many phone makers list typical battery capacity in mAh. As one example, Samsung’s official page lists Galaxy S24 Ultra at 5,000 mAh, S24+ at 4,900 mAh, and S24 at 4,000 mAh, which helps you plug the right numbers into the math (Samsung S24 battery specs).
Quick Reference Table: Estimated Full Charges
This table compares common power bank sizes against popular phone battery ranges. It assumes 3.7 V cells in the bank and a 75% efficiency pick in the middle of the realistic range. Your result can land higher or lower based on gear quality and charging speeds.
| Power Bank (mAh @3.7 V) | Phone Battery (mAh) | Estimated Full Charges |
|---|---|---|
| 5,000 | 3,000–3,500 | 0.9–1.2 |
| 5,000 | 4,000–5,000 | 0.6–0.9 |
| 10,000 | 3,000–3,500 | 1.9–2.4 |
| 10,000 | 4,000–5,000 | 1.3–1.9 |
| 20,000 | 3,000–3,500 | 3.9–4.9 |
| 20,000 | 4,000–5,000 | 2.7–3.8 |
| 27,000 (100 Wh) | 3,000–3,500 | 5.2–6.7 |
| 27,000 (100 Wh) | 4,000–5,000 | 3.6–5.0 |
Why Your Number Changes In Real Life
Cable losses. Long or thin cables waste energy as heat. A short, well-made cable keeps losses down.
Charging speed. Pushing high wattage can raise heat and loss. A moderate PD level often yields better overall energy transfer than the absolute peak your phone can accept.
Phone usage while charging. Screen time, gaming, or background sync subtracts power as it arrives. That turns one “full charge” into less than one, because some energy is feeding live use, not the battery.
Battery health. Older packs lose capacity. A two-year-old phone might refill to “100%,” yet that “100%” sits on a smaller real capacity than day one.
Ambient temperature. Extreme cold or heat hurts efficiency and can trigger throttling. Mid-room temps are friendliest.
Step-By-Step Method You Can Reuse
1) Find The Power Bank’s Energy
Take the printed mAh and multiply by 3.6–3.7. Divide by 1000 to get Wh. Many brands rate at 3.7 V, which is standard for single-cell lithium packs. When a spec sheet lists watt-hours directly, use that number.
2) Estimate A Realistic Efficiency
Pick a fair value in the 60%–80% span. Newer, name-brand banks paired with PD-rated cables often land near the high side. Budget hardware or worn cables skew low. If you’ve noticed the bank feeling hot under load, that’s a sign you should pick a lower value.
3) Convert Your Phone’s Capacity To Wh
Most modern phones sit near 3.8–3.85 V at the cell level. Multiply the mAh by ~3.85 and divide by 1000. If your maker lists Wh, you’re set.
4) Do The Division
Use the equation: Charges ≈ (Bank Wh × Efficiency) ÷ Phone Wh. Round down for a practical count, since you rarely run a bank to absolute empty and you rarely charge a phone from absolute zero.
Worked Scenarios You Can Copy
Compact Bank + Small Phone
Bank: 5,000 mAh @3.7 V → 18.5 Wh. Phone: 3,300 mAh @3.85 V → 12.7 Wh. With 70% efficiency, usable is 18.5 × 0.70 = 12.95 Wh. Charges ≈ 12.95 ÷ 12.7 ≈ 1.0. Call it about one full charge.
Mid-Bank + Mainstream Phone
Bank: 10,000 mAh @3.7 V → 37 Wh. Phone: 4,500 mAh @3.85 V → 17.3 Wh. With 75% efficiency, usable is 37 × 0.75 = 27.75 Wh. Charges ≈ 27.75 ÷ 17.3 ≈ 1.6. Expect about one and a half.
Travel Bank + Big-Battery Phone
Bank: 20,000 mAh @3.7 V → 74 Wh. Phone: 5,000 mAh @3.85 V → 19.25 Wh. With 70% efficiency, usable is 74 × 0.70 = 51.8 Wh. Charges ≈ 51.8 ÷ 19.25 ≈ 2.7. Plan on about 2–3 charges.
Does Fast Charging Change The Count?
Power Delivery and similar protocols raise voltage and current during the session. That can shave time, but the bank still holds the same energy. What changes is heat and conversion overhead. If the session runs hot, real-world efficiency falls and the count dips. If the session stays cool and stable, you stay closer to your paper math.
When The Printed mAh Misleads
Cell vs. output. That 10,000 mAh label lives at 3.7 V inside the bank. When you raise the output to 5 V or 9 V, the outgoing mAh number no longer matches the printed label. Watt-hours make everything apples-to-apples.
100% isn’t always 100%. A bank may shut down before absolute zero to protect cells. Your phone does the same at the low end. Those guards keep the hardware healthy but trim the usable slice a bit.
How To Improve Your Real-World Results
- Use a short, well-made cable. Thick conductors, clean connectors.
- Keep it cool. Shade on dashboards, airflow in pockets or bags.
- Match the charger profile. A PD bank that can meet your phone’s preferred voltage/current curve wastes less.
- Avoid heavy use while charging. Airplane mode or low-power mode turns a top-off into a full refill.
- Charge the bank regularly. Deep storage at empty is rough on lithium.
Common Phone Sizes And What To Expect
Use this simple lookup to sanity-check your math against typical devices. Numbers assume ~75% efficiency in real use. If your bank or cable is sub-par, pick the lower end; if both are high-grade and cool, pick the higher end.
| Phone Battery (mAh) | With 10,000 mAh Bank | With 20,000 mAh Bank |
|---|---|---|
| 3,000–3,300 | ~2.0–2.3 charges | ~4.0–4.6 charges |
| 3,800–4,200 | ~1.6–1.9 charges | ~3.2–3.8 charges |
| 4,500–5,000 | ~1.3–1.7 charges | ~2.7–3.4 charges |
Carry-On Tip: Know Your Bank’s Wh
Airlines and regulators use watt-hours to decide what can fly. A typical 27,000 mAh bank at 3.7 V is right on the 100 Wh line many carriers use. Check your label in Wh, or do the conversion with the same Wh = (mAh × V) ÷ 1000 math before travel.
Mini Calculator You Can Run In Your Head
Quick Path
- Bank Wh ≈ mAh × 3.7 ÷ 1000.
- Phone Wh ≈ mAh × 3.85 ÷ 1000.
- Pick efficiency at 0.6–0.8.
- Charges ≈ (Bank Wh × Efficiency) ÷ Phone Wh.
Why This Beats “mAh vs. mAh”
Energy is what matters. mAh alone ignores voltage. Wh captures both, so it tracks real work delivered to your phone. That’s why spec sheets and calculators lean on Wh for apples-to-apples comparisons (see the watt-hour calculator again).
Real-World Benchmarks To Keep In Mind
5,000 mAh bank: handy for a top-off or one small-phone refill.
10,000 mAh bank: a day out for most users, roughly one to two full phone refuels.
20,000 mAh bank: weekend range, often two to three refuels for big-battery phones.
27,000 mAh (≈100 Wh): multi-day comfort, with travel rules attached in many regions.
Extra Notes For Power Delivery Users
When using PD, the voltage can jump to 9 V or 12 V for a stretch, then drop back down near the end. The bank’s internal step-up and your phone’s step-down both work harder at those levels. If your gear runs warm, expect the charge count to land near the low side of the table ranges. If it stays cool and steady, your real-world result stays closer to the math.
Sources Used For The Math And Specs
The watt-hour conversion relies on the standard Wh = (mAh × V) ÷ 1000 relation used by reputable battery brands and calculators (mAh→Wh formula). Typical Android phone capacities for 2024–2025 include the Galaxy S24 line listed on Samsung’s site (S24 battery capacities), which you can slot into the same equations to get your personal estimate.