Most power banks refill in 2–8 hours, shaped by capacity (Wh), input watts, and the charger-cable combo.
Wondering how many hours your portable battery needs on the wall? Charging time mainly depends on the energy inside the pack (watt-hours), the input speed it accepts (watts), and the gear you plug into it. This guide gives you a clear way to estimate the wait, plus real-world ranges for common sizes and chargers.
How Charging Time Is Actually Calculated
You can estimate the wait with a simple idea: energy divided by power. Convert the pack’s rating to watt-hours, check the input wattage it accepts, then add a small overhead for charging losses and the top-off phase.
The Core Formula
Estimated time (hours) ≈ (Battery energy in Wh ÷ Input watts) × 1.2–1.5
Why the multiplier? Lithium-ion fills fast at first, then slows near full due to the constant-voltage “taper.” That last stretch adds time beyond the math of Wh ÷ W. Battery University outlines this two-stage CC/CV profile and the slower finish at high state of charge.
Converting mAh To Wh
Most packs list milliamp-hours at 3.7 V cell voltage. Use: Wh = (mAh × 3.7) ÷ 1000. EcoFlow explains this conversion and why Wh makes cross-device math easier.
How Long A Power Bank Needs To Recharge (By Capacity & Watts)
Use the table as a quick planner. Values assume healthy cables, a quality charger, room-temperature charging, and a 1.25× efficiency/taper factor. Your unit’s label may list a max input like “18 W” or “30 W PD.”
Typical Full-Charge Windows
| Rated Size (mAh → Wh) | Max Input Watts | Approx. Full Charge Window |
|---|---|---|
| 5,000 mAh → ~18.5 Wh | 10 W | ~2.5–3.0 hours |
| 10,000 mAh → ~37 Wh | 10 W | ~4.5–5.0 hours |
| 10,000 mAh → ~37 Wh | 18 W (PD/QC) | ~2.5–3.0 hours |
| 20,000 mAh → ~74 Wh | 18 W (PD/QC) | ~5–6 hours |
| 20,000 mAh → ~74 Wh | 30 W (PD) | ~3–3.5 hours |
| 26,000 mAh → ~96 Wh | 45 W (PD) | ~2.5–3.0 hours |
These ranges line up with brand guidance you’ll see on product pages. Anker’s 10,000 mAh units often land near 3–6 hours depending on the charger and cable, which fits the Wh ÷ W math plus taper.
What Changes The Wait
Input Wattage Your Pack Can Accept
If the pack tops out at 10 W, plugging it into a 65 W brick won’t make it sprint. The input controller negotiates a profile (like 5 V⎓2 A or 9 V⎓2 A) and sticks to that ceiling.
Charger Protocols And USB-C PD
Power banks and chargers talk before raising voltage or current. With USB Power Delivery 3.1, the ceiling expanded beyond the old 100 W limit and now reaches as high as 240 W for devices that support EPR. That headroom matters for larger packs that accept higher input.
Cable Quality
Under-rated or damaged cables force low current, stretch the timeline, and can trigger protocol fallbacks. For higher PD levels, you need an e-marked cable that matches the negotiated profile.
Capacity Class
More Wh means more time. A compact 5,000 mAh pack can be ready between podcasts, while a 20,000–26,000 mAh unit may need a full evening unless you feed it with a higher-watt PD charger.
Temperature
Charging runs slower when cells are hot or cold. Heat also ages lithium-ion faster, so avoid windowsills or car dashboards during a refill. Battery University documents the stress from heat and high voltage storage.
Pass-Through And Background Loads
If the bank powers a phone while refilling, total time rises. The charger must cover the live load first, then build charge in the pack.
Step-By-Step: Estimate Your Own Charge Time
- Find the real energy. Multiply the mAh rating by 3.7 and divide by 1000 to get Wh. (e.g., 20,000 mAh → ~74 Wh).
- Check the max input. Look for “input: 5 V⎓3 A (15 W) / 9 V⎓2 A (18 W)” or a PD rating on the label or spec sheet.
- Do the quick math. Time ≈ Wh ÷ W. Then multiply by 1.2–1.5 to cover conversion losses and the top-off taper. The taper exists because Li-ion shifts from constant current to constant voltage near full.
- Cross-check with brand guidance. Many makers share typical windows; they should look close to your estimate.
Real-World Examples
Pocket-Size Pack (10,000 mAh)
Energy: ~37 Wh. With a 10 W input, math gives about 3.7 h; add taper and you land near 4.5–5 h. With an 18 W PD input, you’re closer to 2.1 h raw, about 2.5–3 h in practice. This mirrors brand ranges for similar units.
Day-Trip Pack (20,000 mAh)
Energy: ~74 Wh. On 18 W input, rough math says ~4.1 h; with taper you often see ~5–6 h. Give it a 30 W PD brick and it drops near ~3–3.5 h, as the table shows.
Airline-Limit Pack (26,000 mAh class)
Energy: ~96 Wh. With 45 W PD input, math lands near 2.1 h; with taper, ~2.5–3 h. Newer high-end models advertise even faster refills under PD 3.1 when paired with matching chargers and cables. USB-IF’s PD updates enable that headroom.
Fast-Charge Standards In Plain Terms
USB-C PD negotiates voltage and current in steps. Older profiles often sit at 5 V or 9 V; newer EPR adds higher voltages with certified cables. The higher the negotiated wattage, the shorter the refill—until the pack’s own input limit becomes the bottleneck. The USB-IF pages outline the move from 100 W to 240 W ceilings under PD 3.1.
Want the formal spec background? See the USB-IF PD overview for the power ranges and the lithium-ion CC/CV profile that explains why the last few percent take longer. These give context for the Wh ÷ W estimate you’re using.
Troubleshooting Slow Refills
It’s Crawling At 5 V Only
Swap to a PD charger and an e-marked USB-C cable. If the pack accepts 9 V or 12 V, it should jump to a higher wattage after negotiation.
It Stalls Near 80–90%
That’s the CV phase doing its job. The current drops to a small percentage of capacity to finish safely, which adds time. Battery University describes full charge detection by watching current fall to a small tail.
It Gets Warm
Move it off soft surfaces, keep it out of the sun, and avoid charging inside a hot car. Elevated heat slows charging and ages the cells.
Recommended Charger Pairings By Pack Size
These pairings balance speed, stress, and everyday convenience. If your label lists a lower input limit, follow the label.
Charger Picks That Make Sense
| Pack Size (Wh) | Charger Wattage | Why It Works |
|---|---|---|
| ~18–20 Wh (5,000 mAh) | 18 W PD | Cuts refill to a few hours without bulky bricks. |
| ~36–40 Wh (10,000 mAh) | 20–30 W PD | Shortens the evening charge; fits small travel chargers. |
| ~70–100 Wh (20–26k mAh) | 30–45 W PD | Moves from overnight to a compact multi-hour window. |
Safe And Speedy Habits
- Stick to certified gear. Use chargers and cables that match the pack’s input spec. PD 3.1 features need proper cables.
- Charge in a cool spot. Room-temp surfaces help both speed and lifespan.
- Avoid deep depletion. Topping up before the pack is near empty trims heat and stress across the cycle.
- Mind pass-through. If you must power a device while refilling the bank, expect a longer timeline.
Worked Example You Can Copy
Scenario: A 20,000 mAh pack, label shows “USB-C input: 9 V⎓2 A (18 W).”
- Convert to Wh: 20,000 × 3.7 ÷ 1000 ≈ 74 Wh.
- Divide by input watts: 74 ÷ 18 ≈ 4.1 h.
- Add taper/overhead: 4.1 × 1.25 ≈ ~5.1 h. A real-world window of 5–6 h is normal and aligns with brand guidance for similar units.
When Newer PD Levels Matter
Bigger packs that accept higher input can take advantage of PD 3.1’s expanded ceiling with the right charger and cable. The spec update raised the limit up to 240 W, which enables rapid refills on models designed for it. Always check the pack’s input spec; that’s the true bottleneck.
Key Takeaways
- Time hinges on energy (Wh) and input watts; the final stretch slows due to CV taper.
- Expect ~2–8 hours across common sizes with the right charger-cable pair.
- USB-C PD levels set the ceiling; your pack’s rated input sets the pace.