Most power bank charging times range from 2–8 hours; exact duration depends on capacity (Wh), input wattage, and charge efficiency.
Here’s the short answer: how fast a portable battery refills comes down to three numbers—its energy in watt-hours, the charger’s input wattage, and real-world efficiency. With those, you can predict the wait before you’re back to 100%.
Fast Answer: Use Wh ÷ W × 1.15
The best back-of-napkin rule is simple: time ≈ capacity (Wh) ÷ charger input (W) × 1.15. The 1.15 factor accounts for conversion losses and the taper near the top. It won’t match every model, but it lands close enough.
Here’s what those terms mean:
- Capacity (Wh): Many packs list milliamp-hours. Convert to watt-hours with Wh = (mAh × 3.7) ÷ 1000.
- Input wattage (W): Look for the highest “Input” rating near the USB-C port—common values are 10W, 18–20W, 30W, and 45W.
- Efficiency: Lithium-ion charges in two phases. The last stretch slows, so you rarely get a perfect Wh/W hour count.
Typical Charging Times By Size And Input
| Battery Size | Charger/Input | Estimated Time |
|---|---|---|
| 10,000 mAh (≈37 Wh) | 10 W USB | ~4.3 hours |
| 10,000 mAh (≈37 Wh) | 20 W USB-C PD | ~2.1 hours |
| 20,000 mAh (≈74 Wh) | 10 W USB | ~8.5 hours |
| 20,000 mAh (≈74 Wh) | 20 W USB-C PD | ~4.3 hours |
| 20,000 mAh (≈74 Wh) | 30 W USB-C PD | ~3.0 hours |
| 27,000 mAh (≈100 Wh) | 20 W USB-C PD | ~5.8 hours |
| 27,000 mAh (≈100 Wh) | 45 W USB-C PD | ~2.6 hours |
These estimates assume a USB-C input and cable quality. If your pack only accepts older USB currents, add more time.
Why The Last 10–20% Feels Slow
Lithium-ion cells fill fast at first, then the charger eases off to protect the cells. Engineers call this charging lithium-ion using CC/CV: a constant-current phase followed by a constant-voltage phase. In the CV phase, current tapers as the pack approaches full, which is why the final bar takes longer than the first one.
Power Bank Charging Time Variations By Standard
Charging speed also depends on the standard your pack and charger speak. USB Battery Charging 1.2 caps current for legacy ports. USB-C Power Delivery lifts the ceiling and allows far higher input power, which shortens the wait when the pack supports it.
How To Calculate Your Own Time
Grab the label near the input port and note the highest input in watts. Convert capacity to Wh if needed. Then run the math. Two quick examples:
Example 1: 10,000 mAh With A 20 W USB-C Input
Capacity ≈ 37 Wh. Time ≈ 37 ÷ 20 × 1.15 ≈ 2.1 hours. That lines up with many brand claims for small packs.
Example 2: 20,000 mAh With A 30 W USB-C Input
Capacity ≈ 74 Wh. Time ≈ 74 ÷ 30 × 1.15 ≈ 2.8 hours. Real units land around three hours, give or take.
Use the same steps for any model you own. Keep notes to compare chargers.
What Else Can Slow Or Speed Things Up
Cable And Charger Limits
A weak wall brick or a tired cable throttles input power. Use a charger that meets or beats the pack’s stated input and a certified USB-C cable.
Temperature
Extreme heat or cold pushes the control chip to lower current. Room temps help the pack sip power faster and age gracefully.
Pass-Through And Background Loads
If the pack is topping up your phone while it’s refilling, net input falls. Expect longer times when pass-through is active.
Health And Age
As cells age, internal resistance rises. The pack reaches the taper sooner, which stretches the clock a bit.
USB Standards And Realistic Input Power
| Charging Standard | Typical Input Range | What To Expect |
|---|---|---|
| USB BC 1.2 (Legacy) | Up to 7.5 W (5V/1.5A) | Works everywhere, but slow for mid-size packs. |
| USB-C PD (Common) | 18–45 W | Sweet spot for most packs; 20K mAh in ~3–5 hours. |
| USB-C PD EPR | Up to 100+ W | Only on select packs; fastest refills when supported. |
Picking The Right Charger For Your Pack
Match the charger to the pack’s input spec. If the label says “Input 5V⎓3A / 9V⎓2A / 15V⎓2A (30W),” a 30 W USB-C PD brick lets the pack pull its max rate. A lower-rated brick works, just slower, at home too. Newer bricks that support PD 3.1 can deliver higher fixed voltages when devices request them.
Brand guides often publish expected refill windows for their models. Cross-check those times with the Wh ÷ W × 1.15 rule. If your real time is way off, check the cable, the wall brick, or background loads as likely culprits.
Care Tips That Keep Speeds Consistent
- Avoid charging on a hot car seat or under a pillow. Moderate temps help both speed and longevity.
- Use certified USB-C cables rated for the wattage you need. Frayed or unknown cables waste power.
- Don’t obsess over 100%. Topping off to the last sliver adds time; stopping near 90–95% can be kinder to the cells.
- If the pack has a display, check input watts while it refills. It’s the fastest way to spot a weak brick or cable.
Quick Reference Scenarios
Refilling A Small Everyday Pack
A 5,000 mAh unit (≈18.5 Wh) on a 20 W input: about one hour. On a 10 W input: about two hours.
Weekend Phone And Tablet Pack
A 10,000 mAh unit (≈37 Wh) on a 20 W input: roughly two hours. On a 30 W input: under 1.5 hours.
Laptop-capable Pack With USB-C PD
A 27,000 mAh unit (≈100 Wh) on a 45 W input: around 2.6 hours. On 20 W: closer to six hours.
Convert mAh To Wh Quickly
Most packages print capacity in milliamp-hours, yet chargers speak in watts. The bridge between the two is voltage. Nearly all single-cell packs use a nominal 3.7V rating. Multiply mAh by 3.7 and divide by 1000 to get watt-hours. A “20,000 mAh” sticker usually means about 74 Wh. Brands sometimes round the mAh; the Wh number on the spec sheet is the one to trust for math.
If the label already lists watt-hours, you’re set. Many larger packs also print several input modes like 5V⎓3A, 9V⎓2A, 15V⎓2A. Multiply V×A to see each mode’s wattage, then use the highest number the pack accepts.
What The Charge Curve Looks Like
Every refill starts with a steady current phase. The pack soaks up power quickly here. Near the top, voltage holds steady and current trails off. That’s the taper you feel near 80–100%.
Many packs show input watts on a mini display. If you watch that number, you’ll see it peak early and step down as the meter climbs. That’s normal behaviour, not a flaw.
Brand Claims And Real-World Numbers
Spec sheets often quote refill times under ideal conditions: a powerful USB-C PD brick, short certified cable, and a room-temp desk. Real life adds a few minutes. Wall outlets vary. Cables age. Some packs keep tiny cooling gaps inside the shell and throttle when airflow is poor. Expect a small buffer between math and reality, and use the Wh ÷ W × 1.15 rule as your planning baseline.
Troubleshooting Slow Refill
Check The Input Rating In Settings Or On The Label
Some packs accept only 10 W over Micro-USB, even if they can discharge at 18 W. If the label lists a Micro-USB input, expect longer waits. USB-C inputs tend to allow faster modes, including PD profiles.
Verify The Charger’s Capability
Match the brick’s rating to the pack’s highest input mode. A 65 W laptop brick often works well on a PD-ready pack, while a small phone cube may top out at 10–12 W.
Swap The Cable
Hidden cable faults are common. Try a short, known-good USB-C cable rated for 3A or 5A. Many displays jump by 5–10 W after a cable change.
Remove Background Loads
Unplug any phone or earbuds from the pack while it refills. Pass-through is handy on the road, but it steals a slice of the input and extends the wait.
Why Standards Matter
Legacy USB ports follow limits that keep current low. Many older computers raise only 500–900 mA. Dedicated charging ports under Battery Charging 1.2 allow up to 1.5A at 5V, which is still modest. USB-C Power Delivery lets a device and charger agree on higher voltages and currents, which boosts input power dramatically when both ends support it.
Method: How This Guide Estimates Time
The math here uses published electrical limits and a light loss factor. The rule expects a steady phase plus a taper near full. When brands publish measured times, those usually sit close to the estimates once you match the charger and cable. If you want exact numbers for your unit, time a full refill from near empty with a watt-meter inline and repeat once to smooth out noise.
Bottom Line On Power Bank Charging Time
If you know the watt-hours and the max input watts, you can forecast the wait. Keep a good cable and the right wall brick in your bag, and your pack will be ready when you are.