A power bank typically takes 1–8 hours to charge, set by its watt-hours, supported input watts, and the wall charger.
Charging time isn’t a fixed number. It hinges on energy stored inside the battery (measured in watt-hours), the highest input your power bank accepts, the charger’s output, cable quality, and a small energy overhead during charging. This guide shows clear math, practical ranges, and easy ways to speed things up without guesswork.
Typical Power Bank Charging Time By Capacity
Use this quick table to gauge full-charge time from empty. It assumes a modern USB-C charger and about 85% charging efficiency. Real results vary with brand, temperature, and where the charge tapers near the top.
| Capacity (mAh / Wh) | 10W Input Time | 20W Input Time |
|---|---|---|
| 5,000 / ~18.5Wh | ~2.2 hours | ~1.1 hours |
| 10,000 / ~37Wh | ~4.3 hours | ~2.2 hours |
| 15,000 / ~55.5Wh | ~6.5 hours | ~3.3 hours |
| 20,000 / ~74Wh | ~8.7 hours | ~4.4 hours |
| 25,000 / ~92.5Wh | ~10.9 hours | ~5.4 hours |
| 27,000 / ~100Wh | ~11.8 hours | ~5.9 hours |
Where do these numbers come from? Two steps: convert mAh to Wh (for single-cell packs, Wh ≈ mAh × 3.7 ÷ 1000), then divide by the true charging watts reaching the cells. Because the charger and the pack’s electronics waste a little energy as heat, allow ~10–20% overhead. Fast standards like USB-C Power Delivery raise the input ceiling so you can pick a higher-watt charger, cutting hours off the wait.
Fast Math You Can Trust
Step 1: Convert Capacity To Watt-Hours
Most packs list capacity in mAh. For single-cell lithium packs, a handy rule is Wh ≈ mAh × 3.7 ÷ 1000. A 20,000mAh pack stores ~74Wh. Many brands also print Wh on the label for airline rules, so check the shell.
Step 2: Divide By Real Input Watts
Time (hours) ≈ Wh ÷ (Input W × 0.85). The 0.85 reflects typical charging efficiency. So a 74Wh pack with a 20W USB-C input lands near 74 ÷ (20 × 0.85) ≈ 4.4 hours. With a 30W input, that drops to ~2.9 hours.
Step 3: Mind The Taper
Near full, lithium-ion switches from constant current to constant voltage, so current falls and the last 10–20% slows a bit. That’s why “80% in X minutes” claims look faster than “0–100%”.
What Sets The Upper Speed Limit
The Pack’s Rated Input
The spec sheet or silkscreen on the case lists the max input. If it says “USB-C In: 5V⎓3A, 9V⎓2A, 12V⎓1.5A (18W)”, then 18W is your ceiling even if you plug into a 100W brick.
The Charging Standard
USB-C Power Delivery (PD) negotiates higher voltages and currents over a USB-C cable. Newer PD revisions raise the top end for larger devices, and many packs use PD for both input and output. See the USB-IF PD overview for the wattage tiers and how negotiation works.
Charger And Cable Quality
You only get the watts your whole chain can carry. A frayed cable or a 10W phone cube will bottleneck an otherwise fast PD-ready pack. Short, certified USB-C cables help sustain higher current without excess heat.
Why You Don’t Get Perfect Efficiency
During a charge, power runs through converters that step voltage up or down and manage heat. Some energy is lost in those circuits and within the cell itself, so a little overhead is normal. Independent battery references describe the CC/CV profile and how current tails off near the end; that’s the slowdown you see in the last stretch. If your pack feels warm, the management chip is working to keep it safe.
For deeper background on the lithium charge profile and taper behavior, see this primer from Battery University. The piece shows the constant-current phase, the constant-voltage phase, and the end-of-charge cutoff used by chargers.
Capacity Ranges And Realistic Time Windows
Pocket Packs (5,000–10,000mAh)
These live in the 18–37Wh band. With a 10W input, expect ~2–4.5 hours. With a 20W PD input, you’re closer to ~1–2.2 hours. They’re easy to top off during breakfast or a commute.
Day-Trip Packs (15,000–20,000mAh)
Think 55–74Wh. A trickle 10W input means ~6.5–8.7 hours. Move to 20–30W and you’ll see ~3–4.5 hours, which fits a work afternoon or evening wind-down.
Max Airline Size (25,000–27,000mAh)
Most airlines cap carry-on batteries at 100Wh. Packs around 92–100Wh need ~11–12 hours at 10W, ~5.5–6 hours at 20W, or ~4 hours near 30W. A higher-watt PD input trims that further if the pack supports it.
Choosing The Right Charger Wattage
Match or slightly exceed your pack’s rated input. If the shell lists 18W in, buy a 20W or 30W USB-C PD wall adapter so it holds peak output even when warm. PD 3.0 adapters are common; newer PD 3.1 chargers add higher fixed voltages for laptops and multi-device use. The USB-IF page on USB-C PD outlines wattage levels up to 240W; small packs won’t use the top tiers but benefit from the protocol’s smart negotiation.
Why Your Pack Might Charge Slower Than The Label
Charger Is Undersized
Many boxes ship without a wall plug. If you feed a 20W-input pack with a 10W charger, time nearly doubles.
Cable Bottlenecks
Old micro-USB cables or worn USB-C leads can sag voltage. Use short, certified cables rated for higher current.
Warm Room Or Tight Bag
Heat triggers current limits for safety. Give the pack airflow on a desk instead of a stuffed pocket while charging.
Charging While In Use
If the pack is also powering a phone or tablet while it’s plugged into the wall, net charging slows. Pause device charging until the pack refills.
How To Estimate Time For Your Exact Setup
1) Read The Input Line
Check the label near the USB-C port or the spec sheet online. Look for the highest “In” figure in volts and amps, then multiply for watts. Example: 9V⎓2A is 18W.
2) Convert Capacity
Take the printed mAh and convert to Wh using the 3.7V rule. Many travel-size packs also list Wh on the case.
3) Apply The Formula
Time ≈ Wh ÷ (Charger W × 0.85). If your adapter is stronger than the pack’s input ceiling, use the lower pack limit in the math.
4) Add A Small Buffer
Round up 10–20% for taper and heat. If you land near 3 hours on paper, plan around 3.3–3.6 hours in daily use.
USB-C PD, Quick Charge, And What Matters
PD is widely supported and great for both phones and laptops. Some packs also accept Qualcomm Quick Charge over USB-A/C at certain voltages. Either way, the only thing that sets time is how many watts the pack can take in. A 30W input beats an 18W input every time, even if marketing terms differ.
Safety And Longevity Tips While Charging
Keep It Cool
Room temperature is your friend. Don’t stack the pack under blankets or leave it on a hot windowsill during a charge.
Use Certified Gear
Pick chargers and cables from known brands with PD certification. Avoid no-name bricks that misreport voltages or current.
Aim For Smooth Daily Top-Ups
Frequent partial charges are fine for lithium cells. If the pack sits for a week, store it half full and top it off before a trip.
Common Scenarios And What To Expect
Small 10,000mAh Pack, 18W PD Wall Plug
About two hours from near empty, give or take cable quality and room temp.
Workhorse 20,000mAh Pack, 20W PD Wall Plug
Plan on four to four-and-a-half hours. A 30W-input model trims that near three hours.
100Wh Travel-Max Pack, 30W PD Wall Plug
Roughly four hours if the pack accepts 30W in. If it only takes 18W, expect just under six hours.
When Marketing Numbers Don’t Match Your Results
Box claims often quote lab conditions: short cable, cool room, high-watt charger, and no devices plugged into the pack. Real-life setups add cable resistance, charger sag, and the lithium taper near full. The math in this guide gives a grounded baseline; if your times are way off, check charger wattage, swap the cable, and inspect the pack’s input spec again.
Feature Impact On Charge Time
This table sums up how each factor nudges your result. Use it as a checklist when you troubleshoot slow refills.
| Factor | What To Look For | Time Impact |
|---|---|---|
| Rated Input Watts | Label near USB-C “In” (e.g., 18W, 30W) | Higher input cuts hours off large packs |
| Charging Standard | USB-C PD versions; PPS support | Better negotiation holds higher power |
| Wall Charger | Adapter wattage equal to or above input | Undersized plugs lengthen time |
| Cable Quality | Short, certified USB-C cable | Poor cables drop voltage and slow flow |
| Heat | Warm desk, open air | Hot packs reduce current near full |
| Charging While In Use | Pause device charging | Load steals input power from the refill |
Quick Reference: What “Good” Looks Like
Green Flags
- Pack lists PD input at 18–30W or more.
- Wall charger meets or beats that input rating.
- Short, sturdy USB-C cable with clean connectors.
- Pack sits on a desk with airflow while charging.
Red Flags
- Micro-USB input only and a phone-grade 5V⎓1A cube.
- Warm, cramped pocket or bag during a charge.
- Charging another device from the pack at the same time.
Bring It All Together
Figure out your pack’s Wh, check the rated input, match a charger to that number, and run the simple time formula. With a decent PD adapter and cable, most 10,000–20,000mAh packs refill between ~2 and ~5 hours. Larger, airline-limit packs sit near ~4–6 hours when their input supports 30W. Pick your gear with those targets in mind and charging fits neatly into a morning, a work block, or an evening.