A 20,000 mAh power bank charging time is ~4½ hours with 20W USB-C, or ~9 hours with a 10W brick; input caps and losses add overhead.
Let’s pin down realistic charging times and what actually controls them. You’ll see clear estimates for common charger wattages, a simple math method you can reuse, and quick tips to shorten the wait without stressing the cells.
Charging Time For A 20,000 mAh Power Bank — Real-World Factors
Several variables decide how long you’ll be tethered to the wall:
- Input power your pack accepts. Many 20k packs cap at 18–30W over USB-C Power Delivery. Higher-rated bricks won’t go faster if the pack’s input max is lower.
- Energy inside the pack. Most use 3.7V lithium-ion cells; 20,000 mAh at 3.7V is about 74 Wh. That’s the energy you need to refill after a full drain.
- Charging method. Lithium-ion fills in two phases: a fast constant-current stage and a slower top-off under constant voltage. That taper near the end adds time.
- Inevitable losses. Heat and conversion losses mean you’ll input a bit more energy than the rated capacity suggests.
Quick Estimates By Charger Size (Assuming The Pack Accepts It)
Use these ballpark times for a 20k pack from empty. Assumptions: ~74 Wh capacity, USB-C input at the listed wattage, and a ~1.2x overhead to cover losses and the final top-off.
| Charger Wattage | Assumed Input To Pack | Full Charge Time (Approx.) |
|---|---|---|
| 10W (5V⎓2A) | 10W | ~8.9 hours |
| 18W (PD/QC) | 18W | ~4.9 hours |
| 20W (PD) | 20W | ~4.4 hours |
| 30W (PD) | 30W | ~3.0 hours |
| 45W (PD/PPS) | 45W | ~2.0 hours |
| 65W (PD/PPS) | 65W | ~1.4 hours |
| 100W (PD 3.x) | 100W | ~0.9 hours* |
*Only if the power bank’s USB-C input actually negotiates 100W. Many 20k packs accept 18–30W, a few accept 45–65W, and only certain models take 100W on input.
How To Do The Math Yourself
Here’s the simple way to estimate:
- Convert the pack’s capacity to watt-hours (Wh). A 20,000 mAh pack with 3.7V cells stores ~74 Wh (20,000 × 3.7 ÷ 1000).
- Divide by your charger’s real input power to the pack. If the input spec shows 20W, use 20.
- Multiply by ~1.2 to cover conversion losses and the slow top-off near 100%.
Example: 74 Wh ÷ 20 W × 1.2 ≈ 4.4 hours from empty.
That 1.2 factor is a practical rule of thumb for consumer packs, matching what many accessory makers share. The exact overhead varies with the cells, the charger, cable quality, and ambient temperature.
What USB-C Power Delivery Actually Negotiates
USB-C Power Delivery lets the charger and device agree on a voltage/current combo. That’s why a 65W laptop brick can feed a 30W-input pack safely—the pack requests only what it can take. If your pack supports PPS (programmable power supply), it may hold a steadier current as it nears full, trimming a little time. For a spec overview, see the official USB Power Delivery page.
Why The Last 10–20% Takes Longer
Lithium-ion uses a constant-current phase that fills the bulk of the capacity quickly, then a constant-voltage phase to finish the top slice. Near full, current tapers down, so each extra percent takes longer than the first few. If you’re leaving soon, stopping at 80–90% can save time with minimal impact on your day.
Realistic Scenarios And What To Expect
Using A 10W USB-A Brick
It works, but it’s slow. Plan on a full workday on the outlet after a deep drain. If your pack can accept 18W or 20W, stepping up the brick and cable will nearly halve the wait.
Using A 20W USB-C Charger
This is the sweet spot for many mid-priced packs. Four to five hours from empty is normal. If you start at 30–40%, you’ll be ready in two to three hours.
Using A 45–65W Laptop Charger
Only worthwhile if the pack accepts higher input. If the spec says 30W max input, a 65W brick won’t beat a 30W brick. When supported, you’re looking at roughly two hours from flat.
Using A 100W PD Brick
A few performance-oriented 20k models accept 100W input. Those can refill in under an hour. The cable must be a 100W-rated USB-C e-marked cable.
How Cable And Port Choices Change The Clock
- Port matters: Many packs have faster input on USB-C than on micro-USB or USB-A.
- Cable rating: A flimsy cable can bottleneck current. Use a certified USB-C cable rated for the wattage you intend to pull.
- One port at a time: Some packs throttle input when they are also outputting power (pass-through). For fastest charging, avoid topping up other devices at the same time.
Battery Care While Charging
Try to keep the pack in a cool room and away from blankets or cushions while fast-charging. Extreme heat slows charging and ages the cells faster over time. You’ll also see quicker refills if you avoid running output while the pack is trying to charge, since the system has to juggle both flows.
Partial Refill Times (Handy When You’re In A Rush)
Here’s how long common milestones take with a 20W USB-C charger, using the same 74 Wh and 1.2x overhead. The numbers scale linearly for the fast phase and slow a bit near the end, so treat 90–100% as the “patience zone.”
| Target Level | Energy To Add | Time With 20W |
|---|---|---|
| 20% → 100% | ~59 Wh | ~3.6 hours |
| 50% → 100% | ~37 Wh | ~2.2 hours |
| 0% → 80% | ~59 Wh | ~3.6 hours |
| 0% → 50% | ~37 Wh | ~2.2 hours |
| 0% → 20% | ~15 Wh | ~0.9 hours |
Step-By-Step: Get The Fastest Safe Refill
- Check the input spec on the label. Look for “USB-C in: 5V⎓3A, 9V⎓2A, 12V⎓1.5A (18W)” or similar. That number is the ceiling.
- Match a charger to that ceiling. If the pack says 30W input, pick a 30W or 35W PD charger; bigger doesn’t hurt, but won’t speed past the cap.
- Use a proper cable. For 60–100W charging, use an e-marked USB-C cable rated for the job.
- Keep it cool. A cooler pack charges more consistently. Avoid direct sun and hot dashboards.
- Don’t chase 100% every time. Stopping at 80–90% saves time and is gentle on the cells.
Why 20,000 mAh Equals About 74 Wh
mAh numbers are based on the battery’s internal voltage (~3.7V for typical lithium-ion cells). Multiply mAh by 3.7 and divide by 1000 to get watt-hours. That’s why the label on many packs shows both, and why airlines check Wh for flight limits.
Common Questions About Wall-Time
Will A 65W Laptop Brick Always Charge Faster Than A 30W Phone Brick?
Only if the pack’s input supports a higher rate. If the pack tops out at 30W, both bricks will charge at the same speed when using a USB-C cable.
Why Does The Final Few Percent Creep So Slowly?
The charger switches to constant voltage near full. Current ramps down to protect the cells, so minutes per percent increase.
Can I Charge And Discharge At The Same Time?
Many packs allow pass-through, but input speed often drops. For the shortest refill, avoid output while charging.
Further Reading On The Tech Behind The Numbers
If you want to dig into the charging curve and why the taper exists, see Charging lithium-ion. For a high-level view of how USB-C Power Delivery negotiates voltage and current, the USB Power Delivery page is a solid starting point.