Charging a 20,000 mAh power bank takes about 6–20 hours, depending on input watts, the bank’s input rating, cable, and charger.
A 20,000 mAh pack holds a lot of energy, so refill time swings widely based on the wall charger’s wattage, the bank’s USB-C input limits, and cable quality. Below you’ll find a clear formula, quick estimates for common chargers, and tips to shave hours off the wait without hurting the battery.
Charging Time For A 20,000 mAh Power Bank: Real-World Ranges
To ground the estimates, convert capacity to watt-hours (energy). Most power banks use 3.7 V lithium-ion cells. That means 20,000 mAh ≈ 74 Wh using Wh = (mAh × V) ÷ 1000.
Next, divide energy by the charger’s input power (watts), and account for conversion losses in the bank’s charge circuitry. A practical way to ballpark it is:
Time (hours) ≈ (Battery Wh ÷ Charger W) ÷ Efficiency
With ~74 Wh and ~85% effective efficiency, a 10 W input can land near the long end of the range, while an 18–30 W USB-C PD input trims many hours. USB Power Delivery supports higher power levels when both charger and bank agree on a profile.
Estimated Full Charge Time By Charger Wattage
| Charger Input (W) | Typical Time* | Notes |
|---|---|---|
| 5–10 W (USB-A) | 12–20 hrs | Slow; often limited by bank’s micro-USB/USB-A input. |
| 18 W (USB-C PD / QC) | 6–9 hrs | Common on mid-range 20k banks; many vendors cite ~7 hrs. |
| 20–30 W (USB-C PD) | 4–6 hrs | Only if the power bank’s USB-C input supports 20–30 W. |
| 45–65 W (USB-C PD) | ~2.5–4 hrs | Fastest class, but uncommon on basic 20k models; some high-input banks advertise ≈2.5 hrs. |
*Estimates assume ~74 Wh energy and ~80–90% overall charge efficiency. Actual time depends on the bank’s rated input, cable, and temperature.
What Actually Controls The Hours
1) The Bank’s Rated USB-C Input
Your wall adapter can deliver 30 W, but if the bank only accepts 18 W, you’ll still sit near 6–9 hours. Product pages often list input profiles like 5V⎓3A, 9V⎓2A, or 15V⎓1.2A for USB-C, which correspond to 15 W, 18 W, and ~18 W respectively.
2) USB Power Delivery Profiles
USB-C PD negotiates voltage/current levels between the charger and device. Modern PD can supply far more than phone-class power—up to 240 W in the latest spec—though most compact power banks cap input much lower. If your bank supports a 20–30 W PD input, time drops notably.
3) Capacity Measured In Watt-Hours
mAh depends on voltage. Watt-hours do not. Converting to Wh lets you compare apples to apples and compute time for any charger. For 20,000 mAh at 3.7 V, the energy is ~74 Wh using the standard formula published by battery makers and power suppliers.
4) Charging Curve (CC/CV)
Li-ion uses a two-stage profile: a constant-current phase to bulk fill, then a constant-voltage phase that tapers near the top. That taper is why the last few percent feel slow. Expect the first half to move faster than the last stretch.
Worked Examples You Can Reuse
Example A: 10 W USB-A Charger
Energy ≈ 74 Wh. Input power ≈ 10 W. With efficiency at ~0.85, time ≈ (74 ÷ 10) ÷ 0.85 ≈ 8.7 hours for the energy alone; add taper and real-world overhead and you’re in the 12–15 hour band. That aligns with brand guidance that large packs can need up to a workday with a basic charger.
Example B: 18 W USB-C PD Charger
Time ≈ (74 ÷ 18) ÷ 0.85 ≈ 4.8 hours for the energy; once you include taper and handshake limits, expect roughly 6–9 hours. Community tests and brand support pages often cite just under 7 hours for well-known 20k models when paired with an 18 W PD wall adapter.
Example C: 30 W USB-C PD Charger (If Supported)
Time ≈ (74 ÷ 30) ÷ 0.85 ≈ 2.9 hours for the energy; with taper you may land around 4–6 hours. This only applies if your bank’s input spec actually allows 20–30 W PD.
Example D: High-Input Bank (45–65 W)
Premium units advertise far higher input, dropping refill into the ~2.5–4 hour window. One manufacturer markets a 65 W PD input with a claimed ~2.5 hour recharge for a large-capacity bank—illustrating what’s possible when input wattage climbs.
Quick Formula Card
Use this anytime you change chargers or upgrade your bank:
1) Convert capacity to Wh using Wh = (mAh × V) ÷ 1000 (most banks use 3.7 V cells).
2) Compute time: hours ≈ (Wh ÷ charger W) ÷ 0.85 (0.8–0.9 is a realistic efficiency range).
3) Check your bank’s input spec; if it tops out at 18 W, a bigger wall brick won’t charge it faster. For context on PD power levels and negotiation, see the USB-IF overview.
Brand Benchmarks That Match The Math
Well-known 20k models with an 18 W USB-C PD input often refill in about seven hours when paired with an 18 W PD wall adapter and a proper USB-C to USB-C cable, while a basic 10 W charger can take much longer.
Examples From Product Pages & Support
- Typical USB-C input profiles for 20k packs list 5V⎓3A / 9V⎓2A / 15V⎓1.2A, which cap real-world input near 18 W.
- Some high-input designs tout ~65 W USB-C PD input and a ~2.5 hour refill—fast, but a different class than basic 18 W units.
Fast Vs. Slow: Why The Cable And Port Matter
USB-C PD requires a capable cable and a negotiated profile. A mismatched cable or a USB-A port often pins you to 5 V at modest current, which stretches time. PD exists to raise both voltage and current safely when devices agree.
Care, Heat, And The Last 10 Percent
Most charging happens during the constant-current phase; the constant-voltage phase near 100% is purposely slower. That taper protects the cells and keeps temperatures under control. If you’re heading out and the LED is nearly full, unplugging a bit early won’t ruin anything and often saves time.
What About “Charge Efficiency” Claims?
At the cell level, Li-ion can be very efficient under ideal conditions, but once you include DC-DC conversion and thermal management inside a power bank, plan for overhead. That’s why the table ranges include efficiency margins instead of a single neat number.
How To Make A 20k Refill Faster (Safely)
Match The Input Spec
Check the bank’s USB-C input rating. If it supports 9 V/2 A or a 20–30 W PD profile, pair it with a wall adapter that can deliver the same or higher. Going far above the bank’s input limit won’t help, but meeting it precisely avoids a slow fallback mode.
Use A Certified USB-C Cable
Low-quality or wrong-type cables can restrict current or block PD negotiation. Swap in a known good USB-C to USB-C cable when refill time looks suspiciously long.
Avoid Hot Or Cold Rooms
Extreme temperatures can slow charging and stress the cells. If the pack or charger feels unusually warm, give it airflow and avoid stacking devices during fast charging. The CC/CV profile helps manage heat by tapering at the top.
Quick Compatibility Guide: Charger, Cable, And Port
| Setup | What It Means | Time Impact |
|---|---|---|
| USB-A 5V/2A + USB-A Cable | Legacy 10 W path; no PD negotiation. | Longest (12–20 hrs). |
| USB-C PD 9V/2A + USB-C Cable | 18 W if the bank accepts it. | Mid pack (≈6–9 hrs). |
| USB-C PD 20–30 W + E-Marked Cable | Higher PD profile when supported. | Faster (≈4–6 hrs). |
FAQ-Style Clarity (No Extra Section Needed)
Does A Bigger Wall Brick Always Help?
No. The bank sets the ceiling. If its input is capped at 18 W, a 60 W laptop charger will still land near the same time because PD negotiation chooses the shared profile.
Why Do Some Brands Quote Shorter Times?
They often ship high-input designs. A bank with a 45–65 W PD input can refill far faster than basic units, which explains claims near ~2.5 hours.
Why Does The Last Few Percent Crawl?
The constant-voltage phase deliberately slows current to finish safely and balance cells. That’s normal.
Simple Steps To Cut Hours
- Pair the bank with a wall adapter that matches its rated USB-C input (check the product page or manual).
- Use a certified USB-C cable that supports PD negotiation.
- Charge in a cool, open space to avoid heat-induced slowdowns.
- If your model supports higher input profiles, move to a 20–30 W PD charger for a noticeable time drop.
Takeaway You Can Act On
Expect a 20k pack to take 12–20 hours with a basic 5–10 W charger, ~6–9 hours with a typical 18 W USB-C PD input, and ~4–6 hours if your model supports 20–30 W PD. A few high-input banks refill near 3 hours with a laptop-class PD adapter. Convert mAh to watt-hours, match the bank’s input spec, and use a proper USB-C cable—those three steps determine the clock.