A 20,000 mAh power bank typically needs about 6–7 hours with a 20W USB-C PD charger; higher input wattage shortens that time.
Charging time isn’t magic. It comes down to how much energy the battery holds and how fast you can push energy back in. A 20,000 mAh pack stores roughly 74 watt-hours at a 3.7 V nominal cell voltage. If the power bank accepts only a slow 10 W input, you wait a lot longer than if it accepts 18 W, 30 W, or more. The catch: many models cap input around 18–30 W even if you plug in a beefy brick. So the real answer starts with your power bank’s rated input.
Charging Time For A 20,000mAh Power Bank — Quick Estimates
Use this table to set expectations. It assumes about 85% charge efficiency and a 74 Wh pack. Check your box or spec sheet for the exact input supported.
| Charger Input | Typical Time | What To Expect |
|---|---|---|
| 10 W USB | ~8.7 hours | Basic 5 V/2 A bricks and old ports land here. |
| 15 W | ~5.8 hours | Some older quick-charge adapters. |
| 18 W USB-C PD/QC | ~4.8 hours | Common fast-charge input on many 20K packs. |
| 20 W USB-C PD | ~4.4 hours | Popular phone chargers; many banks still cap at 18 W. |
| 30 W USB-C PD | ~2.9 hours | Only helps if the bank supports 24–30 W input. |
| 45–65 W USB-C PD | ~1.3–1.9 hours | High-end banks with high input rates; check specs. |
What Actually Controls The Wait
Battery Energy, In Plain Numbers
A 20,000 mAh pack uses lithium cells rated around 3.7 V. Convert to watt-hours with mAh × V ÷ 1000, so 20,000 × 3.7 ÷ 1000 ≈ 74 Wh. That’s the energy you need to put back. You never get perfect efficiency, so the wall energy you must supply is higher. With an 85% assumption, you feed about 87 Wh into the bank to end up with ~74 Wh stored.
Input Power Limit On The Bank
The speed gate is the bank’s own input limit, not just the charger. Many 20K units top out at 18 W PD (9 V × 2 A). Some newer designs accept 24–30 W or even higher. If the label says “5 V 2 A input,” expect a long night — around ten to eleven hours once taper near full is included.
USB-C PD And Why The Charger Matters
USB-C Power Delivery lets chargers and devices agree on higher voltages and currents so energy moves faster and cooler. A PD brick can advertise 9 V, 12 V, 15 V, or more; the bank then requests the best match. That’s why a 20 W or 30 W PD brick beats a plain 5 V/2 A cube, as long as the bank accepts it. You don’t need a laptop-class brick if the bank caps at 18 W.
For a standards overview, see the USB-IF page on USB Power Delivery; modern PD reaches well beyond phone chargers and now goes up to very high wattage for bigger gear.
How To Estimate Your Own Time
Step 1: Read The Input Line
Look for “Input” on the label or spec sheet. Common lines include “5 V ⎓ 2 A,” “9 V ⎓ 2 A,” or “USB-C PD 24–30 W.” The number you need is watts. Multiply volts × amps when watts aren’t printed. A 9 V ⎓ 2 A input equals 18 W.
Step 2: Convert Capacity To Watt-Hours
Use Wh = (mAh × V) ÷ 1000 with V = 3.7 for most lithium packs. For a 20,000 mAh unit, Wh ≈ 74. This is the common energy figure brands and airlines use.
Step 3: Factor In Efficiency
Charging a pack isn’t loss-free. Converters and the battery chemistry waste some energy as heat. An 80–90% window is common. Using 85% keeps the math realistic without getting fussy.
Step 4: Do The Time Math
Time (hours) ≈ (Wh ÷ efficiency) ÷ input watts. With 74 Wh and 85% efficiency, wall energy is ~87 Wh. Divide by the input. At 18 W, plan on about 4.8 hours. At 30 W, around 2.9 hours — again, only if the bank truly accepts 30 W input.
Why Your Result Might Be Slower
Charge Curve Near The Top
Most chargers taper current near full to protect the cells. The last 10–15% crawls. That’s why you often see a bank go from 0–80% at a steady clip, then linger.
Weak Cables And Ports
Low-grade cables drop voltage under load. A flaky cable can make a PD session fall back to 5 V. Use a decent USB-C to USB-C cable rated for PD and keep runs short.
Thermal Limits
Hot rooms and tight backpacks make the bank warm while charging. Many designs reduce input current when temperature sensors trigger. Give the pack airflow on a table, not under a pillow.
Charger Mismatch
A 65 W laptop brick won’t speed up a bank that only negotiates 18 W. It may still read “fast,” but the input cap rules the show.
Picking A Charger That Makes Sense
If your bank supports PD at 18 W, a 20 W USB-C phone charger is plenty. If it supports 24–30 W, a compact 30 W brick trims your wait without adding much size. There’s no gain in bringing a 100 W monster only for this job unless you also charge a laptop. For the standards view, the USB-IF’s page on USB PD power levels shows how devices request higher power safely.
Real-World Inputs And Times From Popular Models
Brand help pages list recharge times that line up with the math above. These three examples cover the common input limits you’ll run into on 20K units.
| Model | Rated Input | Stated Recharge Time |
|---|---|---|
| Anker 335 Power Bank (PowerCore 20K) | PD 18 W (9 V/2 A) | ~6.5 hours with an 18 W PD wall charger. |
| Anker 325 Power Bank (PowerCore 20K) | 5 V/2 A input | ~10–11 hours with a 5 V/2 A charger. |
| Redmi 20,000 mAh Fast Charge Power Bank | 18 W two-way fast charge | ~6.5–7.5 hours with 9 V/2 A or 12 V/1.5 A. |
For an official example, Anker lists about six and a half hours to refill a 20K pack with an 18 W PD wall charger; see the brand’s page: recharge time for 20K PD banks. For standards background, the USB-IF explainer on USB-C PD power shows how power levels are negotiated and delivered safely.
Worked Examples You Can Copy
Case A: 5 V ⎓ 2 A Input
Input power is 10 W. Energy to feed ≈ 87 Wh. Time ≈ 87 ÷ 10 = ~8.7 hours. Brands with this limit often quote about ten to eleven hours because of taper and overhead.
Case B: 9 V ⎓ 2 A PD Input
Input power is 18 W. Time ≈ 87 ÷ 18 = ~4.8 hours. That aligns with the common six-and-a-half-hour claims once taper and cable losses are included.
Case C: 12 V ⎓ 2.5 A PD Input
Input power is 30 W. Time ≈ 87 ÷ 30 = ~2.9 hours. Only a handful of 20K banks take this. If yours does, you save hours.
Care Tips That Keep Capacity Healthy
Avoid Full Deeps When You Can
Frequent 0%-to-100% cycles wear cells faster than partial top-ups. Topping a bank at 80–90% for daily use is kinder than draining it flat each time.
Charge Cool
Heat ages lithium. Keep the pack out of direct sun during a refill and don’t cover it while it’s working.
Use The Right Mode For Small Gadgets
Many 20K units include a low-current mode for earbuds or watches. Turn it on when needed so the regulator doesn’t waste energy hunting for a larger load.
USB-A Versus USB-C For Recharging The Bank
Older banks accept micro-USB at 5 V/2 A while newer ones add USB-C PD for input. USB-A ports on chargers feed the bank at 5 V only. If your bank lists 9 V or 12 V input, use a USB-C PD brick and a USB-C to USB-C cable to unlock that higher rate. The jump from 10 W to 18 W cuts hours off the wait.
Can You Use The Bank While It’s Recharging?
Some models allow pass-through charging. The bank takes power from the wall and charges your phone at the same time. This is handy, but it stretches the total time since part of the input goes straight back out. Heat also rises during pass-through, which may trigger current limits. For the fastest refill, leave the outputs idle.
Small Checks That Prevent Headaches
Confirm PD Negotiation
Watch the bank’s LEDs right after plugging into a PD brick. If you see no “fast” indicator, swap the cable or try another port on the charger.
Mind The Cable Current Rating
A certified USB-C cable rated for 3 A handles every 18–60 W case you’ll meet with 20K banks. Cheap cables sag and cause slow sessions.
Give The Bank Room To Breathe
Set the pack on a hard surface while it’s recharging. Trapping heat under pillows, beds, or clothes slows things down as the bank throttles.
Field Math You Can Do In Seconds
Keep two numbers in your head. A 20,000 mAh bank stores roughly 74 Wh. You need about 87 Wh from the wall to fill it when you include losses. Divide 87 by your input watts. Seeing 18 W on the label? Call it five hours plus a bit. Seeing 30 W? Call it three hours give or take.
Refill Time Takeaways
A realistic window for a 20,000 mAh bank runs from about five to eleven hours for the mainstream models on store shelves. Feed an 18 W PD input and you land near six to seven hours. Step up to a bank that accepts 24–30 W and you can drop closer to three hours. If your unit only takes 5 V/2 A, budget a full night. Match the brick to the bank’s input, use a solid USB-C cable, and give it some airflow, and you’ll hit the numbers above with ease.