Power bank charge time depends on input watts, supported fast-charge standards, cable quality, and capacity; use watt-hours ÷ watts × 1.2 to estimate.
Buying a portable battery and wondering how long it takes to refill it? The short answer: it varies with the charger’s power in watts, the bank’s supported protocol (USB-C PD, PPS, or Quick Charge), the cable’s current rating, and the battery size. Below you’ll find a clear formula, examples, and quick wins.
Power Bank Charging Speed: The Factors That Matter
Four knobs set the pace:
- Input power (W): The higher the bank’s allowed input, the faster it can refill—up to its limit.
- Protocol: USB-C Power Delivery (PD), PD PPS, and Qualcomm Quick Charge negotiate higher voltages/currents safely.
- Cable: A 5A e-marked USB-C cable is needed for the highest current paths; a 3A cable caps current.
- Capacity: Bigger watt-hours take longer, plain and simple.
The Simple Estimate Formula
Convert the bank’s capacity to watt-hours (Wh), then divide by your charger’s watts and add an efficiency cushion.
Charge time (hours) ≈ Battery Wh ÷ Charger W × 1.2 (the 1.2 factor covers conversion losses and taper near full).
Real-World Time Estimates (Popular Sizes)
This table uses common sizes and inputs. Figures assume PD-class charging with typical efficiency; heat and cables can shift results.
| Capacity (Wh) | Input Power (W) | Estimated Full Charge (h) |
|---|---|---|
| 18.5 (≈5,000mAh) | 18 | 1.2–1.5 |
| 18.5 | 10 | 2.0–2.3 |
| 37 (≈10,000mAh) | 20 | 2.2–2.5 |
| 37 | 18 | 2.5–3.0 |
| 74 (≈20,000mAh) | 30 | 3.0–3.5 |
| 74 | 20 | 4.5–5.0 |
| 99.9 (≈26,800mAh) | 45 | 2.7–3.0* |
| 99.9 | 20 | 6.0–6.5 |
| 111 (≈30,000mAh) | 65 | 2.1–2.5* |
| 111 | 20 | 6.5–7.0 |
*Requires a bank that actually accepts that input wattage and a high-current PD PPS charger.
What “Watts” And “mAh” Mean For Refill Time
mAh on the box describes cell capacity at the cell’s native voltage (usually 3.6–3.7V). For timing and apples-to-apples math, convert to watt-hours: Wh = (mAh × V) ÷ 1000. A 20,000mAh pack at 3.7V holds about 74Wh. When the bank steps that up to 5–20V for USB, some energy is lost in conversion, which is why the estimate above includes a loss factor.
Why Your Cable Choice Changes The Math
USB-C PD 3.1 allows higher power levels, but the cable has to match. For currents beyond 3A, the cable needs an e-marker chip and 5A rating. With a 3A cable, a 20V PD profile tops out at 60W; the same charger and bank with a 5A cable can run at 100W if both sides allow it.
Fast-Charge Standards And What They Do
USB Power Delivery scales from phone-friendly 18–30W up to laptop-class power. The newest EPR modes lift the ceiling well above legacy 100W. Quick Charge is a phone-centric path that also appears on some banks and chargers; many modern devices interoperate with PD PPS. You don’t need to memorize acronyms—just match the logo on the bank, charger, and cable, and check each input spec.
Official Limits And Labels You’ll See
- USB-IF PD 3.1 EPR: Up to 240W with new fixed voltages and PPS ranges. This raises the cap for high-power gear.
- USB-C cable marks: Certified C-to-C cables now carry 60W or 240W icons. A 240W logo signals 5A support.
- Quick Charge 5: Phones and accessories can hit very high power in short bursts and are designed to reach 0–50% fast under the right thermal window.
Step-By-Step: Estimate Your Own Time
- Find the bank’s energy in Wh (spec sheet or convert from mAh at 3.7V).
- Check the bank’s input spec (e.g., “USB-C In: 30W PD PPS”). That’s the upper limit.
- Match a charger that can deliver that wattage on one port, and use a cable rated for the needed current.
- Compute: time ≈ Wh ÷ watts × 1.2. If the bank charges from nearly empty to 80%, you’ll see faster averages; the last 20% tapers.
Worked Examples You Can Copy
10,000mAh Pack With A 20W USB-C Charger
Capacity ≈ 37Wh. Charger = 20W. Estimate = 37 ÷ 20 × 1.2 ≈ 2.2 hours. That matches the “2–3 hour” range many users see.
20,000mAh Pack With A 30W USB-C PD PPS Charger
Capacity ≈ 74Wh. Charger = 30W. Estimate = 74 ÷ 30 × 1.2 ≈ 3.0 hours—if the bank’s input spec lists 30W or higher.
26,800mAh Travel-Limit Pack With A 45W Charger
Capacity ≈ 99.9Wh. Charger = 45W. Estimate = 99.9 ÷ 45 × 1.2 ≈ 2.7 hours—again, only if the bank accepts 45W in.
Common Bottlenecks That Slow Recharging
Charger Power Split
Multi-port bricks share a power budget. If the label says “100W total,” a second device can rob the bank of the top speed. Test with one cable at a time when you care about refill time.
Non-PPS Behavior Near Full
Banks without PPS hold a fixed voltage, so they taper earlier. PPS lets the charger track the bank’s requested voltage and can shave some minutes off the final stretch.
Heat
High ambient temps and cramped spaces cut charge acceptance. Give the pack airflow during a fast refill.
A Quick Look At Protocols
| Standard | Max Power (Single Port) | What It Means For Refill |
|---|---|---|
| USB-C PD (SPR) | Up to 100W | Plenty for small and mid-size packs; needs 3A or 5A cable based on profile. |
| USB-C PD (EPR) | Up to 240W | Enables very high input ratings on banks and power stations. |
| Quick Charge 4/5 | Phone-centric high wattage | Often interops with PD PPS; check bank and charger badges. |
Choosing The Right Charger And Cable
Match The Bank’s Input Number
If the spec says “USB-C in 30W PD,” a 20W cube will cap you at 20W, while a 65W brick won’t help unless the bank accepts more than 30W. Buy to the bank’s limit.
Pick The Proper Cable
For inputs above 60W, use a 5A e-marked C-to-C cable. Labels now show 60W or 240W ratings, which makes shopping easier.
PPS In Plain Terms
With classic PD, the charger offers fixed steps like 5V, 9V, 15V, and 20V. PPS adds fine-grained steps, letting the bank ask for a sliding voltage that keeps heat lower while holding high power. That steady control keeps efficiency steadier across the curve, which trims some time off the last 10–20%.
Troubleshooting Slow Recharging
Check The Brick’s Per-Port Rating
Labels list a combined total plus per-port ceilings. A “140W” charger might limit a single port to 100W. If your bank accepts 65W in, you want at least that much available on the one port you’re using.
Swap The Cable
If you see no change above 60W, the cable might be a 3A type. Try a 5A e-marked cable.
Confirm Protocol Match
A PD-only bank won’t pull QC-only speeds from a QC-only brick. Match logos and you’re good.
Pro Tips To Cut Hours
- Buy for input, not just output: Match the bank’s max input watts.
- One port at a time: Charge the bank alone for peak speed.
- Use a 5A cable when needed: Short, certified cables hold voltage.
Math Corner: Big Pack, Big Charger
Say your pack lists 30,000mAh. Energy ≈ 30,000 × 3.7 ÷ 1000 = 111Wh. Pair it with a PD PPS charger that can dedicate 65W on one port, and a 5A cable. Estimate = 111 ÷ 65 × 1.2 ≈ 2.05 hours. Real life will be closer to 2.2–2.5 hours after taper and temperature.
What The Labels On Boxes Really Mean
“Input: USB-C 5V⎓3A, 9V⎓3A, 15V⎓2A, 20V⎓1.5A (30W)” describes every supported profile the bank can accept. A PPS line might read “PPS 3.3–21V⎓3A.” With a 5A cable, the 20V path can run at 100W if the bank and brick both allow it; without that cable, you’re capped by 3A current paths. Certified C-to-C cables now carry 60W or 240W icons so you can match at a glance.
Where The Standards Sit Today
PD 3.1 EPR raises the cap to 240W with new fixed steps (28V, 36V, 48V) and PPS ranges to match. Quick Charge 5 targets phones and coexists with PD PPS on many chargers.
Safety And Battery Care While Fast Charging
Stick with certified gear. Match the wattage and the cable rating, avoid burying the pack under pillows during a high-power refill, and don’t leave swollen batteries in service. If your power bank or charger labels list PD 3.1 EPR and you’re using a 240W-marked cable, you’re inside the spec that governs these higher power levels.
Bottom Line: Estimate, Match, And Use The Right Cable
Once you know the energy in watt-hours and the bank’s allowed input, the math is simple. Grab a charger that can feed that number on a single port, pair it with a cable that can carry the current, and your wait time drops to the honest limit of the hardware.