Here’s a compact, practical guide to power banks — what they are, how to read specs, safety, and how to pick one for different needs.

What a power bank is

• A portable rechargeable battery pack that supplies power to phones, tablets, laptops and other USB devices.
• Internals are usually lithium-ion or lithium‑polymer cells; output is via USB-A, USB-C, sometimes with an AC outlet on larger units.

Key specs and what they mean

• mAh (milliamp-hours): battery capacity measured at the cell voltage (usually 3.7 V). Useful for comparing like-for-like, but not directly the usable energy at the USB output.
• Wh (watt-hours): actual energy = (mAh / 1000) × cell voltage (3.7 V). Wh is the best single number to compare energy capacity and to check airline limits.
• Output power (W) = voltage × current (e.g., 20 V × 2.25 A = 45 W). Higher W = faster charging / ability to power laptops.
• USB-C Power Delivery (PD): standardized fast charging for many phones and laptops. Check PD wattage (18W, 45W, 65W, 100W).
• Quick Charge (QC): Qualcomm fast-charge standard — common on many Android phones.
• Ports: USB-C can be input/output; look for bidirectional USB-C PD for fastest recharging of the bank and charging laptops.

How to estimate number of charges (simple method)

1. Convert the power bank to Wh: Wh = (mAh / 1000) × 3.7.
2. Convert your device battery to Wh (if you know mAh): phone Wh ≈ (phone mAh / 1000) × phone voltage (~3.7–3.9 V).
3. Allow for conversion/inefficiency (typical usable fraction 60–80%; use ~65% as a conservative estimate).
4. Estimated full charges = (power bank Wh × efficiency) / device Wh.

Example: 10,000 mAh bank → 10,000/1000×3.7 = 37 Wh. A 4,000 mAh phone ≈ 4,000/1000×3.85 = 15.4 Wh. Usable energy ≈ 37 × 0.65 = 24 Wh → ~24/15.4 ≈ 1.6 full charges.

Practical buying guidance by use-case

• Everyday phone backup: 5,000–10,000 mAh, USB-C PD 18–25W (small, light).
• Traveller / multi-day backup: 20,000–30,000 mAh, dual outputs, USB-C PD 30–60W.
• Laptop / power-hungry devices: look for PD 45–100W and Wh large enough to give useful runtime (20,000 mAh+ typical). Make sure the PD wattage matches your laptop’s requirement.
• Camping / AC devices: consider a portable power station with AC outlet (these are heavier, rated in Wh, e.g., 500–1500 Wh).
• Air travel: stay at or below 100 Wh per battery for carry-on (100–160 Wh may need airline approval; >160 Wh generally prohibited). Always check your airline’s current rules before flying.

Safety, quality and shipping rules

• Don’t put lithium power banks in checked luggage; carry in cabin only (airline policies vary—confirm before travel).
• Look for basic safety certifications and protections: overcharge, overcurrent, short-circuit, temperature protection.
• Avoid very cheap, no-name packs without safety info or certifications. They can swell, overheat or fail.
• For shipping or commercial use, UN 38.3 testing is required for lithium batteries.

Features to consider

• Recharging speed of the bank (input watts, e.g., 30W, 60W).
• Pass‑through charging (charges device while the bank is charging): convenient but can stress the battery and is disabled on some banks for safety.
• Number of simultaneous ports and total wattage available when multiple devices are connected.
• Weight and size tradeoffs: higher capacity = heavier.
• PD vs legacy ports: prefer USB-C PD for modern devices.

Care & maintenance tips

• Store at ~40–60% charge if not using for months.
• Keep in cool, dry place; avoid high heat.
• Cycle occasionally (use then recharge) to maintain capacity.
• Replace if it swells, smells, or becomes hot during normal use.

Quick shopping checklist (what to read on the label)

• Capacity in mAh and/or Wh, input and output power (W), supported protocols (USB‑C PD, PPS, QC), number and type of ports, safety/cert markings, weight.

If you want, tell me:

• What devices you need to charge (phone model, laptop wattage), whether you travel by air, and whether you want something compact or high capacity — I can suggest the right spec range or narrow options for your needs.