What Is Inside A Power Bank? | Tear-Down Primer

What’s Inside A Portable Charger: The Core Parts

Open a pocket battery pack and you’ll find a simple idea done with careful electronics. A flat pouch cell or bundled round cells store energy. Small boards measure, charge, protect, and step the voltage to the USB ports. Add ports, LEDs, and a shell, and you’ve got a pocket power plant.

Here’s a clear tour of the hardware inside, what each block does, and how specs like mAh, Wh, and PD wattage map to real use.

Part/Spec	What It Does/Range	Notes
Lithium-Ion Cells (Pouch Or 18650/21700)	Store energy at about 3.6–3.7V per cell; mounted as one big pouch or several round cells in parallel.	Under foam pads or tape; usually wired to the board with a short harness.
Battery Management/Charger IC	Takes 5–20V from USB-C or micro-USB and charges the pack safely with CC/CV control and power-path routing.	Main board near the ports; tied to a thermistor and the cell leads.
Boost Converter (5V/9V/15V/20V)	Raises pack voltage to the USB output level; handles fast-charge profiles and limits current.	On the same board; look for an inductor coil and power MOSFETs.
USB-C PD/Protocol Controller	Negotiates voltage and current with phones, tablets, and laptops over CC pins.	Near the USB-C port; often paired with E-Marker support.
Protection Network	Guards against short, over-current, over/under-voltage, and over-temperature events.	Fuses, MOSFETs, TVS diodes, and sense resistors by the input/output.
Microcontroller & Indicators	Runs button/LEDs, fuel gauge steps, and sometimes passes data to an app.	Tiny MCU plus LED bar; sometimes an OLED on larger packs.
Casing & Thermal Materials	Aluminum or plastic shell, with pads, tapes, and heat spreaders to move heat off the board.	Foam, graphite film, or thermal putty between chips and shell.

Cells, Shapes, And Chemistries

Most packs use lithium-ion cells with a nominal voltage near 3.6–3.7V. The shape varies. Thin units use a flat pouch. Chunkier bricks often use multiple 18650 or 21700 cells in parallel for higher capacity. Shape is form, not chemistry; both styles are lithium-ion at heart.

Round cells fit well in rigid frames and can be swapped in some hobby designs. Pouch cells help slim cases and reduce weight. Capacity, peak current, and heat handling depend on the specific cell model, not the shape alone.

Charging Path: From Wall Port To Pack

Power arrives through USB-C or micro-USB. A charger IC limits input current, routes energy either to the system load or the battery, then follows a constant-current, constant-voltage profile. A thermistor on the cell feeds temperature data so charging can pause if the pack gets hot or cold.

Many designs use an integrated charger with power-path control. This lets the pack charge and serve the port at the same time. When the wall adapter is strong enough, the system draws from the adapter first, leaving the cell less stressed.

Boost And Fast Charge: From 3.7V To USB Levels

The pack sits near 3.7V, but your phone expects around 5V on legacy ports and higher set points when fast charging. A synchronous boost converter raises the voltage and holds it steady as load changes. The controller also shapes current to meet handshake rules for protocols such as QC and PD.

With USB Power Delivery, the source and the device talk over CC pins. They select a profile such as 5V, 9V, 15V, or 20V, and newer gear can reach higher ranges under the EPR rules. The board tracks thermal limits and will step down if parts heat up. Read more on the official USB Power Delivery page.

Ports, Buttons, And Indicators

Most packs keep things simple: one USB-C that moves power in both directions, one or two Type-A ports for legacy cables, a single button, and a row of LEDs. The button wakes the gauge and starts output on some designs. Double-presses may toggle a low-current mode for earbuds or watches.

USB-C brings a protocol layer. The PD controller runs the talk on the CC pins and sets the output level the device asks for. Fixed levels like 5V, 9V, 15V, and 20V are common. Extended range adds higher steps for big loads. You can read more on the official USB Power Delivery page.

Fuel Gauges And Why They Drift

LED bars are simple. A smarter gauge measures coulombs and estimates state of charge with a model of the cell. The estimate drifts with age and heat. A full charge plus a slow discharge helps the gauge learn. That’s why a pack may show odd steps for a while after months on a shelf.

Some chargers expose basic data over I2C to a tiny MCU, which then maps steps to four or five LEDs. High-end units add a small display for pack voltage, current, and Wh in or out.

Protection You Don’t See But Rely On

Shorts and over-current events happen. Good packs include input and output protection, back-to-back MOSFETs to block reverse flow, and TVS diodes across ports. The controller watches for under-voltage and shuts the pack down before the cell gets too empty. That preserves cycle life and safety.

Quality brands test cells and packs against global safety rules. Look for markings like IEC 62133 on documentation and spec sheets. Some teardowns also show temperature sensors pressed to heat spreaders or inductors to catch runaway heat early.

Specs That Matter: mAh, Wh, And Real Output

Capacity is printed in mAh, but that figure lives at cell voltage. Energy in watt-hours is mAh × V ÷ 1000. A 10,000 mAh pack at 3.7V stores about 37 Wh. When the boost stage raises that to 5V or 9V, current changes while energy stays the same, minus conversion loss.

Conversion isn’t perfect. Many boards reach around 85–93% under mid loads. High currents, heat, and cable losses pull it down. That’s why the “rated” USB output capacity is lower than the big number on the label. Brands often publish both energy in Wh and an estimated 5V mAh rating.

Quick Math For Realistic Expectations

Here’s a fast way to set expectations. Take the printed mAh and the typical 3.7V cell level. Get Wh. Divide by your device’s charge voltage. Then shave a bit for losses. Sample math: 10,000 mAh × 3.7V = 37 Wh. At 5V, that’s 7,400 mAh. With 85% conversion, plan on roughly 6,300 mAh delivered at the USB side under light to mid load.

Inside A Pocket Charger: A Safe, Clear Breakdown

Teardowns show familiar building blocks across brands: cells glued or taped into a tray, a main PCB by the ports with a chunky inductor, and a ribbon or short wires to the cell tabs. Better units add heat spreaders and thick pads between the board and the shell to move heat away from the controller.

What Better Design Looks Like

Clean soldering, neat harness routing, and firm strain relief at the ports are good signs. So are cell labels from known makers and a clear spec label with Wh and PD ratings. A metal shell can help spread heat, but plastic with the right pads can work well too.

Heat, Swelling, And Lifespan

Heat is the enemy. High PD loads raise board temperature. Fast top-offs keep cells near maximum voltage. Both stress chemistry. Many makers tune firmware to slow the top of charge or taper early when hot. Leaving a pack full in a warm car is a bad combo that can puff a pouch over time.

Round cells resist swelling better than thin pouches, but they still age when stored hot. Any bulge, sweet solvent smell, or loose rattle calls for recycling. Don’t keep using a pack that shows those signs.

PD, QC, And Other Fast-Charge Flavors

Legacy Type-A ports use simple schemes. They signal a current limit or a named mode by setting voltages on the data lines. That’s why older phones can draw more than 500 mA from some ports.

Common Specs And What They Mean

Part/Spec	What It Does/Range	Notes
Energy (Wh)	20–100+ Wh for larger packs	Best way to compare across brands and voltages.
PD Levels	5V, 9V, 15V, 20V; EPR adds 28–48V	Higher levels feed tablets and some laptops.
Max Output	2–140W depending on model	Shows peak load; sustained power can be lower.
Cell Format	Pouch, 18650, 21700	Shape and packing affect size and weight.
Cycle Life	Hundreds to thousands	Depends on charge voltage, heat, and depth of discharge.
Protection	OCP, OVP, UVP, OTP, short	Built-in guardrails that shut the pack down safely.

How Makers Validate Safety

Most brands test cells and finished packs under standards that check abuse cases: charge faults, shorts, crush, vibration, and temperature swings. Certification marks show up on spec sheets and packaging. Some regions ask for extra paperwork when shipping by air due to energy limits per pack.

Beyond lab stamps, real-world proof comes from field data and responsible current limits. A good design makes the safe thing the easy thing: limits that hold line, boards that shed heat, and firmware that backs off when parts warm up.

Care Tips That Keep Packs Healthy

Keep vents clear and give the pack air when pushing high power. Don’t leave it on a hot dash. Partial charge storage helps; many packs age better when kept around mid-level during long rests. Use snug, short cables for big loads to reduce drop and heat.