Charging Protocols and Fast Charging

A charger may display 65 W while the battery only receives 30 W; this is not necessarily a protocol failure. The device might be powering the screen and processor, the battery may have entered a reduced-current phase, or temperature may be limiting input. The charging system should be divided into four parts: input negotiation, power conversion, battery control, and protection, to be evaluated separately along the energy path.

The Adapter Is Not a Complete Battery Charging Controller

Wall-plug adapters typically convert AC to controlled DC, with some offering protocol negotiation; the device's internal charging IC then controls charging based on the cell, power, and system status. Vehicle power supplies, power banks, and wireless receivers can also serve as input sources, so the statement "a charger must be AC-DC" does not cover all scenarios.

The maximum rated power is merely a capability boundary; you must also check the corresponding voltage, current, and port combinations. A device can only request and draw power within the conditions allowed by the Source, Sink, cable, connector, and power path together. When multiple ports are plugged in simultaneously, the adapter may reallocate power and trigger negotiation changes.

Assume a single port can provide 20 V/3.25 A, but the cable only allows 3 A; this path cannot operate at 3.25 A. If the device only requests 9 V/2 A, the actual contract is 18 W. Both sides supporting PD does not necessarily mean there is a common voltage step that meets the maximum power demand.

What Fixed Voltage PD, PPS, and EPR Solve Separately

Fixed PDOs declare a set of voltage/current capabilities, and the Sink selects and requests them. The Source accepts and switches; power is ready after PS_RDY. See USB and Type-C for the complete connection, role, and negotiation flow. Do not treat new voltage/current conditions as stable just because an Accept was received.

PPS uses Programmable Power Supply capabilities, allowing the device to request voltage and current limits within the range declared by the Source. Common PPS requests have fine-grained steps, but request resolution does not equal output accuracy; the voltage range, maximum current, and available power are determined by the corresponding APDO, not all PPS sources uniformly provide 3.3–21 V, 5 A.

EPR (Extended Power Range) expands the power range, including higher voltage capabilities, up to 48 V × 5 A = 240 W. The port, cable, power path, and protocol status must all support EPR; an old 5 A/100 W cable has current capability but does not necessarily possess EPR voltage and identification capabilities. PD version numbers do not guarantee that a device implements all optional capabilities. USB-IF: Power Delivery

"Direct Charging" Is Not Barely Connecting an Adjustable Power Supply to a Cell

Devices can use inductive buck, buck-boost, switched-capacitor conversion, or controlled bypass structures. Switched-capacitor 2:1 conversion steps down the input voltage to nearly half while, ideally, doubling the output current; real values are affected by losses and control conditions. PPS allows the input voltage to adjust with the battery voltage, enabling the converter to operate in a suitable range.

For example, the TI BQ25980 is a switched-capacitor charger designed for two-series cells; it still includes voltage, current, and temperature protection, and works with a corresponding main charger to complete the charging cycle. This suffices to show that the claim "PPS requires no internal circuitry and generates no heat" is invalid. TI: BQ25980

Assuming an input of 9 V/3 A, a conversion efficiency of 95%, and an output battery branch voltage of 4.2 V, with no system load allocated yet, the output current is approximately 9 × 3 × 0.95 / 4.2 = 6.11 A. This mathematical example shows that an input of 3 A does not equal 3 A at the battery; it is not recommending 6.11 A for a specific cell, and the cell's allowable current and the specific converter's operating range must be constrained separately.

Power Gains Must Be Calculated Along with Cable and Conversion Losses

For the same input power P, increasing the transmission voltage usually lowers the current I = P/V, and cable loss is I²R. Taking 30 W and a loop resistance of 0.10 Ω for comparison only: if transmitted at 5 V/6 A, the line loss is 3.6 W; if transmitted at 15 V/2 A, the line loss is 0.4 W. The former current is not a contract directly usable by standard USB-C; it serves only as a mathematical comparison under the same power.

The device converter after increasing voltage may also increase losses, so you must look at the end-to-end energy ledger and cannot declare maximum efficiency by calculating only cable losses. Dedicated low-voltage, high-current solutions also require matching cables, contacts, and controlled charging paths, and cannot be mixed with standard USB current capabilities.

When comparing "charging power," the same measurement point must be used. The same applies to wireless charging; the receiver output power, charging pad input, and wall input cannot be mixed into a single efficiency denominator.

CC/CV: Why Charging Cannot Be Calculated End-to-End by Dividing Capacity by Peak Current

For many rechargeable lithium-ion cells, charging control includes limited pre-charge at low voltage, normal constant current, constant voltage with decreasing current after reaching a specified voltage, and termination once conditions are met. Specific thresholds, currents, and temperature windows are determined by the cell and charger specifications; not all "lithium batteries" uniformly use 3.0/4.2 V.

Pre-charge handles low-voltage cells with small current under allowed conditions and should not be called trickle charging maintained long-term after full charge. If a low-voltage cell cannot recover within a specified time, the controller may judge it faulty; pre-charge cannot be infinite, nor can every over-discharged battery be treated as recoverable.

The constant current phase constrains the charging current, while the terminal voltage gradually approaches the upper limit. Terminal voltage includes open-circuit voltage, ohmic drop, and polarization contributions; larger currents may hit the voltage limit earlier. Upon entering CV, the controller maintains the voltage upper limit, and the current gradually decreases. Temperature, input capability, or system power limits may also trigger current reduction at any stage.

Termination usually requires conditions such as the battery branch current falling below a set threshold, and may be accompanied by timers and status judgments. Monitoring re-charge conditions according to strategy after termination does not mean continuously pumping small currents into lithium-ion batteries. TI: BQ2407x Charging Status and Termination

Estimate Time Using Segmented Currents, Not Peak Power for the Entire Process

C-rate normalizes current by capacity; for example, 1C for a 3 Ah cell is 3 A. Assume a purely instructional energy account needs to replenish 3 Ah, where 2.4 Ah is replenished at 3 A, taking 0.8 h, and the remaining 0.6 Ah, if at an average current of 0.75 A, takes another 0.8 h, totaling 1.6 h; directly calculating 3 Ah / 3 A = 1 h ignores the current reduction in the later stages.

Real SOC, energy efficiency, and stage allocation are not these fixed numbers. Charging time depends on capacity, chemical system, thermal design, cutoff definitions, and allowed charging strategies; it is impossible to prove that a certain minute count is necessarily feasible or impossible without conditions.

Charge follows peak current or area?

Charge is the area to the left of the cursor. After taper, the gap from the 3 A rectangle grows. Charge is not labeled as SOC, and the exponential is not a universal cell model; it explains the failure of peak-current extrapolation.

$$Q(t)={\int }_0^{t}I(u)du$$

When the System Charges While Using, Power Path Avoids Treating Load as Battery Current

If the system load is directly connected in parallel at the battery node, the output current detected by the charger may include both load and battery current, affecting termination judgments. Solutions with Power Path can manage system power supply and battery charging separately, lowering charging current first when input is limited, and having the battery supplement system power if necessary.

Assume the device's actual available input is 18 W, the system consumes 6 W, and the remaining 12 W goes through an assumed 90% efficient charging path, so the battery only receives 10.8 W; at a battery voltage of 4 V, this is approximately 2.7 A. Increased system load will crowd out the charging budget, so when running high-load applications, "charging slows down" cannot be judged solely by adapter power.

Load management and termination detection must be analyzed based on the specific IC's current measurement position, current limiting loops, and battery replenishment behavior; one cannot assume automatic full Power Path capability after connecting a load to any charging module.

Charging Control, Protection, Measurement, and Balancing Are Not the Same Function

Protection thresholds are typically used for abnormal boundaries; do not write "protection acts at 4.25 V" as "charging should be set to 4.25 V." Series batteries require per-cell monitoring: when the pack voltage is normal, one cell may still be too high; balancing strategies depend on the chemical system, configuration, and system design, and not all multi-series solutions use the same balancing circuit.

Coulomb counting integrates current and is affected by bias, initial SOC, capacity degradation, and temperature; it cannot be called precise solely based on "integration." See Battery Technology for more detailed models and series/parallel energy ledgers.

Compare Protocol Intersections for Private Fast Charging, Do Not Rank Fixed Speeds

QC, manufacturer-specific schemes, and USB PD/PPS may coexist on some products, but compatibility direction, cable identification, certification, and fallback capabilities are determined by specific products. You cannot uniformly summarize brand protocols as "phones don't heat up" or "all have shifted to PPS," nor can you assert that a private 100 W system will necessarily charge faster than another PD 65 W system.

For a group of devices, record their actual input steps, programmable ranges, cable ratings, single-port/multi-port allocation, and charging curves. Higher rated capabilities usually do not force devices to absorb more power during normal compatible negotiation; the key is the common mode and device-specific limitations, not just selecting the largest number.

Wireless Charging: Alignment Reduces Losses, But Protocol and Thermal Limits Still Exist

Qi transmits energy via coil coupling and has corresponding control, identification, and protection mechanisms. Magnetic alignment improves geometric coupling but does not mean the wireless link has no losses; the transmitting coil, receiving rectification, shielding, casing, and foreign objects all affect temperature rise and available power.

Qi2 cannot be uniformly written as max 15 W. WPC released Qi2 25W in 2025, corresponding to capabilities related to Qi v2.2.1; specific phones, chargers, and supporting conditions still need to match, and you cannot infer certification or power from "having magnets." WPC: Qi2 25W Release

If 25 W is measured at the wall and 18 W is measured at a clearly defined receiver side, the efficiency for this segment is 72%; if using battery-end power, the result must also deduct system and internal conversion losses. Without measurement boundaries, writing "wireless efficiency is usually 70%–80%" easily leads to comparing the wrong objects. Rated frequency and power also vary by mode; do not use a single frequency to summarize all Qi products.

Recalculating Power and Charging Time

from math import isclose

p, loop_r = 30, 0.1
low_v_loss = (p/5)**2*loop_r
high_v_loss = (p/15)**2*loop_r
assert isclose(low_v_loss, 3.6)
assert isclose(high_v_loss, 0.4)
assert isclose(low_v_loss/high_v_loss, 9)
assert isclose(9*3*0.95/4.2, 6.107142857142857)
assert isclose(2.4/3+0.6/0.75, 1.6)
assert isclose((18-6)*0.9, 10.8)
assert isclose((18-6)*0.9/4, 2.7)
assert isclose(18/25, 0.72)
print('Line loss, conversion current, segmented time, and system power budget checks passed')

The examples in this article are for understanding boundaries; they do not involve operating power supplies, charging cells, or measuring actual fast charging curves.

Continue Reading

• USB and Type-C: CC, cables, and PD contracts.
• Battery Technology: Chemical systems, capacity, series/parallel, and SOC.
• Protection Circuits: Overvoltage, overcurrent, reverse connection, and protection coordination.