1. Why a Phone Cannot Use Wall Outlet Electricity Directly
On the surface of a charger, small numbers print the input and output specifications. The side connected to the wall uses Alternating Current (AC), while the side connected to the phone uses Direct Current (DC). This difference determines the charger's first job. [1]
AC is electricity where the direction of flow switches back and forth. DC is electricity that flows in one constant direction. An AC adapter (charger) is a device that changes AC into DC. [1] But how is this conversion done? The method used greatly changes the size and weight of the box.
2. How Changing the Conversion Method Makes a Charger Smaller
The first method is the linear method (transformer method). It uses a transformer to lower the voltage, then uses diodes and capacitors to rectify (align the direction) and smooth (flatten the waves) the electricity into DC. Because it needs a large transformer, this method tends to make the charger big and heavy. [1]
The second method is the switching method. It turns AC into DC first, then uses switching elements (like MOSFETs) to turn the current on and off very fast. This creates high-frequency pulses to adjust the output voltage. Because it uses high frequency, it can be made smaller and lighter than the transformer method. [1] Some explanations say that the higher the frequency, the easier it is to make the device small.
This is a bit like adjusting the amount of water coming from a tap by opening and closing it extremely fast. However, in reality, the electricity is chopped into tiny pieces, so it is different from the flow of water itself. [1] This process made chargers small enough to fit in your palm. Then, a new request appeared: to send as much power as possible through this small box.
3. Why Chargers Need More Power Without Raising Voltage Alone
Electrical power is calculated by multiplying voltage (volts) by current (amperes). The standard for USB (BC standard) is 5V with a maximum of 1.5A. Calculating this gives 5V × 1.5A = 7.5W. [2]
To increase power while the rule says the voltage cannot go above 5V, the only option was to increase the current. However, concerns arose about safety, such as currents going outside the standard. Because of this, individual methods like Qualcomm’s Quick Charge were created. Later, USB PD became the standard for delivering high power. [2]
Let’s look at the calculation to see what changes when voltage rises. If you stay at 5V with 3A, that is 15W. But if you use the same 3A at 20V, that is 60W. Even without increasing the current, raising the voltage increases the power. [2]
If you change the voltage, the phone must be able to receive that voltage. So, how does the charger know this?
Non-standard currents and safety concerns led to proprietary methods like Qualcomm’s Quick Charge, and USB PD became the standard for high power. [2]
4. How a USB PD Charger Offers Options and the Phone Chooses
In USB PD, the charger first tells the phone the combinations of voltage and current it can supply (power rules), such as "I can provide 5V at 3A, or 9V at 2A." The phone chooses what it needs from these options. The charger can reject requests that are not appropriate. [2]
Using this example, 5V × 3A = 15W, and 9V × 2A = 18W. This means the phone can choose combinations with different power levels. [2]
The standard voltages are 5V, 9V, 15V, and 20V. In USB PD 3.1, announced in 2021, voltages of 28V, 36V, and 48V were added, allowing up to 240W. [2] [3]
Let’s check the math. 28V for 140W, 36V for 180W, and 48V for 240W are stated. If you divide the power by the voltage, they all equal 5A (140 ÷ 28, 180 ÷ 36, 240 ÷ 48). The previous maximum of 100W is also 5A if using 20V (100 ÷ 20 = 5A). This shows that power is increased by raising the voltage while keeping the current at 5A. Note that only devices compatible with the new voltages can use 240W; ordinary chargers usually stay in the 5–20V range. [3]
Devices only request the power they need. However, PD only works if the charger, cable, and device all support it. [3] So, in what order does electricity enter the battery inside the phone?
5. Why Current Decreases When Battery Charging Switches to Constant Voltage
Charging lithium-ion batteries generally uses a two-stage method. First, electricity is supplied at a constant current (constant current phase) until the voltage reaches a target value. Once the target is reached, it switches to a phase where the voltage is kept constant (constant voltage phase). At this time, the current naturally decreases. [4]
For cobalt-based batteries, the upper limit per cell is 4.20V (with a tolerance of ±50mV). A full charge is defined as the state where the current drops to 3–5% of the battery capacity (Ah). [4] For example, if the capacity is 3Ah (3000mAh), then 3 × 0.03 = 0.09A and 3 × 0.05 = 0.15A, so it is considered full when the current reaches 90–150mA. The 3000mAh figure is a hypothetical number for calculation. [4]
The upper voltage limit differs by battery type; 4.2V is an example for cobalt-based batteries. [4] According to Battery University, lithium-ion batteries cannot withstand overcharging. If the set voltage is exceeded, metallic lithium may appear, or gas may be produced, increasing internal pressure. Therefore, strict control to maintain the voltage limit is necessary. [4]
The decrease in current is a result of maintaining this upper voltage limit and can be read as part of a design to avoid danger. [4]
6. How Qi Wireless Charging Uses Coils and Magnetic Fields
Qi is wireless charging that uses electromagnetic induction. When AC flows through the transmitting coil in the charging pad, a magnetic field is created. This induces an electromotive force in the receiving coil on the phone side, causing a current to flow. Qi is standardized by the Wireless Power Consortium (WPC). If devices are Qi-compatible, they can be used together even if made by different manufacturers. [5]
Efficiency is better when the coils are close, so there are tricks for misalignment. Methods include moving the transmitting coil to find the receiving coil, or arranging multiple coils. [5]
There is also a mechanism to detect foreign objects. The magnetic field from the transmitting coil can cause eddy currents in metal objects like coins or keys, making them hot. Therefore, patent documents explain a mechanism where the phone side reports the received power to the pad. If the difference between sent and received power is large, it judges that metal is present and lowers or stops the power. [6]
7. How Countries Are Standardizing Charging Ports
In the EU, an obligation to unify charging ports to USB-C for smartphones, tablets, earphones, digital cameras, and other devices began by the end of 2024. For laptops, this applies 40 months after the law takes effect. [7]
Buyers will be able to choose between devices that come with a charger and those that do not. The idea is that people who already own chargers do not need to buy multiple copies of the same thing. [7]
With standardized ports, the procedure for safely deciding voltage and current becomes crucial. Agreements like USB PD, discussed in section 4, determine actual usability. [7]
8. How to Calculate Power from the Labels on a Charger
Look at the "Output" section on a charger at home. Multiply the voltage (V) by the current (A) to find the watts. Comparing the included charger with an older one lets you see the power difference in numbers. Do not disassemble chargers or touch outlets with wet hands. [2]
It is also interesting to record your phone’s battery percentage every 15 minutes and make a graph. You can observe if the charging slows down as the battery gets full. However, the slowdown might be for other reasons, and this article’s explanation alone cannot prove it definitively. [4]
At an electronics store, compare chargers marked "PD compatible" or "Qi compatible" to investigate the relationship between wattage and price. [3]
Read the "Input" section on the charger too. It shows the voltage and current it takes from the outlet. The input watts are usually slightly higher than the output watts. This is because some energy escapes as heat during the conversion process. [1]
Disassembling a charger is dangerous. Do not do it. [1]