Interesting things in the world of electronics served as easy to digest articles with a touch of wit.

Every gadget in the world requires electrical energy to function. Your travel bag most probably holds multiple chargers: one for your phone, one for your laptop, one for your bluetooth headphones, smart watch, power bank; you get the idea. The unavailability of these small things can send us into quite a panic when we get the low power mode notification. Let us see what goes inside this gadget.

Basic Circuitry

The basic idea of a battery charger is to provide a higher voltage than the present operating voltage of the battery. Current flows from higher to lower voltage. Our circuit will look like:

A regular lithium cell has a nominal voltage of 3.7V and a maximum voltage of 4.2V. We need to provide 4.2V for charging and to terminate charging once current flow is below a certain threshold. Seems simple enough till you start to implement it. All cells have an internal resistance associated to it, the value depends on the type of cell used as bigger and more powerful cells have a higher internal resistance value than small phone cells. We assume our lithium-ion cell to have 50 miliohms of internal resistance and around 3V when nearly discharged. Let us find the current passing through the circuit:

where V_in is the input voltage, V_battery is the battery voltage, R is the internal resistance of the battery, and I is the current.

That is a huge amount of current to be flowing in a tiny Lithium-ion cell! Looking at the scope also confirms the value. The reduction in voltage is due to the internal resistance of the 4.2V DC source and the negative sign for current is only indicative of direction.

Our next problem is to control current. The best way to charge a battery is to divide the charging regimes into two distinct phases. The first phase is keeping current constant, we control voltage in such a way that we allow a pre-determined amount of current to flow into the battery for optimal charging. The second phase is keeping voltage constant to the maximum voltage of the cell, a voltage difference is what allows current to flow. As the difference reduces, the current goes down till the voltage difference becomes negligible and charging is terminated.

The easiest way to control current in a circuit is to use a resistor, but we cannot control the value of resistance for proper current manipulation. So the resistor fails the first test. The next option is a potentiometer, a component where resistance can be changed by rotating a knob. The problem here is that rotation is to be done manually, so unless one wants to sit by the charging circuit and constantly fiddle with the knob, we need to find an electronic solution.

MOSFETs and BJTs are the option after resistors as they can be electronically controlled and can regulate current. The problem with resistive solutions to current control is that the energy is always dissipated as heat. A small amount of heat dissipation is acceptable but when going towards fast charging devices, thermal management becomes crucial for device health and functioning. A linear current regulator falls into the same pitfall.

This is where some very useful power electronic circuits come into play, buck converters and buck-boost converters are the most useful among these. Instead of using a MOSFET as a voltage regulator, we use it as a switch and the passive components do the rest of the work. The inductor stores energy and releases it as a ripple and the capacitor smooths out the waveform. The only thing one needs to do is change the duty cycle of the transistor. I urge you to go to Falstad circuit simulator and try removing the capacitor and the inductor to see their functioning.

The final block diagram of our charging device looks like this:

The power in our sockets is in the form of AC, so converting it to DC is required for converter operation. The microcontroller is coded with the instructions for converter control by measuring current, voltage and temperature in the cell.

Power Scaling and Unchecked Consequences

To understand the idea of the rapid increase of battery charger powers we shall first look at the increase in appliances utilizing an energy storage device.

Table 1: Charger ratings and battery capacities of various electronic goods

Name	Battery Capacity	Charger Power
Smartwatch	0.5-2 Wh	1-5 W
Wireless earbuds	0.5-3 Wh	1-5 W
Smartphone	10-25 Wh	10-100 W
Laptop	40-100 Wh	45-400 W
Electric scooter	300-1000 Wh	100-500W
UPS	500 Wh – 10 kWh+	300 W – 3kW+
Home inverter and battery	1-10 kWh+	500 W – 5 kW+
Plug-in HEV	5-25 kWh	1-7 kW
Electric Car	30-100+ kWh0	3.3-350+ kW
Electric Bus	200-600+ kWh	50-600+ kW
Electric Truck	200-1000+ kWh	100 kW – 1 MW+

With the exception of an electric bus or an electric truck, most of you have or will own most of these appliances. The range of charging wattage changes rapidly after hand-held devices. The principle of charging remains the same but the scale changes drastically. A small phone charger short circuit will lead to an MCB tripping at most (due to some hard lessons), but an EV fast charger running into an overcurrent failure can lead to quite a disaster.

Some well-documented incidents can be found below:

• DC fast charger arc flash, 2024: On 16 August 2024, in Hope, British Columbia, a Tesla was connected to a non-Tesla commercial 200-kW DC fast charger using an aftermarket adapter from A2Z EV. This adapter lacked an approved certification mark for Canada and was not approved by Tesla or most charging networks. During startup, a short circuit occurred in the adapter/charging interface. This resulted in a large arc-flash that knocked the owner to the ground, blew apart the adapter, and caused heat and burning damage to the vehicle, charging cable, connector, and internal components of the commercial electrical vehicle charger.
• JuiceBox chargers investigated after fires, 2024: In October 2024, the US National Highway Traffic Safety Administration (NHTSA) opened a preliminary investigation into Enel X Way JuiceBox EV Level 2 residential chargers after receiving six complaints involving fires and electrical malfunctions. In one complaint, an individual claimed that charging their vehicle with a JuiceBox led to an explosion in their garage that set fire to a home and rendered it a total loss. Another individual alleged that a car caught fire in their driveway while charging with a JuiceBox. Four other complaints described electrical malfunctions tripping circuit breakers and disrupting electric vehicle charge cycles.
• E-rickshaw charging station fire, 2025: In Shahdara, Delhi, May 2025, a fire broke out at a facility used for parking and charging e-rickshaws. Two people died and four others suffered burns. Delhi Police said the preliminary investigation suggested the fire originated from the e-rickshaw charging setup and spread rapidly. The facility was a tin-roofed structure where vehicles were charged overnight. This case showcases the importance of correct environment and charging protocols due to the absolute failure demonstrated here.

Increasing power requires an increase in safety protocols. Every single failure must be accounted whether it starts from user carelessness or a grid failure. The best cure to a problem is precaution; be mindful of your charger’s surroundings and you will never have to use your charger’s warranty.

Hot and Heavy

Increasing your charger’s power capacity requires increasing thermal management protocols. Heatsinks, fans, blowers, liquid cooling, and other heat transfer solutions require extra power and space. Small phone chargers do not produce much heat and can be packaged in compact form. Moving up the power ladder requires more space for components, safety mechanisms and temperature control apparatus.

Phone chargers range from 10W mini chargers to bulky 100W GaN chargers which have the capability of charging your phone very quickly, that is, if your phone supports such fast charging. Laptop chargers scale in a similar fashion; the power brick keeps increasing in size according to the specifications of your laptop. A laptop meant for working simple tasks and processes does not require a graphics card and possesses a small battery, a laptop of this kind requires a modest power brick of at most 100W. A gaming laptop has a graphics card which takes a lot of power. Gaming laptops are infamous for low battery lives and even lower up time without a charger. The charger will have a power rating of 200W or above if it is a really powerful gaming laptop.

Always read your charger’s specifications before buying or using the appliance. Let us learn to read what is written. In figure 4, after input, we are given quite a few numbers and letters. AC (~) means alternating current (the usual), so don’t plug it into a DC (⎓) power source. 110-240V is the range of input voltage it can take and 50-60 Hz is the range of frequency. USA uses 120V 60Hz and India uses 230V 50Hz, this charger can be used in both the countries, only an external adapter is required for USA due to different sockets. 1.5A is the maximum current rating of the charger. The output portion shows the maximum power rating of the charger and the different ports having their own ratings.

The laptop charger is slightly more detailed in its output description, the current and voltage ratings are given along with the power rating.

The charger in figure 7 is bigger than the one in figure 6, it shows the advances in thermal management solutions over just a few years. Forgive me for the text being unclear, I have had them for quite some time.

The same goes for EVs. They range from slow, low power (3.3-10 kW) chargers which allow you to replenish your EV’s battery overnight, all the way to commercial charging station level of chargers which take 10 minutes to charge your battery to its highest allowed capacity. The size in EV chargers also vary wildly based on energy consumption.

Adapting to the situation

Using a 100W charger with a phone rated for 30-40W charging seems like a bad idea, right? It seems obvious here because the higher current or voltage rating can damage the phone but what if I told you that the two devices agree on terms before beginning the charging cycle? The USB-C has data wires and power wires, the data wires are used to send information about the charging capabilities of the charger and required power for the phone. They both agree on terms and enter a “power contract”. This only works universally with chargers using a USB-C cable as before this all companies had their own charging protocols and phones were expected to run on their chargers respectively. DC barrel jacks have pre-determined voltages.

What happens when the device is not supposed to be detached from the charger? EVs have regeneration braking, which allows energy to be stored during braking. You either have to make separate charging and discharging circuits or make one bidirectional circuit. This same idea is used for home inverters and solar panel energy storage systems. The same circuit can allow power to flow from the battery and to the battery, this helps in system miniaturization.

Duties as a consumer

As a consumer, you should know the devices that you use on a daily basis to reduce the need to go to a repair shop just because of a lack of knowledge. Always use chargers with the correct output ratings. Some devices may have 120V written on them if they were imported from abroad and will fail when coming into contact with 230V.

EVs should always be charged at first party or first party approved chargers. Never take a risk with unapproved or experimental chargers as they may bypass security protocols to provide faster charging.

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Frequently Asked Questions (FAQ)

Ques: How to check your laptop’s battery capacity?

MS Windows: go to command prompt and type powercfg /batteryreport.

Linux: Go to terminal and past upower -i $(upower -e | grep BAT)

MacOS: Open terminal and run system_profiler SPPowerDataType

Ques: How to read charger output?

A charger’s output tells you the voltage (V), current (A), and sometimes power (W) that the charger can provide to a device. For example, an output rating of 5V ⎓ 3A means the charger can provide up to 5 volts and 3 amps, or about 15 watts in DC. Chargers with multiple output ratings may support different charging modes depending on the device connected.

Ques: How to choose a charger for my phone?

Choose a charger based on your phone’s supported charging standard and maximum charging power. Check the phone manufacturer’s specifications for supported technologies such as USB Power Delivery (PD) or other proprietary fast-charging standards. A charger with a higher wattage rating is generally safe if it follows the correct charging standard because the phone and charger negotiate the appropriate voltage and current.

Ques: How to know whether a charger is genuine or not?

A genuine charger should have clear manufacturer information, model and certification markings, electrical ratings, and appropriate safety certifications. Avoid chargers with poor printing, missing specifications, unusually low prices, or unknown manufacturers. For important or high-power chargers, purchasing directly from the manufacturer or an authorized retailer is the safest way to ensure authenticity and proper safety compliance.

Ques: What is a bidirectional circuit?

A bidirectional circuit is an electronic power circuit that allows electrical energy to flow in both directions between two systems. For example, in an electric vehicle, a bidirectional converter can transfer energy from the grid to the battery while charging and from the battery back to the grid when operating in vehicle-to-grid (V2G) mode. These circuits are commonly used in batteries, renewable-energy systems, EVs, and energy-storage applications.