Well, since you're so nice telling me to fuck myself while asking me to teach you, I'll do just that, you dumb little shit. So sit down and shut the fuck up.

Batteries can do two things, both exclusively, meaning the two can't happen at the same time:

• charge
• discharge

Energy either goes into the cell (charging) or goes out of the cell (discharging). The two literally can't happen at the same time, due to the fundamental laws of physics, how electrons behave within the mediums inside a battery cell.

Now that we cleared up, let's discuss how a BMS works. A battery management system, as the name suggests, manages a battery. This is important because the lithium ion chemistry isn't exactly stable, and thus you need to manage it. Like I'm managing all this information for you, you vapid cunt.

So what a BMS does is ensuring that the cell neither goes below a safe voltage (over-discharge), nor does it go above (over-charge), nor does the cell pull or deliver too many amps (overheating issues). As I said, it manages the battery. A BMS is essential in any lithium ion battery application, let it be a phone, an UPS, battery wall, electric scooter/bike/car/airplane/boat, even shitty little RC cars and drones.

Now, the BMS itself handles what you call "bypass charging", aka not charging the battery beyond its safe voltage, cutting off the power supply to the cell when the right voltage is reached so the cell isn't under active use. Every single BMS since the mid-2000s does this for safety reasons. Every. Single. BMS.

So how this works is, the device by default is powered by the battery, the BMS pulling voltage from the cell and providing it to a number of power rails (via converters, let them be buck, step-up or step-down or any other configuration, even voltage dividers - feel free to look these terms up, there should be simple enough explanations that even an absolute doughnut like you can understand). Most devices will have a 5V, 3.3V and 1.8V rail for various applications (most "internal peripheries" like storage chips and sensors require 3.3V, external devices usually use 5V, and SoCs, other core chips use 1.8V). In some cases the 5V rail might not be the main one, but 3.3V, they're freely interchangeable in this though exercise. The point is, there's a primary rail, which will provide power to the rest - and it's usually the highest voltage rail as it requires the lowest amps to deliver higher wattage.

So the battery is nicely chugging along, then suddenly external power is connected. What happens will surprise morons like yourself:

• USB-PD negotiates voltage
• 5V rail is then powered by the external source, either directly or by conversion
• other rails are now supplied by the 5V rail
• this triggers the PMIC to signal the BMS that it can begin charging
• BMS detaches battery from the outgoing power rails, ending the discharge cycle
• BMS takes incoming voltage, steps it down to appropriate charging voltage (usually around 4.2-4.3V), and monitors the battery while it charges, limiting current as and when necessary
• system is now powered via external power

This is all neat, so where does "bypass charging" come in? Well quite simple, my dear simpleton, it happens when, quelle surprise, shocked Pikachu face - the battery is fully charged.

See when the BMS detects the battery is charged - by the fact it reports an internal voltage of appropriate volts - it simply cuts off power to it. Charging voltage is dropped to 0, the battery is essentially disconnected (not fully physically, as there's no internal relay inside, but a similar electric gate does stand in the way, disconnecting the cell from the rest of the circuit), and aside from voltage monitoring at an interval, the battery is idle. It's not charging, it's not discharging, it's just sitting there, fully charged. All the while, the system is powered by the external power supply. The only reason the battery is not fully disconnected is so the battery can kick in the moment external power is removed so your system - whatever it may be - doesn't get interrupted. But up until then, there's no charge or discharge happening to the battery.

Oh, and to cover that firmware update change: most modern devices (emphasis on most) use programmable BMSes where the charging voltage (which, again, microbrain, determines the charge status of a battery) can be set. So instead of 4.2V, which is 100% capacity, it can be set to around 4.12V (which is about 80%, well, approx. 78% actually), so the "bypass charging" cutoff happens there.

Now, would you kindly stop swinging that microdick around and put it away before you prove to even more people just how stupid you are?