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Long rant ahead. Should take about 2-3 minutes to read. So feel free to refill your cup of coffee and take a seat :)

It turns out that the battery in my new Nexus 6P is almost dead. Well not that I didn't expect that, the seller even explicitly put that in the product page. But it got me thinking.. why? Lithium batteries are often good for some 10k charges, meaning that they could last almost 30 years when charged every day! They'd outlive an entire generation of people!

Then I took a look at the USB-C wall charger that Huawei delivered with this thing. A 5V 3A brick. When I saw that, I immediately realized.. aah, that's why this battery crapped out after a mere 2 years.

See, while batteries are often advertised as capable of several amps (like 7A with my LiitoKala 18650 batteries that I often use in projects), that's only the current that they can safely take or deliver without blowing up. The manufacturer doesn't make this current rating with longevity in mind. It's the absolute maximum in current that a given battery can safely handle.

The longevity on the other hand directly depends on the demand that's placed on the battery. 500mA which is standard USB 2.0 rating or 1A which is standard USB 3.0 rating, no sweat. The battery will live for at least a decade of daily charges and discharges like that no problem.

But when you start shoving 3A continuous into a battery, that's when it will suffer. Imagine that your current workload is 500mA and suddenly you get shoved 6 times that work upon you. How long would you last?

Oh and not only the current is a problem, I suspect that it also overvolts the battery to maintain a constant current all the way till the end. When I charged my lithium cells with my lab bench power supply, the battery would only take a few milliamps when it got close to the supply voltage. Quick bit of knowledge: lithium cells are charged at constant current first, then when the current drops below that, it continues at constant voltage - usually 4.2 or 4.35V depending on the battery. So you'd set your lab bench power supply at 4.2V 500mA. But in that constant voltage mode, as the battery's voltage and the supply's voltage equalize, the current drops because the voltage difference becomes lower. Remember, voltage is what causes current to flow. Overvolting at the supply to stay in constant current mode all the way till the end speeds this process up but can be dangerous and requires constant monitoring of the battery voltage.

So, why does Huawei and a bunch of other manufacturers make these 3A power chargers? Well first it's because consumer demands ever more, regardless of the fact that they can just charge at 500mA for the night (8h of sleep) and charge a 4000mAh battery from 0 to 100% no problem. Secondly it's because sometimes you need that little bit of extra juice fast, like when you forgot to plug the damn thing in and you've got only 30 minutes in the morning to pour some charge into it.

But people use those damn fucking things even when they go to bed, making that 3A torture a fucking standard process!! And then they complain that their batteries go to shit?!

Hopefully this now made you realize that the fast charger shouldn't be used as a regular charger ^^