A piece of old-fashioned advice for old-fashioned rechargeable hardware was to completely drain the battery before recharging it, to ‘stretch’ it, the way you’d stretch socks to fit on bigger feet.
Where Did the Myth Come From?
While you’re not necessarily stretching the battery by waiting till it’s drained to recharge it, you are saving its capacity. If you charged your Nickel-Cadmium battery rechargeable phone before it was completely drained, it would lose capacity. Manufacturers were doing the best they could with what they had, and the next generation of battery, the Nickel-Metal-Hydride battery, didn’t have the effect as strongly (although it still had it) but the shelf life of the phone while not in use wasn’t great, so users continued to fully drain their devices before each recharge. To make matters worse, it’s possible to overcharge these batteries too, which also reduces capacity. When overcharged, the battery begins to heat up, and water inside the battery that’s normally carrying electrolytes begins to degrade, meaning it can’t do that job as efficiently. So yes – in these early phones, you had to completely drain it, monitor it while it was charging, and then completely drain it again if you wanted it to last as long as it did when it was new.
This overcharging is part of the reason small electronics batteries (like AA, AAA, 9-volt, etc.) have warnings on them telling you that they’re not rechargeable. While in a purely chemical sense, they could be recharged, the danger of overcharging the battery, having that battery behave unpredictably, and then exploding or leaking battery acid when you’re not expecting it to is simply not worth the risk. And the chemicals inside a battery are nasty – sulfuric acid is one of the critical ingredients. (Seriously. Don’t recharge non-rechargeable batteries!)
And then we got to Lithium Ion batteries. Lithium Ion batteries dominate the market because they hold a lot of charge, they don’t have the memory problem (meaning they won’t lose capacity if you charge them at the wrong time), and they’re lighter and flatter than the other kinds of batteries are at the same capacity. That being said, older devices may still lose their charge faster, not only because the battery is older, but also because it was designed in a time before manufacturers knew how long-lived Lithium Ion was going to be. We’d gone from the infamous brick phone to handheld, lightweight Nokias within a generation. Of course they were designed like they’d only last 5 years, because that’s historically been true!
Be Careful Charging!
Speaking of charging, faulty chargers can do more harm than good. A man discovered that Amazon was not vetting USB-C sellers effectively when he plugged one into his Apple computer and watched it short out the USB port. The phone itself was fine because it’s designed with this problem in mind (and the computer itself wasn’t damaged outside of the now-defunct port) but the USB port was simply designed to put out all the power it could. Normally cords restrict this flow, because little desk trinkets like fans don’t have those same guards. USB ports, however, are programmed to put out as much power as the cord will allow, which is how you get some cords that can quick charge and others that can’t. Old brick- and flip-phone cables also lacked anything telling them to stop when full.
Electric flow needs some sort of resistance. If it doesn’t have any, the flow of electrons across metal or wiring can generate heat and eventually catch fire. This is why you don’t plug two 9-volts into each other even though the bits at the top fit. It will create a lot of heat, and in a worst-case situation, could even catch fire!