When charging a partially discharged NiMH or NiCAD battery, you are actually overcharging it which increases the growth of crystals within the battery. Celebrate a memorable and meaningful Hari Raya. These crystals have jagged edges and sharp corners which can harm the insides of the battery and decrease its capacity. This is called the “memory effect” because it was as if the battery “remembers” how much energy was drawn on previous discharges and would not deliver more than was demanded before.
You must totally deplete your phone’s battery before recharging it in order to keep it healthy. This myth was once accurate, however it is now mostly false due to the widespread use of Lithium Ion (Li-ion) batteries. If you recharged your phone before it was totally drained when it used Nickel Cadmium (NiCAD) or Nickel Metal Hydride (NiMH) batteries (essentially just over a decade ago), you would actually diminish its total capacity.
Restoring the battery’s capacity was simple, as all you would need to do was completely discharge it and recharge it a couple of times. This would break down the crystals thus returning a majority of the lost capacity. Li-ion batteries (the kind most phones use now), on the other hand, do not suffer from the memory effect so recharging your phone while it’s only 50% discharged won’t impact its capacity. Conversely, letting a modern phone drain all the way to zero will damage the battery and shorten its lifespan so it’s actually a good idea to always top it up.
This isn’t a hard and fast rule, however, as most modern phones are built to tolerate such use, it’s just that turning it off once in a while while it’s charging will give your device that little bit more longevity.
Crystal build-up is also the culprit here, it’s just that how they build up in a Li-ion battery is different from NiCAD and NiMH. It’s also a good idea to turn your phone off when charging. It will charge faster as apps running in the background along with the phones WiFi and radio won’t leach power from the battery as it charges. Also, leaving your phone on while it charges can damage its circuit board and other parts as it might get too hot.