Caring for your device's battery: myths and realities

by Thomas Rutter

Unfortunately, there are many popular myths about how to properly care for the Lithium based rechargeable batteries in phones, tablets and laptops. I'm going to attempt to set a few things straight and provide some simple guidelines.

Caring for your device's battery: the short version

  • Don't run the battery right down to fully empty (0% or power off) too often.
  • It's OK to recharge when the battery is partially full.
  • It's OK to leave the charger connected even after the battery is full.

Myths

"Batteries should be fully discharged before you recharge."

Like many such myths, this one has some truth when talking about certain other types of batteries, but it does not apply to the lithium-ion batteries used in mobile devices. In fact, the opposite is true: it is considerably better for battery longevity to top it up regularly and rarely let it become fully discharged. Fully discharging a lithium-ion battery will degrade its capacity more quickly if it is done regularly. If it's only done occasionally, and never left discharged for very long, this won't do as much harm.

Some people argue that a battery needs to be fully discharged occasionally in order to reset the "calibration" in the battery meter. Modern devices should not need this in normal circumstances, particularly if the battery is undamaged. While doing this once every few months won't cause significant harm to your battery, it is unlikely to provide any real benefits. The calibration of the meter won't, for example, influence how much charge the battery can hold.

"Batteries can have memory effect."

True "memory effect" in a battery is a phenomenon that only occurs in sintered-plate nickel-cadmium (Ni-Cad) batteries such as those used in alarm systems or some remote-controlled vehicles. It has never occurred in lithium-ion batteries, or for that matter other battery types such as other forms of Ni-Cad batteries or NiMH batteries.

That said, people frequently use the term "memory effect" when referring to other things - for example, some people use the term to refer to any type of degradation in battery performance over time. While battery performance does degrade over time, it is not helpful to always refer to it as "memory effect" when the actual cause is not related to the original memory effect that happened in sintered-plate Ni-Cad batteries. Doing so falsely implies that all degradation in battery performance has the same cause, when in reality different battery types will degrade for different reasons, and some level of degradation will occur slowly over time even if they are used normally.

"It is bad to leave devices on the charger once they are fully charged."

Lithium-ion batteries and their charging circuits always contain a protection circuit that stops charging the battery once it has reached a certain charge level. This protection circuit fully stops the charging process; there is no trickle charge on a lithium-ion battery.

Therefore, leaving the device connected to the charger after it has charged will not have any further effect on the battery, except that the charger may begin charging again once the device has used a significant amount of charge and its charge level has fallen below a certain threshold.

Battery longevity explained

The rechargeable batteries used in all modern cellphones, tablets and laptops are lithium-ion batteries. This includes lithium-polymer batteries, which are a subset of lithium-ion batteries.

Lithium-ion batteries are sensitive to being charged too far, being discharged too far, and heat. These batteries will last the longest the more they are kept at a medium charge level and stored in a cool environment.

All lithium-ion devices include a protective circuit that prevents further charging once the battery has reached a particular charge level. This overcharge protection is mandatory in a lithium-ion charging device due to the risk of explosion if a lithium-ion cell is charged beyond a certain point. Unlike other battery charging circuits which continue to trickle charge the battery if the battery is left on the charger indefinitely, lithium-ion batteries never trickle charge due to this protection circuit: therefore, leaving the battery on the charger will simply deactivate the charger once it is deemed to be fully charged.

It's not just over-charging that is a problem for lithium-ion batteries, but also over-discharging. That is, if the battery becomes too flat, it can cause damage to the battery. This won't be in the form of explosions, but may reduce the longevity of the battery. To prevent over-discharging, devices that use lithium-ion batteries usually turn their power off when the battery drops to a certain level. At the point at which the battery reaches the device's "0% full" level, the battery is not yet fully discharged, but since the device will not use the rest of the charge below that point, this is factored out of the battery capacity quoted.

If you were to fully discharge a lithium ion battery and then leave the battery in that discharged state for some time - a few days or even weeks - metallic plating would begin to happen inside the battery, permanently degrading its capacity. When storing a lithium-ion battery for a long time without using it, it's important that the battery should be charged to about half its capacity, to avoid damage associated with fully discharging the battery.


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