Why do some phone batteries ‘die’ before reaching 0%?


Why do some phone batteries ‘die’ before reaching 0%?

In: Technology

Cell phone batteries as they get older often have issues with judging 0% to 100%. This in most cases leads to the phone shutting down below 10% as the phone is displaying incorrect battery storage info.

Voltage and Charge are different things. The voltage of the battery goes down when it discharges. If the voltage is to low the device needs to power off to prevent the battery from taking permanent damage. Aged batteries have a higher internal resistance that make the voltage drop further when under heavy load, causing the „emergency“ shutdown to kick in when there is still charge left.

Apple put in a mechanism that reduces peak discharge for old batteries, making them not die suddenly. It does it by reducing the CPU speed, when the battery is low.

While it will vary by battery and with age, as a battery drains it won’t be able to deliver the current (amps) it could otherwise at higher charges or if it was newer. If you try to force too much current out of a battery the voltage drops (this is part of the internal resistance someone else mentioned) and that can cause the phone to spontaneously power off since its incoming power is no longer within its voltage tolerance. If you have an old device that suddenly shuts itself off when put under load (eg: playing games) this may be why, and it’s a big part of why Apple implemented that feature that slows down your phone with age.

Also Lithium Ion batteries if over-drained will permanently die. So there is an incentive to shut them down a little bit early to keep them outside of that danger zone.