Every rechargeable lithium-ion battery, from a phone cell to an electric-vehicle pack, holds less energy as it ages. This explainer covers what happens inside the cell, which conditions speed the decline according to US government researchers and a major device maker, and which habits slow it.

1. What a battery does when it charges

A battery stores electricity as chemical potential energy, according to the US Department of Energy (DOE) Office of Science. It has two terminals, the cathode and the anode, separated by a chemical material called an electrolyte. Electrons flow through the external circuit while ions, atoms or molecules with an electric charge, move through the electrolyte to balance them.

In a rechargeable battery that traffic runs in both directions. The catch, DOE explains, is that the chemical processes inside “do not completely reverse” as the battery is charged and discharged. Over time, that incomplete reversal can change the chemistry and structure of the battery materials, which can reduce both performance and safety.

2. Two kinds of aging: time and use

Researchers at the National Laboratory of the Rockies (NLR), a DOE national laboratory, write on the lab’s battery-lifespan page that batteries “degrade with use and storage.” Their life-prediction model therefore captures degradation from both calendar time and cycle aging: a battery sitting unused is still aging.

The lab lists the conditions such models have to account for: temperature, operating windows, charge and discharge rates, storage environment and cycling patterns. The physical processes it models include the formation and growth of a “solid/electrolyte interphase” layer, and mechanical stress at both micro and macro scales, coupled with electrochemistry and temperature.

3. Heat does lasting damage; cold does not

Apple’s battery guidance gives 62° to 72° F (16° to 22° C) as the ideal comfort zone for its devices. The company says ambient temperatures above 95° F (35° C) can permanently damage battery capacity, so the battery will not power the device as long on a charge. Charging in the heat can damage it further, and even storing a battery somewhere hot can damage it irreversibly.

Cold behaves differently. Apple says a very cold environment can shorten battery life, but the effect is temporary: once the battery returns to its normal operating range, performance returns too. It lists 32° to 95° F (0° to 35° C) as the best ambient range for iPhone, iPad, iPod and Apple Watch, and 50° to 95° F (10° to 35° C) for MacBook.

Climate shows up at vehicle scale as well. FuelEconomy.gov, which is administered by Oak Ridge National Laboratory for DOE and the US Environmental Protection Agency, says a DOE study suggests electric-vehicle batteries may last 12 to 15 years in moderate climates and 8 to 12 years in severe climates.

4. Charging speed and the top of the tank

NLR notes that fast charging can cause stresses in a cell’s electrode stack that shorten its lifespan, and that its multiphysics models are meant to help designers reduce those stresses. Device makers manage the same trade-off in software. According to Apple, its batteries fast-charge to 80% of capacity and then switch to slower trickle charging, a combined process the company says extends battery lifespan. Software may also limit charging above 80% when recommended battery temperatures are exceeded.

5. What a charge cycle actually is

Apple counts wear in charge cycles. One cycle is complete when a battery has discharged an amount equal to 100% of its capacity, though not necessarily in one go: using 75% one day and 25% the next adds up to a single cycle. Capacity diminishes slightly with each complete cycle. Apple says its lithium-ion batteries are designed to retain 80% of their original capacity for a high number of charge cycles, a figure that varies by product.

6. Storing a device for months

For long-term storage, Apple’s recommendations are to charge the battery to around 50%, power the device down, keep it somewhere cool and dry below 90° F (32° C), and top it back up to 50% every six months if storage runs longer than that. A battery stored fully discharged can fall into a deep discharge state and become incapable of holding a charge, while one stored fully charged for a long time may lose some capacity.

Common problems

  • Draining the battery before every recharge. Apple says there is no need to let its lithium-ion batteries discharge 100% before recharging; partial charges simply add up toward cycles.
  • Sudden battery loss on a freezing day. Apple describes cold-weather loss as temporary, unlike heat damage, which can be permanent.
  • A phone that gets hot while charging. Apple says charging inside certain styles of case may generate excess heat, which can affect capacity, and suggests taking the device out of its case if it gets hot.
  • A device that will not start after months in a drawer. Apple warns that it may be in a low-battery state and may need 20 minutes of charging with the original adapter before it can be used.

Corrections and tips: email the editor using the address on the Contact page.