Why Lithium-Ion Batteries Lose Capacity Over Time

Lithium-ion batteries wear out because of three things: how many times they are charged and discharged, how hot they get, and how high a voltage they are held at. Battery University, a reference site on batteries, says environmental conditions often matter more than cycling alone. This article gathers the numbers it and Apple publish, with one caution: figures vary by maker and by test.

Depth of discharge: shallow cycles last longer

Depth of discharge (DoD) is how much of the battery's capacity you use before recharging. Battery University's table gives approximate numbers of cycles before capacity falls to 70 percent. The smaller the dip, the more cycles the cell survives.

• NMC cells: about 300 cycles at 100 percent DoD, 400 at 80, 600 at 60, 1,000 at 40, 2,000 at 20 and 6,000 at 10 percent.
• LiFePO4 cells: about 600 cycles at 100 percent DoD, 900 at 80, 1,500 at 60, 3,000 at 40, 9,000 at 20 and 15,000 at 10 percent.

NMC (nickel manganese cobalt) and LiFePO4 (lithium iron phosphate) are two common chemistries. The site cautions that real-world life is often shorter than laboratory estimates.

Voltage: the cost of a full charge

Charging a cell to a higher voltage stores more energy but wears it faster. Battery University lists roughly 300 to 500 cycles for a cell charged to 4.20 volts, and says that each 0.10-volt reduction in peak charging voltage is said to double the cycle life. The cost is lower usable capacity. The site's prose and table are not fully consistent on the exact cycle counts at slightly lower voltages, so treat the figures as a rough guide.

This is one reason some devices offer options that stop charging short of 100 percent. Whether a particular device does so is up to its maker.

Heat does lasting harm

Battery University warns that temperatures above 30 degrees Celsius and sitting at full charge stress the cell. Its estimate of the capacity recoverable after a year of storage at 100 percent charge is about 80 percent at 25 degrees, 65 percent at 40 degrees and 60 percent at 60 degrees (after three months at that last temperature). At 40 percent charge and 40 degrees, it estimates about 85 percent after a year.

Apple's guidance on its devices agrees on the main point. It gives an ideal ambient range of 16 to 22 degrees Celsius and says exposure above 35 degrees can permanently reduce battery capacity, and that charging in high heat causes further damage. Cold, by contrast, temporarily reduces battery life, which returns once the battery warms up.

Storage

For a battery to be stored for a long time, Apple suggests charging it to about 50 percent: fully charged storage can reduce capacity over time, while a fully drained battery can fall into a deep-discharge state where it may not hold a charge again. Apple also advises powering the device off, keeping it in a cool, dry place below 32 degrees Celsius, and topping it up to 50 percent every six months if stored longer than that.

Other stresses

Battery University also lists high currents and ultra-fast charging among factors that shorten life. Apple says built-in charge management software aims to reduce battery aging, so some of the wear control happens without any action from the owner.

Summary of the main points

• Smaller discharge swings give more total cycles.
• Lower peak voltage lengthens life but reduces capacity.
• Heat is the main outside threat; a cool environment helps.
• For long storage, a half charge in a cool place is the guidance from Apple.
• Published cycle counts are approximate and depend on the cell and the test.

Sources

• BU-808: How to Prolong Lithium-based Batteries, Battery University
• Maximizing Battery Performance, Apple