# Your battery holds less every year. Where does the charge go?

- Series: [How Computers Work](https://miu.fm/howcomputerswork.md)
- Published: 2026-10-21
- Page: https://miu.fm/howcomputerswork/your-battery-holds-less-every-year-where-does-the-charge-go

## Story

Your battery holds less every year. Where does the charge go?

Not used up. Much of it is still inside, locked away.

Take a laptop a few years old (our example): over a thousand charge cycles, and full now means 75% of what it held when new. Apple designs a Mac laptop's battery to keep up to 80% of its charge at its maximum cycle count. One cycle is a whole battery's worth of use in total, not one plug-in: Apple's own example is 75% one day and 25% the next.

Inside, lithium moves between two sides: into the graphite when it charges, back out when you use it. That back and forth won the 2019 Nobel Prize in Chemistry (Whittingham, Goodenough and Yoshino): "lithium ions flowing back and forth between the anode and cathode", not chemical reactions that break the electrodes down. But while it's charged, the liquid around the lithium slowly reacts with it and builds a thin film on the graphite. The film traps lithium so it can't carry charge any more. Engineers call it loss of lithium inventory. It isn't the only way a battery wears, but at ordinary temperatures it's a big one. Heat speeds it up, and so does sitting full.

What helps: charge whenever you like, there's no need to drain it first. Keep it cool, and don't keep it at 100%: that's what the 80% charge limit and optimized charging are for.

The racks and beads are our own simplified drawing of the idea, not of any real cell.

What's your battery health at right now?

Sources: Royal Swedish Academy of Sciences, The Nobel Prize in Chemistry 2019, press release; S. E. J. O'Kane et al., Lithium-ion battery degradation: how to model it, Physical Chemistry Chemical Physics, 2022; P. Keil et al., Calendar Aging of Lithium-Ion Batteries I, Journal of The Electrochemical Society, 2016; Apple, Batteries: Why Lithium-ion and Maximizing Performance; Apple Support on charge cycles, cycle counts, Optimized Battery Charging and Charge Limit.
Sounds and music: made by us.

## Watch

- Instagram: https://www.instagram.com/reel/DewTl60kgFs/

---

Under copyright law, a quotation must name its source: Miuradio, https://miu.fm/howcomputerswork/your-battery-holds-less-every-year-where-does-the-charge-go

© 2026 Miuradio, https://miuradio.com