25th
Marco wrote a good post about what you should know about your laptop battery.
It is false to believe that you need to empty your battery and charge it back regularly to extend its life. And this is true for all the lithium-polymer batteries used in any modern laptops these days.
To extend your battery life, you should plugged your laptop much as possible and run it on battery for a while at least once per month. It’s pretty difficult to own a laptop and not do this!
Here’s how lithium-ion batteries behave:
Due to their chemistry, their capacity slowly diminishes with age. Laptop batteries usually lose most of their useful capacity 2-3 years after manufacture (not initial use).
If you use the laptop on battery power a lot, the battery lifespan will be shortened. This “wearing out” effect is much less severe than with older battery technologies, but is still present. This is why you should plug it in if it’s convenient.
When plugged in, the battery is not in use. The laptop’s power circuitry bypasses the battery unless it’s needed.
Apple recommends that you run it on battery for a while at least once per month. The recommendation for monthly battery usage isn’t just for capacity preservation: it’s mostly so the charge indicator can maintain accuracy as the battery’s capacity decreases naturally over its lifespan.
In conclusion: You shouldn’t technically leave your laptop plugged in all the time, but you certainly don’t need to deep-cycle it “a few times a week”.
