Battery life and your laptop (cellphone, too!)

March 15, 2012 in hardware by Gerry Frawley

There is a lot of bad information out there about battery life when it comes to charging/not charging.

I blame it on the Internet.

The probably with the internet is once it is out there, it is out there forever. I can still find tracks of things I did on the internet way back in the days when we used things like gopher, archie, usenet, etc.

As such, old ideas tend to get repeated over and over again, even when they are no longer relevant (talk to me sometime about fixed page files).

This is certainly true about batteries.

Back when laptops were about as expensive as a new car and about the size of a suitcase (ok, ok exaggerating a bit), they used NiMH and NiCd batteries. People would always caution you to fully charge and then run out your batteries completely once a month. The rationale behind this idea was that these types of batteries had a memory effect–if you kept a battery at 70 percent charge it would eventually set that as the new ceiling.

Today, in most current electronics that have batteries (cellphone, tablets, laptops) we use Lithium-Ion batteries. Unlike the old NiMH and NiCd batteries, these batteries last longest if the battery is frequently charged; actually fully discharging them will degrade their capacity relatively quickly. A partial discharge, however, reduces stress and prolongs battery life.

It should be noted that high temperatures and high currents also affect cycle life negatively, so keeping batteries cool and charging them slowly is better (every time I see someone with a cellphone on a dashboard I want to shake them).

There is a caveat, here, though. When storing lithium batteries, they actually degrade more while fully charged than if they are only 40% charged. Again, degradation also occurs faster at higher temperatures. So, if you’re not going to be using a battery for a while, run it down to about 40 percent, remove it from the laptop, and put it in a cool place.

One more thing I get asked about laptop batteries all the time: what happens when I leave the laptop plugged in all the time? The good news is you don’t have to worry about this anymore. Under normal circumstances the charging circuit in your modern laptop will turn off once the lithium-ion battery is full. You might have noticed when you hover over a battery icon on your laptop that it says “fully charged.” That means your laptop knows to stop charging it. You might also have noticed that when it is charging, your battery gets warm, but once fully charged it is no longer warm. Leaving your laptop plugged in should be fine.