Know Your Laptop Battery
- A battery cannot be overcharged. There’s no danger of a battery being overcharged if you leave it plugged in all the time. As soon as it hits 100%, it will cease charging and won’t start again until the voltage falls below a certain level.
- Fully discharging a battery will damage it. Having a battery fully discharged for an extended period can put it into a deep discharge state, from which it might never recover.
So, based on this, do we conclude that you should simply leave your laptop plugged in all the time? Not quite.
Things That Damage Lithium Batteries
- Charge/discharge cycles. Every battery has a finite number of times it can be charged and discharged.
- Voltage level. The higher the charge level (measured in volts per cell), the shorter the battery’s life.
- High temperature, over 30 degrees Celsius. This can cause irreparable damage.
The last two are the ones that we’re most concerned with here. A comprehensive study by Battery University highlights how voltage level and high temperatures will shorten the life of a battery in isolation, and even more when they combine.
Lithium-ion batteries charge to 4.20V/cell, which amounts to 100% of its capacity. At this level, the battery will have a lifespan of 300-500 discharge cycles.
Every 0.10V/cell reduction in the charge doubles the number of discharge cycles, until the optimum is reached: 3.92V/cell, with 2400-4000 discharge cycles. Unfortunately, at this level the battery is only 58% charged, so the runtime will be little more than half of a fully-charged battery.
And then there’s heat. Elevated temperatures, typically classified as being over 30 degrees celsius, will shorten the life of a battery irrespective of any other factors. Simply leaving your laptop in your car on a summer’s afternoon is a bad idea.
When the stress of high temperature combines with the stress of high voltage, the effects are even greater.
Charged to 100% the capacity falls to 65% under the same conditions. For a fully charged battery at 60 degrees the capacity plummets to 60% in just three months.
The evidence seems clear. Keeping the battery permanently charged at 100% will slowly shorten its life. Keeping it at 100% and exposing it to high temperatures will shorten it much quicker.
Should You Remove The Battery?
If heat is such a danger, it begs another question. Should you remove the battery altogether when using your laptop on AC power?
Where they are replaceable, the answer seems to vary from one manufacturer to the next. Acer, for instance, suggests removing the battery at all times. When Apple produced laptops with removable batteries, it advised against ever taking them out.
If you do choose to remove the battery, ensure that you store it properly. This means charged to between 40% and 70%, and kept at room temperature.
Curiously, the industry as a whole doesn’t seem to have settled on a single answer for the question about whether to use your laptop on AC or battery power.
We’ve seen that Acer recommends removing the battery when on AC power. HP draws the line at two weeks of continuous charging. But Dell says there’s no problem leaving the laptop plugged in at all times.
Apple’s advice is no longer on its website, but you can still read it online. The company recommends against leaving a laptop plugged in all the time. Instead, it suggests:
An ideal user would be a commuter who uses her notebook on the train, then plugs it in at the office to charge. This keeps the battery juices flowing
Leaving your laptop plugged in will not cause short term damage, but if you only ever use it on AC power you’ll almost certainly find that after a year the battery’s capacity has been significantly reduced. Similarly, if you only ever use it on battery power you’ll get through the battery’s discharge cycles quicker.
So, the best solution is something of a compromise between the two: use it on battery power some days, and keep it plugged in on others. And in all cases, you’ll want to ensure it doesn’t get too hot.