Thursday, April 7, 2011

Laptop Battery Life: Avoid Heat and Storage at Full Charge

How to prolong and maintain the life of your Lithium (Li-Ion) laptop or phone batteries? Battery University, a battery-specific knowledge site affiliated with Cadex Electronics, discusses the life of Lithium-based batteries. The conclusion is surprising: you should expect them to die quickly in normal conditions of use. It is possible, however, to make them last for the life of the laptop by making some minor changes to your working habits.

Battery University bases its findings on research done by Cadex Electronics, some of which was published in book format by its founder Isidor Buchman. Some of the data presented is striking.

Keep the battery as cool as possible
Going from an operating temperature of 75°F to 140°F doubles -or more- the yearly loss in battery capacity, depending upon the average charge. While the statement that heat damages batteries is not very original, the data given by Battery University is shocking:

A laptop battery functioning at at 140°F, when normally left at full charge, will drop to 36% of capacity in two years, or to 42% if working at 104°F. On the other hand, if we are able to keep it cooled to 77°F, it will still have 64% of its original capacity left after 2 years. How likely is it for laptop batteries to run significantly hotter than 77°F? Using our trusted lab thermometer gun, we did an informal survey around the office, and found that most of our laptops' batteries were at or above 104°F, and some of them were close to 140°F. Based on Battery University data, this means that most laptops may lose 60% of their capacity in two years: shocking!

Battery University also advises against leaving the laptop in the sun, and warns about using laptops in a bed or pillow due to restricted air flow. It suggests putting objects such as rulers under the laptop to improve heat transfer. We find this suggestion impractical -ever tried typing on an unstable keyboard?- but figure that a good laptop cooler should be standard operating procedure when at home or in the office. Some open space near the laptop intake and exhaust is obviously beneficial.

When not using your laptop, leave it at partial charge
We just discovered that, even if we are able to cool our laptop to 77°F at all times, as long as we store the laptop at full charge (i.e. plugged in), we are still left with a pithy 64% of battery capacity after two years. Can we improve this number? Leaving your laptop at 100% capacity when not in use can triple the rate at which your lose your maximum  battery capacity: "Lithium-ion suffers stress when exposed to heat and kept at a high charge voltage." While the site does not suggest that disconnecting your laptop is necessary, it is not clear to us how else to leave it at partial charge (40% recommended). The good news is that, based on Battery University data, it should be possible to keep a laptop battery in very good shape, and usable for 4 years or more, by keeping its operating temperature around 77°F, and by always storing the laptop at partial charge:

Based on the conclusions from the articles, it appears that a good practice would be to disconnect your laptop some time prior to terminating use for the day, and leave it disconnected while not in use -unless, of course, you need your battery topped off the next time you use it.

Deep discharges shorten your battery life
Despite all the advice we have heard in the past, it is NOT good practice to fully discharge your battery: "similar to a mechanical device that wears out faster with heavy use, so also does the depth of discharge (DoD) determine the cycle count. The smaller the depth of discharge, the longer the battery will last. If at all possible, avoid frequent full discharges and charge more often between uses." The data is clear and impressive:

There is no room for ambiguity - if we want to prolong the life of a Li-Ion laptop battery, we had better be very careful about deep discharges.

Lithium batteries do not suffer from memory effect
There has often been some concern expressed ("memory effect") about specific battery types where partial discharges may decrease total battery capacity. Battery University dispels that concern for Li-Ion: "Partial discharge on Li-ion is fine; there is no memory and the battery does not need periodic full discharge cycles other than to calibrate the fuel gauge on a smart battery."

Beware wireless chargers
Battery University discusses the higher temperatures generated by wireless charging: "Batteries are also exposed to elevated temperature when charging with wireless chargers. The energy transfer from a charging mat to the portable device is 70 to 80 percent and the remaining 20 to 30 percent is lost mostly in heat. Placing a cellular phone on the heat generating charging mat stresses the battery more than if charged on a designated charger."

How to easily measure your laptop temperature
You can easily check out your laptop temperature without any temperature sensor. First put your hand below your throat, right under your shirt. Then touch your laptop casing around the battery. If it feels warm to the touch, it is likely to be at or above 104°F. It it feels hot to the touch, it is likely to be at or above 140°F.

Beyond the pure physics discussed by Battery University, Lifehacker also has some good advice on how to make your battery last longer.

  • Use a laptop cooler when possible
  • Disconnect your laptop AC charger sometime before finishing work for the day, and store it at partial charge
  • Avoid deep discharges
  • Only use wireless chargers when no traditional charger is available

Battery University via Lifehacker

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