After seeing that the capacity of my iBook battery has fallen by 10% over the course of four complete cycles of discharging and charging, I went and read up on lithium-ion batteries. My previous conceptions about them turn out to be almost entirely wrong. Since almost all cellular phones, laptops, and music players with rechargeable batteries run on this sort, it is worth knowing how to keep them going for as long as possible.
1. Discharging completely, then charging completely, is not the ideal approach
Unlike other kinds of batteries, there is no ‘memory effect’ with Li-ion systems. Batteries that suffer from memory effects ‘forget’ how much charge they can hold if they are not completely drained and then completely recharged. As such, the strategy to keep them alive for the longest time is to always follow that pattern.
With Lithium-Ion batteries, full discharging is not only non-ideal, it is actually harmful. This is because it strains the weakest cell. Since a battery is composed of several cells, the failure of any one will mean the failure of the whole system. All lithium-ion rechargeable batteries have systems to prevent cell voltage from dropping too low (a microcontroller cuts it off before it reaches that point), but draining them to the point of cutoff is still harmful.
2. Temperature matters most
The biggest factor in battery life, especially for laptops, is the temperature at which the battery is kept. Judging by the figures from iStat Pro, mine is consistently at more than 40Â°C when the computer is running. Between reading, writing, listening to music, and just hanging around on Skype, that is probably more than twelve hours a day.
Just keeping the battery at 40Â°C will result in capacity loss of more than 15% over the course of one year, compared with a 2% temperature based loss if the battery is kept at 0Â°C and a 4% loss if it is kept at room temperature (about 25Â°C).
The most practical upshot of this is that it is intelligent to keep your battery outside of your computer when you are using it plugged into the wall. The most important reason for this is that it will thus be living at a much lower temperature, and thus for much longer. Since a laptop with no battery will shutdown instantly (and incorrectly) with any interruption in the external power supply, the best bet is probably to use a battery on its last legs (but still good enough for a few minutes) when plugged in, and a better one when working off battery power.
3. Storage or using at 100% charge is harmful
For reasons too complex for me to understand, a charge of about 40% is best for the long-term storage of Li-ion batteries. A Li-ion battery kept at 100% charge and 40Â°C will lose about 35% of its capacity in a year.
4. Li-ion batteries fail over time, regardless of anything else
According to Wikipedia: “At a 100% charge level, a typical Li-ion laptop battery that is full most of the time at 25 degrees Celsius or 77 degrees Fahrenheit, will irreversibly lose approximately 20% capacity per year.” This loss is because of oxidation (over and above heat damage, as I understand it), which causes cell resistance to rise to the point where – despite holding a charge – the battery cannot provide power to an external circuit.
For more information see Wikipedia and this page. The especially bold can learn how to rebuild depleted Li-ion batteries. Anyone with background in electrochemistry is strongly encouraged to comment on the accuracy of the above information.
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