Lithium Ion Batteries Are Sensitive to Heat

Over 15 million students are enrolled in fall college classes across the United States according to a US Census Bureau study in 2004. Using that number as a base it can be projected that the fall of 2008 should see a slight up-tick in college enrollees. Interestingly the number of college students that are going to college with laptop computers have increased by 28% compared to 42% of college students in 2004. This means that nearly 70% of enrolled students are using laptop computers. In real numbers that represents 10,500,000 laptop computers.

Now listen up college students – your laptop battery is more than likely a Li-ion battery and if it is then there is a natural tendency to keep your laptop plugged into a wall outlet when you are close to one. You may also find that you are actually “plugged” in to a wall outlet more than you are not and there in lies a problem. When your laptop is plugged into a wall outlet your battery heats up big time and heat and lithium do not mix well together.

Hold on! You have to charge battery. Yes that is true, but you do not have to keep your laptop plugged into a wall outlet for the entire school year! But won’t that reduce my battery life if I’m constantly powered from the battery?

Your battery will diminish in capacity – the ability to charge and power your laptop. That is a fact and a natural consequence of batteries today. This diminishing power performance is called battery degradation and power loss. I have written on this topic before and you can read about it on my blog but on a high level a battery over time degrades and eventually stops working, this is no surprise, and it occurs due to the following technical processes: declining capacity, increasing internal resistance, elevated self-discharge, premature voltage cut-off on discharge.

So should you constantly keep your battery charged at 100% capacity? No you should not. Why? To answer that question let’s look at what is occurring when you charge a battery. When charging your battery you are forcing electrical current into a battery cell from a charger. The force of electrical current causes temperature increases.

Now it is true that contained within your laptop battery are integrated power management circuits that are designed to protect against over-voltage and under-voltage conditions that increase heat in the battery but one factor of how well a battery is being protected during a charge depends on the ratio of the heating rate versus the dissipation rate. If the heating rate is higher then the dissipation rate then thermal runaway will occur (leaking, smoking, gas venting, flames).

Now don’t go into panic mode since the integrated circuits are really good at keeping the heating rate lower than the dissipation rate and you are in extremely minimal danger of thermal runaway occurring.  But the practice of keeping your battery charged continuously can negatively affect your battery’s longevity. So charge your battery and then run your laptop on battery power until you have to charge it again.

Until next time Dan Hagopian
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