Postby EricTheFred » Thu Feb 07, 2008 1:24 pm
Fair warning. The link in the last post, while it does treat certain myths, has several pieces of misinformation. This is probably due to the misimpression that Li-Ion batteries are one kind of battery. There is actually lots of different chemistries out there, and the technology has evolved dramatically in the last ten years. Often, something is true for one kind, and gets into the public knowledge base as if it were true for all kinds.
My credentials are having worked in the past with the battery management / power management chip design group in my company. I'm out of date since I have worked in more recent time with other sectors, so I can't reliably quote numbers, but I do have enough knowledge to make the above statement.
First of all, please note that there is no such thing as a memoryless dry rechargeable. The myth that Li-Ion batteries are memoryless probably comes from the fact that NiCads are extremely sensitive to short cycles, and in comparison, Li-Ions seem 'memoryless'. You are still best off with a fairly deep cycle in the long run. However, as I tried to point out, there isn't enough benefit versus the hassle, because they are very good at short cycles too.
The idea that Li-ion batteries deterioriate regardless of use is not precisely true. Li-ion batteries in regular use deteriorate due to oxidation in 2-3 years. They last longer on the shelf, and less use will cause less oxidation. (But shelf storage doesn't prevent oxidation, just slows it down. So the statement isn't entirely wrong, either. Just a half-truth.)
If one did purposefully keep one's laptop on when you aren't using it, in order to discharge the batteries, yes, that would shorten the life, so don't do that.
Full discharge of Li-ion batteries causes damage to the cell, and for this reason, people will tell you that you are better off not letting your laptop run all the way down at all. What you need to understand is that your laptop has a management circuit that is actually cutting you off and refusing to run before your battery gets down this far. "Out of battery" on a laptop isn't really "empty", but simply below the spec. That's why most laptops can power up, tell you you're out of battery, then shut off again, if you attempt to turn them back on in this state.
You actually NEED to let your computer get down to this point once in a while, though. I think most manufacturers put the number at every thirty cycles. This is not for the battery but for the 'gas gauge', the chip that is monitoring your level of charge. It needs to recalibrate, or it literally loses track of your battery's real status. If it doesn't, then it will begin mis-managing your battery, which also leads to shortened life-time.
This is because Li-Ion batteries are much more erratic than NiMH batteries. It's for this reason that Prius Hybrids don't use Li-Ion batteries. NiMH batteries can go through many more short-cycles (as they tend to in a hybrid) and not develop this problem. However, even they have memory, and eventually must be replaced, and even they are better off with deeper cycles.
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