Rate of change for a charging battery.

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the tree
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Rate of change for a charging battery.

Postby the tree » Wed Dec 01, 2010 11:43 am UTC

(maybe this should have been put in Fleeting Thoughts - but the Science one of that is old and empty so I figured it wouldn't hurt to create a new thread)

I noticed that when charging my phone, it seems to slow down as it gets closer to 100%, that is that it gets from 0 to 50% in a few minutes but takes twice as long to get the rest of the way. (I haven't actually measured this, maybe I could do that). I guess it goes a bit like a logistic curve.

I really can't remember much about the workings of batteries, I think I did some electrolisisis..oh whatever it was in chemistry at GCSE level but that was never detalied enough to mention anything about what'd affect the rate of change.

Not really knowing any science I'm trying to come up with some sort of intution for this, so far what I've made up is while the battery is completely uncharged it's easier to find some lithium that hasn't been ionised, but as it gets closer to being fully charged the electricity is less likely to find some lithium to ionise and just flies of as heat instead. As I said, I'm making this up so I could be monumentally off the mark. Is it something like that?

Another plausible explanation is that the phone is wrong and actually it doesn't go in a curve at all, I guess.

Moose Hole
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Re: Rate of change for a charging battery.

Postby Moose Hole » Wed Dec 01, 2010 3:31 pm UTC

If I remember correctly, rechargeable batteries generally maintain almost constant high voltage until they are at the end of their capacity. That makes it hard to detect how much capacity is remaining in the battery based on voltage (it's easy for alkalines because they steadily lose voltage). I believe cell phones instead just guess at the remaining capacity based on charging times, but that doesn't explain how they can detect how full the battery is when you insert a new, half charged battery (though I don't know if that works anyway).

My guess is that the manufacturer doesn't give an accurate picture of the current charge and just shows a happy picture to make the user happy. If that's the case, they might make you happy by reinforcing your charging efforts when the battery was almost gone by showing it filling quickly, and then slow down later when the real work begins. This is similar to when you're installing files and it looks really fast, but then when you're at 99% you're waiting forever for it to finish.

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thoughtfully
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Re: Rate of change for a charging battery.

Postby thoughtfully » Wed Dec 01, 2010 4:20 pm UTC

Google "(dis)charge curve", optionally with lithium/ni-mh or whatever you're interested in. There's loads of stuff.
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