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This topic comprises 2 pages: 1 2
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Author
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Topic: Rechargeable batteries
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Mike Olpin
Chop Chop!
Posts: 1852
From: Dallas, TX
Registered: Jan 2002
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posted 09-29-2003 10:53 PM
First of all, True memory loss in a battery (known as memory effect) almost never happens, and when it does it only happens in NiCd batteries, although many argue that NiMH can also exhibit signs of memory effect.
Memory effect occurs when the battery is discharged to the exact same place every time, then recharged. Even the slightest change in time between when the battery is chargedand then recharged will not lead to true memory effect.
Consequerntly memory effect only occurs in exact controlled conditions such as laboratories.
The phenomanon that everyone refers to as "memory effect" is actually called "Voltage Depression".
If you remove the battery from the device your using, before its finished draining, over time, some of the cells in the battery pack become weaker, and while they sill hold the same capacity , they no longer can hold the same voltage . This becomes a problem for advanced digital devices such as cell phones, palm piolots, and laptops, which monitor thier batery level using voltage as a reference for an expected battery capacity. The device flashes a low battery warning and in some cases executes a timer that will automatically shut off the device after a given period of time after the warning (especially common in laptops). This gives the impression that the battery has become empty, when in fact it has the ability to power the device for much longer. Draining the battery almost compleatly will "re awaken" the dead cells, allowing them to store the correct voltage again, and thus curing this condition. This hapens in all NiCd and NiMH batteries, however some people are recently beggining to report this in Li-Ion batteries as well.
PLEASE NOTE, it is importaint never to compleatly drain the battery, as many people recomend. Drain them to about 1.2 Volts per cell, then charge them. If you go lower, some of the weaker cells will reverse (charge from the other end, if you will). This causes them to compeate with the healthy cells renderring them, and the cells they are compeating with, compleately useless. This is verry bad, and usually results in a battery which appears not to hold a charge at all. Some of the high end chargers, automatically drain the battery to 1.2vpc before charging.
As for Nokia, the manual recomends doing this probably because they want to make sure that all the cells are "fully awakened".
Love the new signature Ken. Anyone who hasn't become aware of the latest craze in internet geekdom should head over here.
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Ken Lackner
Phenomenal Film Handler
Posts: 1907
From: Atlanta, GA, USA
Registered: Sep 2001
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posted 09-29-2003 11:34 PM
LOL! Thanks for the comment about the sig. Strong Bad rules! Thanks for the info, Mike. How do you know when you're at 1.2 volts per cell? For that matter, how do you know how many cells are in a battery? With my phone, I'm just using all the talk time until it shuts off, then I'm charging it overnight. (Doing this for three cycles, as the manual suggests. Then I will just continue to plug it in at night or when I get home from work, etc., without fully draining it first.)
So I am correct that only NiCad batteries have the memory effect, not Li-Ion, right? But am I reading you correctly that you can undo the effect by completly draining and then recharging a NiCad?
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Mike Olpin
Chop Chop!
Posts: 1852
From: Dallas, TX
Registered: Jan 2002
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posted 09-30-2003 02:08 AM
Phill is right, its actually 1vpc, however, from what I understand, in order to prevent cell reversal, one should stop drainage at 1.2vpc to be safe. Mabey it was 1.02 vpc? Oh well, can't remember right now.
Anyway, to answer the cell related question, inside your nokia battery, (or more correctly, battery pack), there are several small cells. If you cross-section the battery pack, oreo cookie style, you will note them inside the battery. About measuring voltage, I guess a standard volt meter should do the trick, but I have never actually done this. The information was more for technical info than for practical application, and the Film-Tech manual dissclaimer (read this and learn -but dont you dare try it) certainly should apply to that. Just use your Lithium Ion battery as the phones manual suggests and you should get 1000 cycles out of it with no problem. After 1000 cycles its best not to try to resurect them. Batterys do eventually die.
quote:
So I am correct that only NiCad batteries have the memory effect, not Li-Ion, right? But am I reading you correctly that you can undo the effect by completly draining and then recharging a NiCad?
Yes, in theory only NiCds suffer from the scientificly correct "memory effect", but it seems that almost all rechargeables suffer from voltage depression. Both can be corrected by a disscharge to near empty, providing that the cells havent reversed.
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More pointless ramblings about strongbad
Whenever a film burns at my theatre, I always blame it on Trogdor Inhabiting the booth.
Trogdor the Burninator WITH MAJESTY!
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John Hawkinson
Film God
Posts: 2273
From: Cambridge, MA, USA
Registered: Feb 2002
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posted 09-30-2003 07:56 PM
This is a sufficiently complicated topic that it's probably not going to reach succesful completion here. But a few comments:
The NiCd memory effect is caused by crystalization within the cells, and a full discharge helps to break down the crystals.
Mike said, "Anyway, to answer the cell related question, inside your nokia battery, (or more correctly, battery pack), there are several small cells." This is generally not true for LiIon. Cells can be manufactured as "prismatic cells", which are basically rectangular extrusions. They are single-cell, and thus have the LiIon nominal voltage of 3.6V.
Ken asked why 3 charge/discharge cycles are necessary. The chemistry of the battery is such that it does not reach its full capacity until it has been used a few times. Some vendors may run these 3 cycles in the factory, or as part of manufacturing, others may instruct the user to do so.
It's also important to remember that all batteries, especially NiCd, NiMh, and LiIon chemistries, have a limited number of charge/discharge cycles, generally "a couple hundred." After that , the battery will stop performing adequately.
For more than you could ever possibly want to know, see the excellent online resource, Batteries in a portable world.
--jhawk
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