From: spamtrap42@jacob21819.net
On 2012-07-03, Adam wrote:
> Robert Riches wrote:
>> On 2012-07-03, Aragorn wrote:
>>> On Tuesday 03 July 2012 04:22, Adam conveyed the following to
>>> alt.os.linux.mandriva...
>>>> I read that Li-ion batteries, unlike
>>>> NiCd/NiMH, should /not/ be allowed to discharge completely if
>>>> possible.
>>>
>>> No, that is not correct. NiCd batteries _must_ be periodically
>>> discharged completely
>>
>> Yes and no.
>
> Thanks for the detailed reply, Robert! Does what you wrote apply
> equally to the NiMh rechargeable batteries that have replaced NiCds in
> the U.S.?
I don't know enough about NiMh to say. NiMh wasn't around when I
still subscribed to Pop'Tronics.
>> NiCd cells must be discharged or they develop the memory
>> effect
>
> Isn't just leaving them alone for a few months effectively discharging
> them? I read somewhere that NiCds lose about 1% of their charge per day
> when they're not used.
That would work, if you let them sit long enough. If
self-discharge is linear (1 percent of full charge per day), then
three months and ten days would suffice. However, if the
self-discharge is exponential (1 percent of _current_ charge per
day), it would take quite a bit longer to get them close to fully
discharged. Letting a fairly expensive investment sit idle for
months just doesn't feel right.
>> However, if you fully discharge a series chain of cells (a
>> battery of cells, aka a battery)
>
> I assume that includes a "9V" rechargeable battery, which is 6 1.2V
> cells in series (so actually 7.2V total).
I think I have seen 8.4V on a "9V" rechargeable, which would most
likely imply 7 cells, but I don't remember whether that was NiCd
or NiMh.
>> you run the risk of reverse
>> charging the weakest cell in the chain. Reverse charging a NiCd
>> cell results in a dead short caused by metal whiskers inside the
>> cell.
>
> How does one detect that? Will the cell actually have a negative voltage?
The whiskers form pretty quickly (I think less than a minute or
so) when it goes negative. Once the whisker has formed, voltage
is exceedingly close to flat zero.
>> The cure for such a shorted cell is to charge up a
>> substantial electrolytic capacitor (at least a couple milliFarads
>> aka a couple thousand microfarads) to around 12 Volts or more and
>> _ZAP_ the cell in the positive polarity.
>
> So the +ve terminal on the charged capacitor would go to the +ve
> terminal on the battery? For how long?
A very small fraction of a second is sufficient. The capacitor
will discharge far more quickly than a manually operated switch
could disconnect it. When the capacitor leads make contact with
the cell, you will see a spark and hear a -ZAP- sound. Once the
sound reaches your ears, the action has pretty much finished.
>> The only problem with the zap cure is it reduces the capacity of
>> the cell slightly, and it was already the weakest cell in the
>> chain. The weakest cell gets weaker.
>
> And, I assume, eventually won't hold a charge at all. Is that the same
> thing as a NiCd cell failing (won't hold a charge, even when charged
> separately) from normal use, after numerous charge/discharge cycles?
As I understand it, it's not so much that they won't _hold_ a
charge but that the cell's capacity shrinks so much that it's
effectively worthless.
> BTW do you have any comment on the "controversy" here about discharging
> Li-ion batteries?
>
> Adam
Li-ion was also not around when I still subscribed to
Pop'Tronics, and I don't know much about their care and feeding
other than that their coulombic efficiency is about 67%, explains
why they heat up so much while charging, which heat build-up is
at least part of the reason they seem to like to burst into
flame.
--
Robert Riches
spamtrap42@jacob21819.net
(Yes, that is one of my email addresses.)
--- SoupGate-Win32 v1.05
* Origin: you cannot sedate... all the things you hate (1:229/2)
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