"BillW50" <BillW50@aol.kom> wrote in message
news:h9bg5e$rp9$1@news.eternal-september.org...
> In news:4ab74dcf$1_1@glkas0286.greenlnk.net,
> M.I.5¾ typed onMon, 21 Sep 2009 10:56:50 +0100 :
>> "BillW50" <BillW50@aol.kom> wrote in message
>> news:h95ed7$es$1@news.eternal-september.org...
>>> In news:4ab6318a$0$83248$e4fe514c@news.xs4all.nl,
>>> Linea Recta typed on Sun, 20 Sep 2009 15:43:35 +0200:
>>>> In the past I have replaced the CMOS power cell of my PC, but I was
>>>> wondering how is this done in a laptop? Is the cell accessible at
>>>> all?
>>>
>>> Most laptops and netbooks use Ni-HM batteries for the CMOS/RTC which
>>> the computer routinely recharges. They rarely fail so I wouldn't
>>> worry too much about them. I would worry more about the florescent
>>> lamp, inverter, hard drive, and main battery myself. Although if you
>>> are still interested, it usually requires total disassembly to get
>>> to it.
>>
>> While some use Ni-MH, they are nowhere near universal. Around 50% or
>> so still use a lithium coin battery.
>
> I haven't seen 50% using lithium coil cell at all. In fact, I never seen
> any but just heard that a few do exists. They have been Ni-Cads or Ni-MH.
>

All our comapny laptops use CR-2032 coin cells (HP). Both of my laptops at
home also use CR-2032 coin cells (and one of them is under a flap so is
easily user replaceable.

>> Ni-MH batteries do routinely
>> fail as the succumb to Nickel whisker growth.
>
> Are you sure you are not thinking of Ni-Cads? As those, I used to zap them
> with a 12v car battery to fry the whiskers in two. Although even this they
> were short lived. As once the whiskers start, it is like cancer. Those
> cells their days were numbered. Supposedly Ni-Cads can only work
> chemically for about 7 years and then they were done. I personally have
> gotten far longer out of some of them. 5% to 10% of them I have had them
> still going after 35 years. Although the capacity is only like 5% of what
> they were originally.
>

The whisker growth is a property of the nickel and is nothing to do with
what type of battery the nickel is in. Ni-MH do suffer nickel whisker
growth but improvements in separator materials and design have reduced the
liklihood of it happening - but it does still occur. A 12 volt battery is
the wrong way to remove them as it does not provide enough current. A
47,000 MFD capacitor charged to 40 volts is th way to go (capacitors
designed for flash guns are best for this).

> Ni-MH I never had a whisker problem. In fact, I never had a Ni-MH go bad
> on me yet. Only two bad things I know about them.
>
> 1) They can't take vibrations too well. So I am told from the RC camp
> anyway. I never had a problem myself.
>

We use them in real aeroplanes, but have not had a problem.

> 2) They like lithium gets very hot if you over charge them. Although I
> never heard of them exploding or anything. They just gets hot enough to
> cause something to catch on fire. Of course, when they have been heated in
> this way, they don't last too long anymore. Well I should say their
> capacity drops down very quickly. As they continue to work for many years,
> but at a lower capacity.
>

Unlike Ni-Cd, the charge reaction in Ni-MH is exothermic (gives off heat).
Charged quickly they can get very hot. As you note, there is a temperature
limit to charging and any properly designed rapid charger should have
temperature sensing. Doesn't always happen though. Some battery packs have
a built in thermal limiter.