Rechargeable CMOS/RTC Battery was a Bad Idea
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April 11th 15, 10:24 PM posted to microsoft.public.windowsxp.hardware
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Rechargeable CMOS/RTC Battery was a Bad Idea
Found this on searching for the battery in the Gateway Solo 3350:
Ni-MH type. Yep, a rechargeable battery. Couldn't search on the CMOS
battery for the "HP laptop" since that obviously gives no details, like
a model number.
Of course, even an old and weak alkaline CMOS battery will case
corruption of CMOS settings. It's not the chemistry per se that is the
problem. It's letting the CMOS battery get too weak. I replace
alkaline coin cells at about 3-4 years. Since they are also chemical,
NiMH batteries are not eternal so they have to get replaced, too.
NiMH will self-discharge. That means not using them will eventually
leave dead a once fully-charged battery. The low load on a CMOS battery
is not a good application for a NiMH battery, and equally so for any
applicable where idle storage (non-use) is a consideration.
Were these laptops bought new or used by you?
The HP laptop model is N3390.
Since both laptops had defective main batteries, and I always use the AC
adapter anyway, I removed the rechargeable CMOS batteries. I soldered
wires to the battery contacts, routed the wires into the area of the main
battery (I had removed the contents). In the main battery case(s), I
installed two AA alkaline batteries (HP), two AAA alkaline (Solo).
I added a diode in series with the battery due to the charging circuit.
Note: The voltage drop caused by the diode in series did not reduce
the voltage that much.
The CMOS battery is 3 V. Two AA or AAA batteries in series would give 3
V (2 x 1.5 V) but the .7 V would bring that down to 2.3 V. Why was the
diode needed? There should be no charging circuit .... oh wait ...
yeah, this was a stupid design with rechargeable NiMH. So what was the
count of swear words you spewed during the mods?
Wouldn't it be better to run the wires for the old CMOS battery coin
cell contacts to a new coin cell holder with a heatshrink covered inline
diode and use a CR2032 wafer battery (at the proper 3 V)? That might be
thin enough to lay inside the main battery compartment and still have
room for the main battery. If not enough room, see if any of the
existing cover plates or the one you add has room enough underneath to
glue on the coin cell holder.
Although you say you will always use the laptop on A/C power, there's
also always the chance you or someone to whom you give the laptop might
want battery operation (portability). Even if it's too old and slow for
you, someone else might like it for simple stuff, like e-mail and
Wordpad. I try to donate my old stuff rather than have it occupy space
in the landfill. Just put it on Craigslist for free and someone will
pick it up. Donate to the Goodwill and let them make some money off
The HP N3390 laptop CMOS battery was located below the mother board with
no access to it!!!! You would have to remove the mother board to get to it.
I used a dremel tool since someone in a NG provided a link to an image of the
bottom of the mother board. This allowed me to know where to cut a square
opening at the proper location.
I've done that, too, because the battery was one the wrong side of the
mobo (topside instead of bottomside with an access cover). A dremel at
slow speed (to cut instead of melt the plastic) with a cutting wheel
works well. Reuse the cut out piece by epoxying a couple tabs on each
side to screw down the just-made cover plate back onto the case. In one
case, I managed to get at the battery with a pair of forceps. Another
time I had to partially open the case (lots of screws of varying lengths
to remember which went where). I'd run wires to a new coin cell holder
held with epoxy (insulator) to the bottomside of the mobo. Of course,
there was a ton of cursing out the laptop maker while doing the mod for
their ****ty design.
Dremels sure are handy but I wish those cutting discs were more sturdy.
A light angle or bounce and, zing, flies off a piece of the wheel or all
of it except a tiny donut around the anchor screw and I have to put on
another one. Zzzzzz, zing, replace & grumble, and repeat. That's why I
always wear safety glasses when using a cutting wheel.
When you said the CMOS batteries were rechargeable, I didn't believe you
at first. Nah, he's gotta be wrong. Guess I've been luck(y|ier). I've
had to do the mods to help others keep using old laptops (that weren't
always powered on and on a good-sized UPS) but never yet had to change
the chemistry type. You never know the **** you find inside. I
remember replacing the carpet in my car and finding discarding moulding
pieces underneath embedded in the felt sound deadener. Hey, I got a
grill insert replacement for the front of my car (unpainted though).
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