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'[EE] Battery flipped polarity'
2008\10\15@212201 by Jinx

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Just doing some maintenance on a device and notice that one battery
out of three is no good

It's a prototype product, based on a 12F675 powered by 3 AAA
alkalines

For the vast portion of the day the PIC is sleeping and using next to no
current. Only for less than a second a day is it required to do any work,
and draws just a few mA for that time. I predicted that the batteries
should last close to their 10 year shelf life

This one has been running fine for over 2 years, but stopped working
a couple of days ago. Not expecting it to be the batteries I checked
connections, sensor etc. Apart having from a little verdigris and a spider
web (it's installed in a sheltered spot outside) the circuit seems OK and
does still work

Two of the batteries measure 1.48V, the third measures -0.2V

Any ideas what may have caused this ?

Also, is something like WD40 or CRC going to be alright as an after-
spray to prevent verdigris (and spiders) ?

TIA

2008\10\15@231944 by Xiaofan Chen

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On Thu, Oct 16, 2008 at 9:21 AM, Jinx <spam_OUTjoecolquittTakeThisOuTspamclear.net.nz> wrote:
>
> Two of the batteries measure 1.48V, the third measures -0.2V
>
> Any ideas what may have caused this ?

Is the battery expired? Is it from a known good source?

Xiaofan

2008\10\15@234358 by Jinx

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> Is the battery expired? Is it from a known good source?

Well, as sure as you can be about any battery these days. It does
say Made In China, and came from the same blister as all the others
that didn't suddenly go flat. They seem a reasonable weight compared
with carbon-zincs and other alkalines. If that's anything to go by. I just
don't know whether to feel insecure, as I've seen this many times with
NiCd but not an AAA pack

I've put a new one in place of it, left the other two and will monitor
what happens to them

2008\10\16@005208 by Dr Skip

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I wouldn't do that. Always match. It's a real problem with rechargeables. They
will self-discharge at a different rate than the others, and they won't be used
equally. I see that all the time and am meticulous about keeping 'matched
sets', and even then, one will discharge differently and I'll either charge it
separately for a few times (it often helps) or turn a 4-set into a 3-set and be
done.

Sanyo has a new series out called Enerloop (I think - or something like that)
that keeps a charge for years - a few % per year loss. It might be a good
candidate.

The reason might have been it wasn't matched in packaging (an older cell got
put in perhaps by a worker in China) or it had a defect, or maybe, since there
was corrosion, there was moisture and the physical configuration gave that cell
a watery discharge path from condensation or rain or such.

A couple mA a day makes the charge go away... ;)

-Skip


Jinx wrote:
{Quote hidden}

2008\10\16@010906 by Jinx
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> I wouldn't do that. Always match. It's a real problem with rechargeables

Good advice for rechargeables, as you say. This particular set are just
straight alkalines. Maybe I will put a whole new set in and use the older
other two for remotes

2008\10\16@012651 by Vitaliy

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face
Dr Skip wrote:
> A couple mA a day makes the charge go away... ;)

Is anyone recording this and similar gems of engineering folklore?




2008\10\16@083618 by Peter

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The first battery to get weak will get reverse polarized and thereafter look
like the one you have. It is possible for the battery to be a dud, but it is
more likely that the pic was glitched or that the circuit began conducting due
to the verdigris and other stuff.

In general pot or conformal coat any low current circuit that must be exposed to
weather. Tropical conformal coats are available (and usually nasty). Covering
the whole thing in Silicone coating (conformal) should do the trick. Do not use
anything oily like WD, it is a magnet for dirt and other stuff. Wax works for
short time testing (but not for long time).

Peter

2008\10\16@130652 by Brooke Clarke

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Hi Jinx:

You didn't say how the batteries are mounted.  For example are you using metal
clips on the battery diameter or a plastic battery holder?  I ask because I've
seen a number of cases where metal clips have nicked the thin plastic label on
Alkaline cells and caused a short across one or more cells.  Note that was not
a problem with Zinc Carbon cells since the cylinder and the negative end were
the same, but with Alkaline cells the positive terminal and the cylinder are
the same.

If that was not the problem it may be that that cells were mismatched when you
installed them.  When the lowest capacity cell runs out of charge it will be
driven in reverse polarity by the other cells in series with it, hence the
negative polarity.

You can check the state of charge simply by applying a load and measuring the
cell voltage.  This is one of the very few cases where that method works.  For
some load values see the BA-58 at: http://www.prc68.com/I/TS183.shtml#DtTbl

--
Have Fun,

Brooke Clarke
http://www.prc68.com/P/Prod.html  Products I make and sell
http://www.prc68.com/Alpha.shtml  All my web pages listed based on html name
http://www.PRC68.com
http://www.precisionclock.com
http://www.prc68.com/I/WebCam2.shtml 24/7 Sky-Weather-Astronomy Web Cam

2008\10\16@135922 by Steve Smith

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face
The battery has failed...(high resistance). The reverse polarity is due to
the other batteries and load still being connected... This happens on ups
equipment that has not been well maintained poor batteries. When a load is
called for you end up with 300-400v backwards across a single block as the
current demand ramps up reasonably quickly there have been some spectacular
results.... !

Steve

{Original Message removed}

2008\10\16@140058 by Steve Smith

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Transistors protect fuses...!

-----Original Message-----
From: .....piclist-bouncesKILLspamspam@spam@mit.edu [piclist-bouncesspamKILLspammit.edu] On Behalf Of
Vitaliy
Sent: 16 October 2008 06:25
To: Microcontroller discussion list - Public.
Subject: Re: [EE] Battery flipped polarity

Dr Skip wrote:
> A couple mA a day makes the charge go away... ;)

Is anyone recording this and similar gems of engineering folklore?




2008\10\16@170323 by Jinx

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Peter, thanks

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