Resistence of dirt on battery contact

[Alex Coleman]

I have a cordless phone with rechargeable NiMH cells. The instructions
point out I must wipe clean the ends of the cells and also the contacts.

Is it really likely that "normal" dirt (in other words invisible dirt )
may affect performance?

ISTR that an NiMH and a NiCd have an internal reistense of about 0.24
ohm. How much resistence would someone's finger grease have?

Are there other residues which can build up on the battery and and
contacts from normal use? Perhaps from gas vebting from the cells or
just a reacton with the atmosphere.

 

[Long Ranger]

I have a cordless phone with rechargeable NiMH cells. The instructions
point out I must wipe clean the ends of the cells and also the contacts.

Is it really likely that "normal" dirt (in other words invisible dirt )
may affect performance?

ISTR that an NiMH and a NiCd have an internal reistense of about 0.24
ohm. How much resistence would someone's finger grease have?

Are there other residues which can build up on the battery and and
contacts from normal use? Perhaps from gas vebting from the cells or
just a reacton with the atmosphere.

It also depends on how well engineered the contact set is, and what they are
made of. I have an old Dell pocket PC that gives me a "low battery" alert
from time to time. Sometimes it's the main cell, sometimes the backup. I
have to take the batteries out, and clean them and the contacts, then it
works fine for another month or two. I don't know what the main cell
contacts are made of, but they always leave tiny black smudges on the
battery wipe area, which I assume is some sort of oxide build-up. I've tried
various anti-oxidants, and a faint trace Corrosion-X Marine seems to give
the longest lasting results. I think the corrosive effect of finger smudging
is probably a bigger concern than the inital resistance of it. It doesn't
take much to ruin the crappy little contact schemes of some units. (Like
mine).

 

[Mike Lamond]

It also depends on how well engineered the contact set is, and what they
are made of. I have an old Dell pocket PC that gives me a "low battery"
alert from time to time. Sometimes it's the main cell, sometimes the
backup. I have to take the batteries out, and clean them and the contacts,
then it works fine for another month or two. I don't know what the main
cell contacts are made of, but they always leave tiny black smudges on the
battery wipe area, which I assume is some sort of oxide build-up. I've
tried various anti-oxidants, and a faint trace Corrosion-X Marine seems to
give the longest lasting results. I think the corrosive effect of finger
smudging is probably a bigger concern than the inital resistance of it. It
doesn't take much to ruin the crappy little contact schemes of some units.
(Like mine).

With my last cell phone, the charger base would occasionally blink on error
when I set the phone into the base. Once I cleaned the battery's exposed
charge contacts, it was fine. That hasn't happened yet with my current
phone, which suggests that the spring contacts in the base must have a
better wiping action against the battery contacts. Cordless phones just
drop into the base instead of snapping in like my cell phones, so there's
not as much pressure between the contacts.

When I opened up a pair of solar powered sidewalk lights, I found a
fair amount of corrosion on the NiCd battery terminals. That was after
several summers outside. After cleaning they seemed to be brighter
(relatively speaking for a white LED) but now one of them has failed
completely.

Mike

 

[GregS]

I have a cordless phone with rechargeable NiMH cells. The instructions
point out I must wipe clean the ends of the cells and also the contacts.

Is it really likely that "normal" dirt (in other words invisible dirt )
may affect performance?

ISTR that an NiMH and a NiCd have an internal reistense of about 0.24
ohm. How much resistence would someone's finger grease have?

Are there other residues which can build up on the battery and and
contacts from normal use? Perhaps from gas vebting from the cells or
just a reacton with the atmosphere.

I see problems all the time. Finger grease can improve conduction, at least
tempoarily, until other contaminants cause crorosion. The contact material
on many devices, nickel, is just plain hard to get good conduction, unless
a fair amount of force is present. Cleaning helps get rid of corrosion, and
dry films can build up causing problems. Water or water alcohol combinations
will dissolve most films. Lubrication will help conduction, but can cause
film and dirt collection. Most problems I see are not the batteries, but the
other contacts involved. When batteries start oozing stuff out, severe damage
and corrosion will occur. Gassing could be a minor problem, not sure.
On enclosed batteries, I try to spray or coat something on before insertion.
Light cleaning sprays, like Caig DeOxit will work, and for extreme situations
Vaseline can help. For exposed connections, just wipe with water and alcohol.
I actually like rubbing alcohol, and it contains a light oily base which
should not cause problems.

greg

 

[Evgenij Barsukov]

Actually, resistance of 10um fat layer to a voltage of 1.5V can be 1MOhm. The only reason
why normally you don't see such resistances is that contact is pressurized
so you "squeeze out" the fat from the contact area. However, if contact
is badly designed, pressure is low or contact area is too small, you
can easily have something that appears like open circuit even
with barely noticeable impurities.

Note that battery contacts are operating at low voltage, so other mechanism how
contact can be estabished at 110 or 220V - by electrostatic break-through
across the thin insulating film, will not take place at 1.5V.

So it is all up to removing the insulating layer
completely, or squeeze it out temporary by high pressure...

Regards,
Evgenij

 

[Ted Edwards]

I have a cordless phone with rechargeable NiMH cells. The instructions
point out I must wipe clean the ends of the cells and also the contacts.

Is it really likely that "normal" dirt (in other words invisible dirt )
may affect performance?

I use AA NiMH cells in my GPSR and Camera. Whenever I replace cells, I
wipe the contact ends on my pants leg then insert the cells. If I
should forget, I will often get much less out of the batteries. I have
never had a problem with wiped ones.

Ted

 

[CS]

I have a cordless phone with rechargeable NiMH cells. The instructions
point out I must wipe clean the ends of the cells and also the contacts.

Is it really likely that "normal" dirt (in other words invisible dirt )
may affect performance?

ISTR that an NiMH and a NiCd have an internal reistense of about 0.24
ohm. How much resistence would someone's finger grease have?

Are there other residues which can build up on the battery and and
contacts from normal use? Perhaps from gas vebting from the cells or
just a reacton with the atmosphere.

Could be anything in the atmosphere. Ionized particals sticking to the
contacts, a whiff of greasy smoke from the BBQ outside, fecal particles from
the dog passing gas...

My Logitech mouse wouldn't charge unless I jiggered it around in the
charger. It got worse until it wouldn't charge at all, about a month after
I bought it. I finally got out my trusty Universal Electrical Contact
Repair Tool (pencil eraser) and took this mouse to school. Worked
flawlessly after that.

Strange thing, it wasn't grease from my filthy meathooks which caused this.
It was some sort of laquer type material that took some work to remove.
Apparently it came from the charging cradle. Either way, in the past 8
months, the problem hasn't reappeared.

Is the need to wipe cells & contacts equally applicable to silver oxide
cells? Does it depend on the application?

Can't hurt.

CS

 

[daestrom]

Actually, resistance of 10um fat layer to a voltage of 1.5V can be 1MOhm.
The only reason
why normally you don't see such resistances is that contact is pressurized
so you "squeeze out" the fat from the contact area. However, if contact
is badly designed, pressure is low or contact area is too small, you
can easily have something that appears like open circuit even
with barely noticeable impurities.

Note that battery contacts are operating at low voltage, so other
mechanism how
contact can be estabished at 110 or 220V - by electrostatic break-through
across the thin insulating film, will not take place at 1.5V.

Yep. We had an application where the engineers used nice, robust switches
designed for 120/240 VAC. But the switch is only operated in an emergency
or bi-annual testing. To test the system, normal power was removed and
low-voltage (2 D-cell batteries) were hooked up along with a chart recorder.
When we turned the switch, the recorder didn't record contact closure
because of oxide on the contacts.

After cycling a few times, the wiping action of the switch cleaned the
contacts enough for the 3VDC to pass through. When we spoke with the
engineers about how unreliable this switch seemed to be, they pointed out
that at 120 VAC the contacts would have worked the first time.

daestrom

 

[Don Kelly]

----------------------------

Yep. We had an application where the engineers used nice, robust switches
designed for 120/240 VAC. But the switch is only operated in an emergency
or bi-annual testing. To test the system, normal power was removed and
low-voltage (2 D-cell batteries) were hooked up along with a chart
recorder. When we turned the switch, the recorder didn't record contact
closure because of oxide on the contacts.

After cycling a few times, the wiping action of the switch cleaned the
contacts enough for the 3VDC to pass through. When we spoke with the
engineers about how unreliable this switch seemed to be, they pointed out
that at 120 VAC the contacts would have worked the first time.

daestrom

----------------
I had problems with normal (i.e. cheap but decor is nice and it doesn't go
"clack") 120V AC switches with a millivolt thermostat relay for a gas
fireplace. The contact wiping is insufficient at low voltage and, as you
indicate, it doesn't take much to build up a high resistance layer. I
replaced the switch with one that has better contacts (- nickel plated-
next step up would be gold plated) I also paralled it with a cheap
radio/automotive switch intended for DC -for backup but haven't had to use
this as the nickel contacts are still working well after 10 years. The now
non-existent (except in some thermostats) mercury switches would also have
worked well. Simply put- I spent $5 to replace a <$1 switch- in order to get
better contacts. Well worth it.

 

[Alex Coleman]

Yep. We had an application where the engineers used nice, robust
switches designed for 120/240 VAC. But the switch is only operated
in an emergency or bi-annual testing. To test the system, normal
power was removed and low-voltage (2 D-cell batteries) were hooked
up along with a chart recorder. When we turned the switch, the
recorder didn't record contact closure because of oxide on the
contacts.

After cycling a few times, the wiping action of the switch cleaned
the contacts enough for the 3VDC to pass through. When we spoke
with the engineers about how unreliable this switch seemed to be,
they pointed out that at 120 VAC the contacts would have worked the
first time.

daestrom

Are there any other terms for this effect than "elctrostatic
breakthrough" used by one of the posters in this thread?

I can't find much info on this.

 

[Alex Coleman]

I see problems all the time. Finger grease can improve conduction,
at least tempoarily, until other contaminants cause crorosion. The
contact material on many devices, nickel, is just plain hard to get
good conduction, unless a fair amount of force is present. Cleaning
helps get rid of corrosion, and dry films can build up causing
problems. Water or water alcohol combinations will dissolve most
films. Lubrication will help conduction, but can cause film and
dirt collection. Most problems I see are not the batteries, but the
other contacts involved. When batteries start oozing stuff out,
severe damage and corrosion will occur. Gassing could be a minor
problem, not sure. On enclosed batteries, I try to spray or coat
something on before insertion. Light cleaning sprays, like Caig
DeOxit will work, and for extreme situations Vaseline can help. For
exposed connections, just wipe with water and alcohol. I actually
like rubbing alcohol, and it contains a light oily base which
should not cause problems.

greg

If some battery contacts on the device look as if they have started
to become a bit roughened then would it be ok to rub then with a very
fine abrasive paper (such as 600 grit)?

I am not sure what the contacts are made of. ISTR someone saying in
this thread that the plating is usually nickel. But I don't know
what the underlying spring metal is. Perhaps it's nickel all the way
through? I don't want to sandpaper a contact and then find it is
going to go into decline even faster!

 

[Graham W]

If some battery contacts on the device look as if they have started
to become a bit roughened then would it be ok to rub then with a very
fine abrasive paper (such as 600 grit)?

I am not sure what the contacts are made of. ISTR someone saying in
this thread that the plating is usually nickel. But I don't know
what the underlying spring metal is. Perhaps it's nickel all the way
through? I don't want to sandpaper a contact and then find it is
going to go into decline even faster!

I wouldn't use 600 grit but try scotch-brite scouring pad material.
I regularly rub the battery ends on my trousers since I like to keep
them clean - the batteries, that is. Slightly more aggressive is a type
of carpet tile - a quick rub on a tile will 'wipe' the film off and
restore
the shine. These latter two cleaning methods don't remove any metal.

HTH

 

[Lostgallifreyan]

Are there any other terms for this effect than "elctrostatic
breakthrough" used by one of the posters in this thread?

I can't find much info on this.

Breakdpwn voltage? Investigate for dielectrics, oils, polymers...

 

[Lostgallifreyan]

If some battery contacts on the device look as if they have started
to become a bit roughened then would it be ok to rub then with a very
fine abrasive paper (such as 600 grit)?

I am not sure what the contacts are made of. ISTR someone saying in
this thread that the plating is usually nickel. But I don't know
what the underlying spring metal is. Perhaps it's nickel all the way
through? I don't want to sandpaper a contact and then find it is
going to go into decline even faster!

I'd use brass or silver polish. It's very fine grit, extremely so, so it
will smooth out rough grain or scratches, and the solvent part of the
liquid will remove grease. A wipe with a clean rag or paper will remove any
remaining smear of polish. Once it's been done a few times and is shiny, it
can be maintained replacing the polish with a freshly shaken 30/70%
water/isopropanol (or white spirit) mix bound as an emulsion by a drop or
two of washing up liquid, and again, wipe with clean rag afterwards.

That's overkill, maybe, but easy enough to prepare and use, and will get
you a cleaning method that can't be easily improved on without the kind of
care taken with laser optics. (lens tissue damped with methanol or acetone,
wiped once lightly across surface).

 

[Findor]

I'd use brass or silver polish. It's very fine grit, extremely so,
so it will smooth out rough grain or scratches, and the solvent
part of the liquid will remove grease. A wipe with a clean rag or
paper will remove any remaining smear of polish. Once it's been
done a few times and is shiny, it can be maintained replacing the
polish with a freshly shaken 30/70% water/isopropanol (or white
spirit) mix bound as an emulsion by a drop or two of washing up
liquid, and again, wipe with clean rag afterwards.

That's overkill, maybe, but easy enough to prepare and use, and
will get you a cleaning method that can't be easily improved on
without the kind of care taken with laser optics. (lens tissue
damped with methanol or acetone, wiped once lightly across
surface).

Isn't brass polish quite greasy? Whenever I have spilt a bit of it
on my hands the result feels quite oily.

Does silver polish have any abrasive in it?

 

[Lostgallifreyan]

Isn't brass polish quite greasy? Whenever I have spilt a bit of it
on my hands the result feels quite oily.

Shouldn't be greasy, unless cheap turps is used to suspend the fine
abrasive. If it is, use a bit of alcohol on the clean cloth used after the
polish. Fine abrasive even in non-greasy liquid might feel oily.

Does silver polish have any abrasive in it?

Yes, as fine as possible. Might not be so important if you're not cleaning
actual silver.

 

[Fred Kasner]

Are there any other terms for this effect than "elctrostatic
breakthrough" used by one of the posters in this thread?

I can't find much info on this.

One of the real problems of my doctoral thesis work was to maintain a
very steady current (4 or 5 significant digits) to a heater in a
calorimeter (I was measuring temperature changes to tne nearest
millionth of a degree in a chemical dilution.) The current was allowed
to stabilize by running it through a dummy heating coil load (same
resistance as the heater in the calorimeter. And after the system
stabilized I would use a penulum clock driven switch to transfer to the
calorimeter load for ten seconds. Rarely could get a stable current
measured to about 5 sig. fig. Why? The lead straps connecting the wet
cells to one another could not make reliable contact even if cleaned
just before the equipment was turned on. Appears that little vibraions
in the building (a heavy truck driving by would produce an effect) would
shake the battery set and vary the resistance. Finally had to drill
holes in the lead terminals and solder the units together with low emf
solder to get a reliable electric contact. Typical of such problems in
this work that cost me 6 months to solve. No wonder my doctoral research
stretched out to about 6 years. Constancy of electric contacts is a
difficult thing for high precision work.

FK