Resistence of dirt on battery contact

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

---------------- 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.
--

Don Kelly snipped-for-privacy@shawcross.ca
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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.
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Breakdpwn voltage? Investigate for dielectrics, oils, polymers...
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Alex Coleman wrote:

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
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wrote:

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
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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!
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Alex Coleman wrote:

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 -- -- Graham W http://www.gcw.org.uk/ XP1800+ Page added, Graphics Tutorial WIMBORNE http://www.wessex-astro.org.uk/ Wessex Astro Society's Website Dorset UK Info, Meeting Dates, Sites & Maps
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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).
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Alex Coleman wrote:

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
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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.

Can't hurt.
CS
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