Cleaning battery contacts

[bkn42]

I got lucky (I think) at a thrift store and picked up a new looking HP-48GX with box and manuals. It's the ASEE conference version -- I paid $50 (USD). However, the previous owner left batteries in it and they corroded. I got the old batteries out and tested it with new batteries before buying.

My question is: what is the best way to clean the battery contacts?

Thanks,
Brian

[sylvandb]

I remove the majority of the deposits by scraping and blowing. Try to not let pieces fall into the calculator, and do not get them in your eyes.

Alkaline batteries are alkaline, so I use household vinegar on a cotton swab to dissolve and neutralize what is left.

If the deposits do not react (a bit of fizz and bubble) in vinegar, perhaps they were simple carbon-zinc cells. Those are acidic and a saturated baking soda in hot water solution could be used to neutralize those deposits.

Usually the nickel plating on the contacts will be gone but they will function OK. If you have the contacts out then you could use some solder (lead-free if you like) to tin the contacts. This requires substantial heat and beware it could easily melt plastic or desolder the contacts from the board.

Usually I'll coat the contacts with petroleum jelly (e.g. Vaseline) to prevent further oxidation.

[anetzer]

(04-13-2014 05:23 PM)sylvandb Wrote:  If you have the contacts out then you could use some solder (lead-free if you like) to tin the contacts. This requires substantial heat and beware

My case is a HP-71: Corrosion. Cleaned the contacts thoroughly. Now I have intermittent conductivity between the discs affixed to the spring pairs 1-2 and none on 3-4 and on the metal band between 2-3. Just to find out, whether the machine works, we strung strips of aluminum foil between the contacts and the calc booted just nicely. So yes, it's the contacts...

I read somewhere, that covering battery contacts with a layer of solder is only advisable in devices with low power consumption? IF that is true, I would guess that every calculator covered here falls into that category...

What would the high council of the forum advise:

As it is rather easy to free the battery contacts from the plastic housing on an HP-71, is it advisable to give them a fresh layer or solder or would you go for other solutions (e.g. conductive silver pen, pole grease; contact spray)

Thanks a lot!

a.

[jebem]

(06-20-2014 03:08 PM)anetzer Wrote:  

(04-13-2014 05:23 PM)sylvandb Wrote:  If you have the contacts out then you could use some solder (lead-free if you like) to tin the contacts. This requires substantial heat and beware

I read somewhere, that covering battery contacts with a layer of solder is only advisable in devices with low power consumption? IF that is true, I would guess that every calculator covered here falls into that category...

I don't like to use solder as the condutive material for power contacts, but that is one valid solution.
Solder having Pb will oxidize and create a high contact resistance hence lowering the available voltage for he calculator. However, as the current here is so small the voltage drop should be minimal.
However it requires very high temperatures to force the contact metal material to merge with the solder and in the end it will look ugly as the solder covering will not be uniform.
For that reason I just use a metallic brush to remove the oxidation covering and then apply electrical contact cleaner (dry version withou oil, in spray bottles) to remove all remaining acid/base/dirt components, and it should be fine for the next year or so. A bottle will last for many years of usage.

[anetzer]

(06-20-2014 05:48 PM)jebem Wrote:  Solder having Pb will oxidize and create a high contact resistance

Looks might be an argument, but to be honest: In a battery compartment I will value good conductivity higher.

So if one where to use lead-free solder it should be a good albeit ugly long-term solution?

Thank you so much!

a.

[jebem]

(06-20-2014 06:24 PM)anetzer Wrote:  

(06-20-2014 05:48 PM)jebem Wrote:  Solder having Pb will oxidize and create a high contact resistance

Looks might be an argument, but to be honest: In a battery compartment I will value good conductivity higher.

So if one where to use lead-free solder it should be a good albeit ugly long-term solution?

Thank you so much!

a.

I would say yes, but it depends how badly the contacts are after the battery leaking exposition. Usually soldering is a extreme measure to be used if all other solutions have failed. I'm not sure if the end result will be ugly or nice, it all depends on our ability (and luck) when applying the solder

As Sylvandb mentioned in Post #2, great care is need to do the soldering procedure to make sure the plastic supports will not be damaged with the heat. I would remove the contacts in order to solder them in the best conditions.

[sylvandb]

(06-20-2014 06:24 PM)anetzer Wrote:  

(06-20-2014 05:48 PM)jebem Wrote:  Solder having Pb will oxidize and create a high contact resistance

Looks might be an argument, but to be honest: In a battery compartment I will value good conductivity higher.

Yeah, an argument... Pb oxide is not a concern.

Aluminum oxide on your foil is a very good insulator and forms quickly when aluminum is exposed to air at room temperature. Aluminum oxide is also very hard to penetrate, yet your foil conducted, did it not?

Pb oxide is going to form very slowly at room temperature (if ever) and further it is an unexpectedly reasonable conductor. If it were not, your car battery would not deliver the huge starting current necessary.

The lead itself is a poor conductor compared to other metals (and yet it works for battery terminals carrying hundreds of amps). Of course, your leaded solder is likely only 30% to 40% lead and nearly all of the rest is typically tin. It will be fine. And it is easier to work with and workable at lower temperatures.

Silver is the best conductor, but lead-free plumbing solder has only small percentage of silver along with a whole stew of others. Silver oxide is also an insulator.

Or maybe you could solder it up nice and neat (it can be even, just takes practice and proper flux) and then burnish gold leaf into the surface to prevent corrosion.

Really, the biggest problem with soldering contacts is not causing heat damage. The solder will oxidize less and cause fewer problems than the spring steel exposed from under the nickel plating. Even so, if the contacts are in the calculator I would leave them as bare spring steel rather than solder them. Too much heat!

[anetzer]

Thank yo very much!

I'll try to sum up the tips and arguments I read here and elsewhere:

1.
Thorough mechanical cleaning of metal parts. If they are corroded to far, they become brittle and might break. Here soldering in new contacts might become inevitable. Be prepared for very hard and resistive residue layers.

2.
Neutralize residues:
- For Alkaline residues, use household vinegar to neutralize
  (Soaking thoroughly, if possible). It will fizzle a bit on Alkalines...
- For carbon-zinc residues mix baking soda and water to a paste and
cover contacts, better yet - if possible - submerge in the paste.

3.
Reworking (after cleaning surfaces with an abrasive method, e.g. fine sandpaper):
- Leave unchanged: Risk of renewed corrosion of the raw surface
- Apply contact cleaner: See above
- Cover contacts with vaseline: Risk of reduced conduction, oil is not a conductor!
- Soldering: Any solder will corrode eventually
  (lead too, but is not that much of an issue)
- Soldering and applying a gold-leaf to the final surface: Very cute, thank you!
- Electroless (chemical) nickel plating: Also very interesting...!
- Electrically conductive adhesive (e.g. silver): Rather high resistance
- Electrolytic nickel: Can I do that myself? Seems rather complicated.

Did I get all the techniques?

I am finished cleaning so far and the bridge contacts of early 71's can be easily removed (and those where luckily the most affected in the whole compartment). Machine works now, so I have got time for the next step).

a.

[jebem]

Or you may acquire a broken one for spares... look to this one.

Cheers.