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Old 9th Jul 2018, 9:28 pm   #1
Biggles
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Default Corrosion on HT battery connections.

I have a few home-made 90V HT batteries in the shed for powering various radios and have noticed over the years that the negative connections tend to collect a greenish corrosion and sometimes a powdery deposit. The shed is probably a little more humid than the average house, but I was just wondering if this was a problem with HT batteries "back in the day". If left unchecked, the corrosion or verdigris eventually causes a bad connection in my experience. Could the type of metal that the connections are made from make a difference, or is it just one of those unavoidable things?
Alan.
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Old 10th Jul 2018, 8:38 am   #2
ionburn
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Default Re: Corrosion on HT battery connections.

I cannot remember the details of any reason, but this sounds like the familiar 'black wire corrosion'. I would suspect this has come up here before, and there will likely be a thread. It happens at far lower voltages too. I had a model control Tx which had a low voltage battery pack, which had this problem.
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Old 10th Jul 2018, 8:43 am   #3
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Default Re: Corrosion on HT battery connections.

At a quick look I have found this external link:

http://www.batterybusiness.com.au/bl...wire-corrosion
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Old 10th Jul 2018, 9:15 am   #4
Boater Sam
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Default Re: Corrosion on HT battery connections.

From reading the available literature on the web it would seem that this is corrosion of bare copper negative wires (carrying DC current) in the presence of water vapour or liquid. It does not occur on tinned wire or in completely dry environments.
I have experienced a similar corrosion in the braid of aerial coax cable many tens of feet away from the open end. I often wondered how water could get so far down a cable especially as it occurs where gravity could not take the water.
It seems that humidity is sufficient.
Black wire corrosion is named so as the copper is corroded to copper oxide which is black but it also seems relevant that the wire is the negative, i.e. black insulation!
Copper corrosion due to acids is usually blue or green, black is unusual.
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Old 10th Jul 2018, 11:08 am   #5
broadgage
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Default Re: Corrosion on HT battery connections.

Conductors at a negative voltage with respect to the surroundings are at greater risk of corrosion and in extreme cases insulation breakdown.

In the days of three wire DC mains, negative to earth faults tended to outnumber positive to earth faults.

The reason is said to be as follows. In all but perfectly insulated and perfectly dry conditions, a leakage current will tend to pass between conductors at different voltages.
Electrons may be considered for practical purposes as little bits of negative electricity. Therefore leakage currents consist of electrons moving towards the positive pole, whilst positive ions drift towards the negative pole.
This results in dirt, dust, and damp tending to build up on or around the negative wire or terminal.

Modern vehicle practice is to earth the battery negative, so as to reduce the amount of corrosion on electrical terminals, and leakage therefrom that would discharge the battery when not in use.
Off grid homes with a battery bank tend to earth the negative side of the battery for the same reason.
Telecoms practice is to earth the positive side of the battery, non optimum for the reasons already given, but now much too well established to alter.
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Old 10th Jul 2018, 6:20 pm   #6
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Default Re: Corrosion on HT battery connections.

Quote:
Originally Posted by Boater Sam View Post
I have experienced a similar corrosion in the braid of aerial coax cable many tens of feet away from the open end. I often wondered how water could get so far down a cable especially as it occurs where gravity could not take the water.
Isn't this due to capillary action? The water wicks along the braiding.
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Old 10th Jul 2018, 8:20 pm   #7
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Default Re: Corrosion on HT battery connections.

I did think of capillary attraction but over 6 metres and upwards on a vertical cable? That was on the land.
My boat aerial is on a 3 metre pole, sealed at the top, thoroughly. I had to isolate the pole from the boat else the braid on what passes for coax these days rotted away in months, the boat is negative ground off the battery bank and the inverters.
Coincidence?
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Old 10th Jul 2018, 10:44 pm   #8
Biggles
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Default Re: Corrosion on HT battery connections.

The deposits seem to form on batteries which are not connected to anything too, so I assume it is due to moisture in the atmosphere. It doesn't happen with my other batteries of up to about 12V or so. The deposits have a green tinge generally, but sometimes are powdery and white. I guess this must be influenced by the type of metal the connectors are made from or coated with. The positive earth on telecoms equipment was to reduce corrosion I understand. Considering most of the telecoms gear I have worked on in the past happened to be parked on top of a damp mountain with a basic cabin wrapped around it I am not surprised that it was a problem. Nowadays it's all in nice cool air conditioned cabinets. It's a shame the same luxury isn't extended to the staff accommodation!
Alan.
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Old 11th Jul 2018, 1:25 pm   #9
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Default Re: Corrosion on HT battery connections.

Quote:
Originally Posted by Biggles View Post
I have a few home-made 90V HT batteries in the shed for powering various radios and have noticed over the years that the negative connections tend to collect a greenish corrosion and sometimes a powdery deposit.
Alan.
I noticed exactly this on the nickel plated brass terminals on a 90V battery I made for a battery valve radio. It surprised me because I'm much more used to seeing corrosion around a positive terminal (as in lead acid car battery connections) and one would think that a surplus of electrons would inhibit the corrosion rather than promote it. Also it seems only to happen on the 90V battery. On a low voltage battery I made, with the same terminals, no corrosion on either terminal. So I think the notion of the negative terminal attracting positive ions which are corrosive to the surface, seems to make sense.
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