Solder on battery contacts causing problems?

[Hellbore]

OK I am having a strange problem.

I have an Elly modded to run a Cree p4 LED direct-drive off an unprotected 14500 lithium ion cell. Inside the head, where the driver circuitry used to be, I installed a lithium ion battery protection circuit, so it can safely run unprotected cells.

The point where the positive battery terminal contacts the driver board is coated with solder. I don't remember if I did that or if it came that way. Anyway, a while back I put a solder blob on the positive terminal of one of my 14500 batteries to make the cell fit better in one of my flashlights, because the unprotected cells are a bit shorter than AA batteries.

So now, if I put this cell into this light, the positive battery contact is solder against solder. It is silver solder.

Anyway, here's the point of this long story. With this light, if I shake it around or pump it, the light flickers. Afer while it will start to flicker when I first turn it on. It looks like a flicker due to contact issues... and it is. If I remove the battery, the positive terminal of the battery (where the solder is) is all gray and.. oxidized? burt? Not sure what to call it. There is also a smell that reminds me of the smell when I'm soldering.

So, it appears that solder is a poor choice for battery contacts... it looks like when the light is bumped there is some arcing that is causing the solder to oxidize or something, I don't know what. Over time it gets worse and the battery terminal gets more and more burnt and has more and more contact issues and causes more flickering.

Can anyone explain why this happens? I guess I need to carefully remove the solder from the battery terminal, and possibly throw the cell away.

 

[DM51]

I'll move this to the Batteries section for you. Soldering battery terminals is not recommended unless they have tabs, and I don't know whether that may be part of the problem or whether you just have a loose connection. The battery gurus will advise you...

 

[Mr Happy]

I think you have a problem of poor contact between the battery and the battery contact, as your analysis suggests.

Solder to some other metal is OK as an electrical contact, since the button on the base of a light bulb is made of solder and it works fine. However, I'm not certain about whether solder to solder connections are good.

When people want good electrical contacts they usually use gold plating, but this might be overkill for a simple battery and light application.

Firstly I would look at the spring pressure in the battery holder. Is the spring strong and long enough to hold the battery firmly against the contact? It seems like when you shake the light around the battery is free to move and produce arcing at the contact. Arcing will definitely produce contact erosion and high contact resistance.

(When you look in most battery holders and/or flashlights, the battery fits in the holder with very tight pressure. This is deliberately designed to prevent any battery movement and momentary loss of contact.)

 

[Lynx_Arc]

I have soldered blobs on flat top AA nimh and they work fine and I didn't use silver solder. I would measure the length of your battery comparing it to an alkaline one it is possible it is too short and arcing at times. Either you need to stretch the spring or make your blob taller if that is the case.

 

[Greg G]

I soldered two Eneloop AAA's together in series with wires for my cordless phone here at work. It has been several months and they work great. Much better than the junk battery that came in it.

The way I do it is like this: Using a large soldering tip, I apply some solder to the hot tip, then hold the tip of the iron against the battery until the sodler "just flows" onto the battery. Then I immediately dip the just soldered end of the battery into a cup of water to cool it off right away. It doesn't take long at all. Never heat up the battery then apply solder. That will take too long and will likely damge the battery.