Soldering to batteries - a word of warning...
Mr Ian Primus
ian_primus at yahoo.com
Mon Mar 9 09:31:54 CDT 2009
Well, I did something mildly stupid recently, and I did it even though I _knew_ the dangers. Some people might not know these dangers (or think about them), so I figured I'd post a word of warning, since this is bound to come up in classic computing.
So, I was recently givin a copy of "Mario All Stars" for the Super Nintendo. It played, but the save game feature doesn't work. These cartridges use a battery-backed SRAM chip to store save games. So, my assumption was that the battery was simply dead.
Opening the cartridge revealed the problem. The battery (a solder-tab style CR2032) was actually good, but the positive terminal 'tab' had come off. The spot-welds just let go. It must have been poorly made, and then got dropped at some point. I don't have a replacement battery with solder tabs, nor do I have a way to get one quickly. But, this seems simple enough, I'll just solder the tab back on.
Now, I've soldered to coin batteries before and never had a problem. The trick is to rough up the surface of the battery so the solder can stick, and work quickly to avoid damaging the battery. Well, this battery wasn't having any of that. The solder simply would not flow. It was then that I thought "I can't heat a lithium battery too much - it might explode!". So I donned my full face mask and got back to work.
More attempts to get the solder to stick were failing. I tried piling on more solder, so that the flux might have a chance to get it to flow. It looked like it was just about to stick and.... PFOOOF!!
Yes. Lithium batteries _do_ explode when heated too much.
The battery exploded sending bits of black crud and molten solder (that never stuck to the battery) everywhere. It suprised the hell out of me, that's for sure. I heard a little *plink* as the top cover of the battery landed behind me. I just stared at the bottom of the battery on the bench for a minute, startled. I took off the face mask and went to get the vacuum cleaner to clean up the mess.
After cleaning up the debris and finding the battery tab and the battery cover, I was trying to figure out how to replace this battery - and if I should try soldering to another cell (more quickly, this time). I then looked over at my face mask on the table. It was covered with solder spashes, and quite a lot of it too (remember, I was trying to get the flux to help the solder flow). Had I not been wearing that mask, I would have gotten all sorts of crud in my eyes.
I've always worn this mask when drilling, dremeling, using the table saw, and yes, when soldering to batteries. This is the first time one has actually exploded though. Yeah, it was my fault, I heated it too much - I should have known better. But it's easy to get frustrated and forget. But never forget to put on safety equipment when working with stuff like this. Givin the choice, I'd always rather have a piece of impact-resistant polycarbonate between me and flying shards of hot metal.
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