Originally posted by JamesArluck
The hotter the iron and higher wattage and greater thermal mass allows you to melt the solder more quickly
Mmm..not quite. Thermal mass in this case is the same as real mass. A hotter iron that only has a few ounces of metal in the tip will have a much harder time than an iron that has 2x the actual mass and is 50 degrees cooler. I use a huge old 100W Weller pencil for my cells, with a 210/260W gun doing backup duty.
If it takes more than three seconds
fromt he time you touch the iron to the cell and the time the joint is complete, you need a bigger iron. Same goes for connectors. Anything longer will start to seriously bork the cell or melt a connector housing.
As for Solder, 63/37 is the best for batteries, becasue it melts at the lowest point (183C) of all tin-lead blends, allowing the lease amount of solder time to be used, heating the cell the least.
The normal prep for soldering cells isn't to grind, buff, scrape, cut, or anything else. It's adding flux. Flux is the stuff inside the solder that cleans the joint. It liquifies at alower temperature, and will mostly vaporize during soldering. It acts to clean all the gunk (at a microscopic level) from the metal. I use Kester flux pens, that I picked up at my local full line electronics store. If you hit a search engine, you'll find a ton of places to get them. Make sure you only use rosin core solder. Acid core solder or acid flux will keep earing the connection and it will eventually fail. Bad news.
When I solder my cells it works like this:
Flux positive cell top, load with solder. Flux negative cell top.
Place battery bar, heat positive side until bar "falls" into place (<=3sec).
Solder negative side of bar.
I've soldered hundreds of cells like that over my RC career, and I've never killed one. Not coincidentally, it's a very close relative of the procedure that nuclear cetrified solder technicians use.
<who spent way too long analyzing solder joint failures under a microscope>