Solding most wiring connections can be done with almost any old soldering gun or pen. However soldering batteries is a different story. The battery draws heat out of the soldering gun quickly which makes it the most difficult soldering job there is.
Over the years I've tried all kinds of soldering guns. I prefer soldering guns because they are safer since they are only on when you squeeze the trigger. I can't tell you how many times I almost burnt myself on someone else's soldering pen that is constantly hot at the race track. Well, I've tried so many different kinds of soldering guns from 50 to 150 Watts. Then I finally found the Mac Daddy of all soldering guns.
Snap-on makes the R490 soldering gun with replaceable ceramic tips. It's not cheap ($98), but I don't care. I've gone through too many of today's plastic junk soldering guns.
This Snap-on gun starts at 400 Watts until it reaches 1000 degrees F. Then it drops automatically to 150 Watts to maintain temperature. This smaller tip is good for everyday soldering.
But if you want to put together your own battery packs, I recommend getting the $78,
330 / 550 Watt tip. This baby is huge! As my friend Dave put it, "The batteries have no choice". There is so much heat and mass to this tip, that the batteries simply don't fight you any more. With this tip, you can make an outstanding soldering connection in 1 to 2 seconds. No more struggling. No more overheating the batteries. You put the tip on, you take it off, you are done in no time.
It's easy to swap out the tips too. 1 screw holds them in. Anyway, I'm very happy with it, and I thought I'd share, and give credit where credit is due. Good job Snap-on.