You said that you don't want the soldering iron too hot because you don't want to put too much heat into the battery. There's actually a strong argument for using a heavier iron with the temperature set right up for this kind of job because THIS tends to put less heat into the component. What you want is to get the solder to melt ASAP so that you're not slowly getting the whole component (In this case the battery) up to the solders melting point. You want to be done so quick that by the time the heat from the solder joint starts to warm the main part of the component, you've already finished, so no more heat is added. It's like if you were trying to solder the end of a large copper rod. If for example, you were using solder with an melting temperature of 300'F, and you applied a low powered iron at 350'F to the end, by the time THAT points got to 300'F the whole bar will be pretty hot, but if you hit the end with an iron that has a large thermal mass, and is set to 450+'F, it'll dump heat into the rod faster than it can conduct that heat away, so the job can be done in seconds, and as you've only got a small portion of the rod up to 350'F, once that heat gets conducted and spread into the whole rod it will be significantly lower than if a small low powered iron was used for a longer duration. Personally, for most electronics work I've got a little 24v pen type soldering iron because it's easy to get around a board, but for anything like this battery I use a much larger butane powered iron with a 5mm wide "Bevel tip", because it can dump huge amounts of heat into a joint fast and I'm back away from it in seconds. Apart from this one criticism, the rest of the video is brilliantly put together. Informative and with an air of professionalism. Well worth the like I gave it.