I copied someone else's plan to do this. This is my attempt. It's working for me and my Canon S100, S110 and ELPH 300HS cameras for my bird photography. With two factory batteries it supplies 4.10v, which apparently the cams don't mind as they haven't melted...yet. Anyway, I know little to nothing about electrical things and terms so if I can make it work you can too. I made a video of what I did. There are two parts. Part 1 is about 11 mins long and Part 2 about 5 mins long. I've built and tested two of these, one for the ELPH and the other for the S100 and S110. I realize that there are many other ways to do this but I liked this way because it used cheap chargers (I paid about $3 for each) and the standard batteries. I have one outside right now taking pics of winter birds. I got 4hrs and 10mins from both packs as I tested them each. That's about double the time I used to get with just one battery in the camera the normal way. Also, in hindsight, I didn't need to order Canon S100 and Canon ELPH 300HS specific chargers as the bottom line here is the 3.7v, which both NB-4L and NB-5L provide so I could've just ordered one type and went with that. Oh well, like I said, I don't know this stuff.

Part I:

Here are two recent birdy pics using this method.