I screwed up.
I apologize for my mistake.
-I thought that you are in the USA and you are not. I mix up because normally in our forums there are people of the US, a few Canadian and that's it. In this forum there are a lot of Australian. You among them.
-About the use of power tool batteries, I like the idea, but I use DeWalt batteries in my portable tools and the good ones are $90-$100 USD /each.
-I like the idea of a solar charger too.
Enjoy your vacation.