"Hey, can I borrow that charger?"
Translation:Hei, voinko minä lainata tuota laturia?
You can say voinko lainata tuon laturin. The meaning is a little bit different though. In this case you are asking permission to take it with you and keep it for a longer time whereas when you use the partitive voinko lainata tuota laturia it is understood that you are using it just the one time and returning it right away
I'd think because borrowing is an incomplete action, but I'm not sure. Wiktionary says: (borrow): In questions of permission e.g. "may/can I borrow it?", the partitive case is often used for the object instead. So maybe this is just a usage thing, like certain verbs going with certain cases.