Random off shoot of another thread.
The topic was about the liquids found in people.
So I took it to google, and found a bunch of articles on bio-batterries. Thought you all might like to see.
Find an article HERE.
There’s a bunch more if you google “human bio battery”.
And… clearly it must be safe. They tested it on rats.
Using bodily fluids as a battery electrolyte is a Really Bad Idea [tm]: firstly the electrodes get dissolved into the electrolyte as the battery generates power - meaning you’re going to pump yourself full of metallic ions over time. Secondly, the electrodes get depleted and you’ll have to replace them when they’re spent, which isn’t what you want for an implant.
Unfortunately that article is full of crappy crap just to fulfill the “blood sweat and tears” motif they were going for. The best most promising thing I’ve seen so far is a glucose fuel cell… but it’s huge, produces only microwatts, and the membrane inside lasts only a few hours… but glucose is one of the most energy dense molecules out there - that’s why our body uses it for fuel… I’m waiting for this.
There are problems with other stuff, but I don’t have the energy to write my own article on what those are or why… at least not right now 
Well, that sucks.
Still, if the tech ever develops fully. It’d be a game changer for sure.