I have a
3 PCB that I keep around to periodically swap into my DK Jr cabinet, and I've noticed that the DK3 board has a spot to insert two AA batteries on it. I glanced through the original Nintendo instruction manual and I didn't see it mentioned, so I was wondering what it's for. I don't have the guts to put batteries in it to investigate myself because I'm afraid I would break it.
The battery compartment is for book keeping stuff. You should not insert batteries as there is no benefit in doing so and you risk damaging your board from acid damage if they start to leak.