Low Magic Mouse/Keyboard charge warning?

Is it my imagination or is there a way to get BT to notify you before it's too late?

I had it set up on my previous Mac but can't find it now.

TIA

Okay, I found it. So I'm not crazy (in this way).

But it didn't warn me when the mouse was down to 16%. Why not?

TIA

Can you share a preset?