When my Laptop runs low on power it pops up something like, "You're running out of power." They clearly have the technology to detect if the battery itself is going bad. Why can't my phone boot into that mode and say, "It looks like your battery's life is at X% would you like to slow your phone down to concerve power? [Yes] [Yes, always] [No, ask again when it's more dead] [No, Don't ask any more]"

You know... let the user know what's going on.