Quick Tip: BS5839:6 2019 Has been updated. The new grade is grade F2. This is the lowest rating on the detector scale. This is a user replaceable battery typically 9v and is often seen in many of our homes (usually yellowed and non-functional) because the battery has been removed or it was "Chirping" (because the battery was failing). Another Quick Tip: Landlords Please Take Note; These are NOT suitable for the rental market.