Smart Padlock

Nothing here is fitted to a door. A hardened shackle passes through a hasp, chain or locker loop, and a small motorized or solenoid mechanism releases it after a code, a fingerprint or a phone. Power comes from an internal rechargeable cell charged by USB, usually lasting 6 to 12 months, or from a coin cell. Many models have no keyway at all, which removes picking and adds a hard rule: charge it, or carry the emergency power cable.

Every other style in this group works on a hung door with a frame. The padlock protects things a door cannot reach: a side gate, a shed, a garage roller, a trailer coupling, a site container. Compared with a Key Lock Box, which stores a key for someone else, a padlock is the lock itself.

Contractors, delivery drivers and cleaners can be given a code that dies on Sunday night, which beats a key that lives on someone's ring forever. Allotments, boat lockers and storage units suit it, as does anyone tired of a rusted padlock that no longer takes a key. It is a poor fit where the padlock lives underwater, in salt spray, or in a place you cannot reach to recharge.

Shackle diameter is the number that decides how it fails. 6 mm cuts with a small bolt cutter, 10 mm and up in hardened boron steel resists most portable tools. Look for an IP65 or better rating, where the 6 means dust-tight and the 5 means protected against water jets, and check the stated cold limit if it hangs outside all winter. A shrouded or closed shackle removes the gap a cutter needs. Fingerprint readers should have a code fallback for wet fingers.

Rinse road salt and grit off the shackle and spray it with a dry lubricant twice a year, then charge before winter rather than during it. Remember that the hasp and the staple it screws to are usually weaker than the lock, so bolt them through with carriage bolts, not wood screws.