Most smart locks run on the same batteries you put in a TV remote, and they last months rather than weeks. The figure that actually varies — sometimes by a factor of ten — is how many months, and almost all of that difference comes down to what the lock is doing while nobody is at the door. Understanding the draw helps you pick hardware that fits the door and avoid the surprise of a lock that dies after a season.
What actually drains the battery
There are two real consumers inside a lock, and they behave nothing alike.
The first is the motor. Every time the bolt throws or retracts, the lock does real mechanical work, and that is the single largest pulse of current the device sees. A household door might cycle a dozen times a day; a busy short-term rental or office entrance can cycle hundreds. The motor draw scales with how busy the door is, and there is no way around it short of choosing a lock with a more efficient drive.
The second is the radio, and this is where locks diverge most. A lock that maintains its own Wi-Fi connection is essentially never asleep — it holds a link to the access point all day, every day, and that continuous draw dominates everything else. A Bluetooth-only lock, by contrast, sleeps until a phone or a gateway wakes it, so it spends most of its life drawing almost nothing. Moving that always-on radio off the battery is the single biggest lever on battery life, which is why the architecture matters as much as the battery itself. The trade-offs between those architectures are laid out in offline vs cloud access control.
After the motor and the radio, everything else is small. Keypad backlights, fingerprint sensors waking to listen, the occasional status sync — these nibble at the battery, and they only matter on a lock that is already efficient elsewhere. This is also why firmware updates can quietly change battery life: manufacturers tune how deeply the device sleeps between events.
Numbers worth holding onto
Rules of thumb, not promises, because real life depends on traffic and temperature.
A Bluetooth-only lock, or one that talks to the cloud through a gateway, commonly runs eight to twelve months on four AA cells on a residential door. The same lock on a high-traffic entrance might land closer to four to six months. A lock that holds its own Wi-Fi connection, with no gateway in the path, typically runs one to three months — the always-on radio is simply expensive. Coin-cell or small-form-factor locks trade convenience for shorter runs and may need attention every few weeks. Cold weather pulls every one of these numbers down, because battery chemistry delivers less usable capacity as the temperature drops.
The practical reading of all this: if a lock advertises a year of battery life, it is almost certainly a sleeping-Bluetooth design, not an always-connected one, and that difference is worth confirming before you buy.
Choosing cells
Alkaline AAs are the default and are perfectly adequate for most indoor doors in mild climates. They are cheap and easy to find, and their main weakness is voltage sag in the cold — a lock on an exterior door in winter can report low battery well before the cells are chemically spent.
Lithium primary cells (the 1.5 V Li-FeS₂ AA type) hold a flatter voltage curve and handle cold better, so they last longer and behave more predictably on outside doors. They cost more, and on a quiet indoor door you may never notice the difference.
Rechargeable NiMH cells are the awkward case. Their nominal voltage is 1.2 V instead of 1.5 V, and some locks read that as a permanently low battery and nag you even when the cells are full. Check the lock's documentation before committing to rechargeables; a fair number of models simply do not support them cleanly. Whatever you choose, fit a full set of identical, same-age cells — mixing fresh with half-used ones shortens the life of all of them.
Habits that extend run time
A few choices make a measurable difference.
Prefer a gateway-bridged design over a lock with built-in always-on Wi-Fi, so the radio that never sleeps runs on wall power instead of batteries. Keep firmware current, since sleep behaviour is one of the things updates quietly improve. If the lock exposes it, shorten the keypad-backlight timeout and disable continuous remote polling you do not actually use. On exterior doors in cold climates, budget for a winter cell change the way you would for a smoke alarm. And treat low-battery warnings as a few-weeks-of-margin notice rather than an emergency — most locks announce them early, which is the window covered alongside other common failures in the troubleshooting guide.
When the warning comes
Almost every lock gives both an audible chirp and an app notification well before the cells die, leaving days to weeks of normal operation. Keep the path to the batteries simple and keep a spare set near the door, because a flat set of cells always feels urgent at the moment it happens. Replace all cells at once, not just the weakest. And before it ever matters, confirm the backup: most locks keep a mechanical key override or external battery contacts so a dead set of cells never locks you out.
| Lock type | Typical life on a residential door | Main draw | Notes |
|---|---|---|---|
| Bluetooth-only (sleeps) | 8–12 months | Motor, briefly | Longest run; needs a phone or gateway nearby |
| Gateway-bridged (BLE + hub) | 6–12 months | Motor; radio sleeps | Cloud access without an always-on radio on the lock |
| Built-in always-on Wi-Fi | 1–3 months | Continuous radio | Shortest run; trades cells for no extra hardware |
| Small form-factor / coin cell | A few weeks to months | Small reservoir | Convenience over run time |
The door cycle and the radio together explain almost every battery-life number you will see on a box, and once you know which one dominates a given lock, the rest is cells and temperature. The wider picture of how the credential, the motor and the radio fit together is in how smart locks work, and the role of that offboard radio is worth reading in what a smart lock gateway does.