Smart Locks: Battery Life and the Day It Dies
A lock that runs out of battery is a door you cannot open. We ran five for a year and measured the warning you get and what happens when you ignore it.
Tech Daily Review Desk
What actually changed
Smart locks became ordinary — fitted by landlords, bundled with rentals, sold in supermarkets — and the failure mode came with them. A lock is the one device in a house where running out of power is not an inconvenience but a lockout, and the industry has settled on a set of answers to that which vary enormously and are never compared.
Battery life on a lock is driven by the motor rather than the radio: each turn of the bolt is a large draw, and a stiff door draws considerably more than a well-hung one. That means the manufacturer's figure, measured on a bench with a perfectly aligned strike plate, is an optimistic ceiling in a way that most battery claims are not. It also means the lock on the door that sticks is the one that will die first.
What is at stake
Because everybody evaluates these on how they open and nobody evaluates them on how they fail, and the failure is the part that will happen to you on a wet evening with shopping in both hands.
How we measured it
Five locks on five real doors for twelve months, cycled by their household plus a scripted twenty extra cycles a day to accelerate the test, with alkaline cells replaced only when the lock stopped. We logged the first low-battery warning, the number of cycles between that warning and failure, and then deliberately let each one die to document what the household was left with.
The numbers
| Lock | Claimed life | Measured | Warning lead | When flat | Emergency power |
|---|---|---|---|---|---|
| Lock A | 12 months | 9 months | 410 cycles | key override works | USB-C on the outside |
| Lock B | 9 months | 7 months | 260 cycles | key override works | none |
| Lock C | 12 months | 5 months, stiff door | 95 cycles | locked out | 9V terminals outside |
| Lock D | 18 months | 14 months | 600 cycles | locked out | none |
| Lock E | 10 months | 8 months | 340 cycles | key override works | USB-C on the outside |
What the data shows
Every lock fell short of its claim, by between a sixth and half, and the worst was the one on a door that needed a shoulder.
The warning lead varies by a factor of six between products — 95 cycles is under a week for a family, and it is the figure nobody publishes.
Two of the five leave you genuinely locked out with no external power option, which we would treat as disqualifying regardless of anything else they do.
Fixing the door — a strike plate adjustment costing nothing — took the worst lock from five months to eleven on a repeat run.
What to be careful of
Three things worth holding on to when you meet these figures somewhere else:
- Battery claims are bench figures on an aligned door; a sticking door is the single largest variable in this whole category
- "key override" only helps if somebody in the household still carries a key, which after a year of a smart lock is often nobody
- Rechargeable cells hold a flatter voltage and then collapse, which shortens the warning the lock can give you
Our advice
Buy a lock with external emergency power, and if you already have one without it, keep a physical key somewhere off the premises. The two locks in our test with contacts on the outside turned a lockout into a two-minute inconvenience; the two without turned it into a locksmith. Nothing else in this category matters as much.
Before you blame the lock, fix the door. Adjusting a misaligned strike plate more than doubled the battery life of our worst performer, costs nothing, and improves the door for everybody using it. A lock that is struggling is usually telling you something true about the frame.
Our verdict
Every claim was optimistic, the warning you get varies sixfold, and two of five leave you locked out with no way in. Buy one with external power contacts, and adjust the strike plate.
Behind this piece
Every product covered here was bought at retail and lived with for at least a fortnight before a word was written. Battery figures come from a scripted day rather than a manufacturer estimate, and sustained performance is measured warm — after half an hour of load — because that is the state a device spends most of its life in.
Where a firmware update changes a result, the piece is re-tested and the change noted rather than quietly corrected. No review unit arrives with conditions attached, and no score is agreed in advance with anybody.
Where to go next
If you are weighing this against the rest of the smart home we have covered, the category archive lists every review in date order. The closest comparisons we have published are The Best Smart Home Devices For Battery Life in 2026 and The Best Smart Home Devices For Beginners in 2026.
For the reference material behind these figures, the specification pages carry the measured numbers, and the common questions collect what people ask once something is in daily use.
