What should I check before using a microSD card for 24/7 security camera recording?
Check three things before anything else: whether the camera writes continuously or only on motion, how hot the housing gets, and what happens when the card fills or the power fails. A 24/7 camera is the same endurance problem as a dashcam but with two additions — it may run for years without intervention, and it is often the only copy of footage that matters after an incident. Choose for endurance and power-loss behaviour, and verify that the camera can actually read the card you standardise on.
Two recording modes, two different workloads
Continuous recording writes constantly and fills the card on a fixed cycle. Motion-triggered recording writes in bursts, which sounds gentler but has its own problem: the files that matter are written at unpredictable moments, often immediately after a power event or a camera reboot.
Ask which mode the deployment uses before choosing capacity, because the arithmetic is completely different.
| Mode | Write pattern | What it stresses |
|---|---|---|
| Continuous | Constant, 24 hours | Endurance and heat |
| Motion-triggered | Bursty, unpredictable | File system integrity at power loss |
| Hybrid | Continuous low-rate plus bursts | Both, and buffer behaviour |
Power loss is the quiet killer
A security camera is often on a circuit that also feeds other equipment, so it can lose power abruptly. If power drops mid-write, the card can lose not only the file being written but the file system structures that map the whole card.
The result is a card that looks full but mounts as unreadable — discovered precisely when someone needs the footage. Cards and hosts that implement power-fail handling mitigate this; cheap combinations do not. It is worth asking specifically what happens on unexpected power removal, because "it works" is not the same answer.
Heat, and why the housing decides
A camera inside a metal housing in the sun runs warmer than the room it is installed in. Continuous writing adds its own heat. Together they push the card toward thermal throttling, which shows up as dropped frames or gaps in the recording rather than an error message.
If the deployment is outdoors or in an unconditioned space, test the card in the actual housing at the actual temperature. A card that records flawlessly on a bench can produce gaps in a sealed outdoor unit.
Capacity: size to the retention requirement
Retention is usually the driving requirement — how far back does someone need to be able to look after an incident. Work backwards from that, not from the largest card available.
If a site must retain 30 days of continuous recording, the required capacity follows from the bitrate and the recording mode. Buying a card twice that size adds cost and does not improve reliability, though it does spread writes over more cells.
Sources
- SD Association — endurance and power-fail behaviour
- SD Association — application performance classes
Figures quoted on this page are from the published reporting listed above. Where a number is an estimate rather than a published figure, it is described as approximate.
Frequently asked questions
Is a card designed for phones suitable for a security camera?
Usually not for continuous recording. The workload is closer to a dashcam than to a phone: sustained writes, elevated temperature and years without intervention. If the camera records continuously, specify a card rated for that duty rather than one chosen on speed or price, and confirm the operating temperature range covers the installation environment.
What happens when the card fills up?
It depends entirely on the camera firmware. Some overwrite the oldest footage in a loop, some stop recording, and some do something worse — record a placeholder while appearing to work. Confirm the behaviour before deployment rather than discovering it after an incident. This is a host behaviour, not a card property, so the card choice cannot compensate for it.
Should I use one large card or several smaller ones?
One card is simpler and avoids the risk of a card being removed and not replaced. Several cards spread wear but introduce a human process that fails quietly. For most deployments a single endurance-rated card sized to the retention requirement, with a monitoring alert on card health, is more reliable than a rotation nobody maintains.
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