Read a spec sheet for a mini camera and you get one number: eight hours, or two hundred hours, or thirty days on standby. It is almost never wrong, it is just answering a different question from the one you asked. Eight hours of battery on a card that fills in three is three hours of device.
Pick a device class to load typical figures, then replace any of them with the real numbers from your own hardware. The result is two independent countdowns and a plain statement of which one ends first.
Picking a device refills every box below with a typical figure for that class. Change any of them.
A cell is empty for the electronics before it is empty on paper. 85 is a fair working figure, lower in the cold.
| Battery lasts | |
|---|---|
| Card fills after | |
| Average draw across the cycle | |
| Energy in the cell at 3.7 V |
Two independent sums, shown so you can check them. Battery: hours = mAh × usable% ÷ average mA, where average mA weighs the recording draw and the idle draw by the duty cycle. Card: hours = GB × 0.93 × 8192 ÷ (Mbps × 3600) for video, or the same with kbps for audio. The 0.93 is the space a card loses to formatting.
Where the numbers come from
Current draw is the figure you will struggle to find on a spec sheet, and the presets here are middle-of-the-road values for each class rather than a measurement of any particular model. If you own a USB power meter, measuring your own device while it records takes two minutes and makes everything below exact.
The usable share of the cell defaults to 85 per cent. A lithium cell is empty for the electronics well before it is empty in the chemical sense, and cheap devices cut off earlier than good ones. In cold weather, drop it to 70 and see what happens to the figure. Winter runtime surprises people every year.
Trackers work differently
Select GPS tracker and the card fields disappear, because there is nothing being recorded. What decides a tracker life is how often it wakes up, gets a fix and uploads it. Each of those cycles is counted here as 35 seconds awake, which is a fair figure for a unit with a warm almanac and decent signal. A cold start under a metal roof can take four times that, which is why a tracker in a garage dies so much faster than the same tracker on a driveway.
The practical consequence: going from a report every two minutes to one every ten is not a small saving. It is most of the battery.
The loop recording trap
When the card fills, most devices do not stop. They start overwriting the oldest footage, and they do it quietly. People discover this at exactly the wrong moment, when they go looking for something that happened four days ago on a card that holds three. If the calculator tells you the card fills first, that gap is the real length of your recording history, not the battery figure on the box.