The Two-Year Expectation
A remote control in a glovebox is expected to work when the owner finally needs it, which is often two years after it was put there. That expectation sets a hard constraint: the standby current has to be small enough that a coin cell survives the interval, and the design has to tolerate the voltage drop that comes with it.
Where the Current Actually Goes
| Path | Behaviour | Why it matters |
|---|---|---|
| Quiescent / sleep current | Continuous, dominates the total | The single biggest lever |
| TX burst current | Short, averaged over years is small | Sets peak capability |
| Button switch leakage | Continuous when idle | Often overlooked |
| Pull-up / pull-down resistors | Continuous | Value and platform selection |
| Voltage regulator ground current | Continuous | A common hidden cost |
The Sleep Current Is the Whole Game
The button press is not the expensive part. A transmission burst lasts a few hundred milliseconds and draws a few milliamps; averaged over a year it is irrelevant. What matters is what the circuit does when nobody is pressing anything.
Every microamp the quiescent path draws is multiplied by the full shelf time. A design that idles at 5 µA may look fine on the bench and still show an obviously flat battery after a year in a hot car, because the elevated temperature accelerates the self-discharge of the cell on top of the standing load.
Designing for the End of the Cell’s Life
The failure mode is not “battery runs out.” It is “battery runs out at an inconvenient moment because the brown-out threshold was set too high.” A remote that needs a fresh cell to be 2.8 V but refuses to transmit below 2.6 V will strand a customer whose cell has drifted to 2.5 V, and the customer’s conclusion is that the remote is faulty.
- Set the brown-out threshold below the cell’s usable curve, not at its nominal point.
- Design for a voltage range, not a voltage. The same unit should transmit at 3.0 V and at 2.2 V.
- Consider the temperature coefficient. A cell in a dashboard in July behaves very differently from one at 20°C, and the resistance of the contacts rises as the grease in them ages.
- Give the user a way to tell. An LED that flashes differently for “weak battery” and “not paired” prevents the two most common support calls from being confused.
How to Test It Honestly
Measure standby current over a real interval rather than extrapolating from a minute on the bench. Pull a sample of units from a production run, fit them with a known cell, log the current at the bench supply, and hold them at temperature for weeks. A design that passes a 24-hour bench soak can still fail a two-year glovebox test, and the only way to know early is to test early.
Battery complaints are rarely about chemistry. They are about the threshold at which the design decides the battery is dead.
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