Understanding Quiescent Current: The Vampire That Kills Batteries

Executive Summary

If you put a smart lock on a shelf and never touch it, the batteries will still die. Why?

Quiescent Current ($I_Q$) is the tiny trickle of electricity required to keep the brain "alive" so it can wake up when you press a button. It is the metabolic rate of the device.

In 99% of a smart lock's life, it is asleep. Therefore, optimizing Quiescent Current (measured in Micro-Amps, µA) is often more important than optimizing the Actuator Current (measured in Amps).

The Math of "Doing Nothing"

Let's run the numbers on a standard AA battery pack (2500 mAh).

Scenario A: The "Good" Design (Zigbee)

• Quiescent Current: 20 µA.
• Daily Sleep Drain: 0.48 mAh/day.
• Years to Drain (Idle): 14 Years (Longer than the shelf life of the battery).
• Result: The battery is used almost entirely for locking the door.

Scenario B: The "Bad" Design (Cheap Wi-Fi)

• Quiescent Current: 200 µA (Due to cheap LDO regulators).
• Wakeups: Wakes often to check Wi-Fi (High Duty Cycle).
• Daily Sleep Drain: 50 mAh/day equivalent.
• Time to Drain (Idle): 50 Days.
• Result: The battery dies in 2 months even if you never use the lock.

Where does the power leak?

1. The LDO (Linear Regulator)

Batteries provide 6V. The chip needs 3.3V. A cheap LDO burns the excess 2.7V as heat.

• Efficient Lock: Uses a Buck Converter (95% efficient).
• Cheap Lock: Uses an LDO (50% efficient). Half your battery is turned into heat inside the door.

2. The "Touch" Sensor

Capacitive touch keypads are energy hogs.

• Mechanism: The chip must energize the keypad surface 10 times a second to detect a finger "hover."
• Cost: This adds ~50µA to the sleep current. This is why keypads often have shorter battery life than "App Only" locks.

3. The "Polling" Trap

The lock isn't always to blame.

• Z-Wave Polling: If your Smart Hub queries the lock status every 30 seconds, the lock enters a "Light Sleep" instead of "Deep Sleep."
• Drain: Current jumps from 20µA to 20mA. Battery life drops from 1 year to 3 weeks.

Measuring Vampire Draw

You cannot measure this with a standard $20 multimeter.

1. Burden Voltage: Standard meters add resistance. When the lock wakes up (drawing 1 Amp), the voltage drops across the meter, resetting the lock.
2. Dynamic Range: A meter set to "Amps" cannot see "Micro-Amps." A meter set to "Micro-Amps" will blow its fuse when the motor fires.
3. The Tool: You need an Otii Arc or Joulescope—specialized energy analyzers that can switch ranges in nanoseconds.

Related Tools

• Battery Life Estimator: Input the µA and Motor Load to predict your exact lifespan.
• Protocol Power Draw: See which protocols have the lowest sleep current.

Frequently Asked Questions

Does cold weather increase Quiescent Current?

No. The chip actually draws less current when cold (lower resistance). However, the Battery Chemistry fails. Cold increases the Internal Resistance of the battery. So even though the lock is sipping tinier amounts, the battery can't deliver it without the voltage collapsing.

Why do smart locks utilize 4 AA batteries instead of 2?

Motor Torque. A smart lock needs to physically turn a deadbolt. That requires 6 Volts (4x 1.5V) to generate enough torque to overcome door friction. 3 Volts (2x 1.5V) is not enough to shove a sticky bolt.

What is the ideal Quiescent Current for a Smart Lock?

< 50 µA. Anything under 50 is excellent. Anything over 100 means the firmware is sloppy or the hardware is cheap.