Diagnosis
The Breville Smooth Wave BMO850BSS is designed with a volatile memory system for its clock function. Unlike a traditional wall clock or some high-end ovens, this microwave does not utilize a long-term CMOS battery or a replaceable coin cell to maintain the time during a power disconnect. Instead, it relies on a capacitor-based backup system that can only hold the clock memory for a very short window—typically less than 30 seconds. When you unplug the unit, the capacitor drains almost instantly. Because there is no permanent battery backup integrated into the control board, the microprocessor loses its volatile memory and reverts to the default "0:00" state. If you are seeing this happen even during a momentary flicker of power, it indicates that the capacitor on the control board has degraded or that the unit is simply performing as designed by clearing its cache upon loss of AC current. You can confirm this by plugging the unit in, setting the time, and then unplugging it for just 10 seconds; if it resets, it is the standard behavior of the BMO850BSS circuitry.
How to Fix It
Since there is no user-serviceable battery to replace, the "fix" involves ensuring a stable power connection to prevent unnecessary resets and properly re-initializing the clock using the specific sequence required for the Smooth Wave interface.
- Ensure the power cord is seated firmly in a grounded 120V outlet. Avoid using extension cords or power strips, as voltage drops can sometimes trigger a logic reset on the control board.
- Press the Clock button on the control panel. The display will begin to flash.
- Use the Dial (the rotating knob) to adjust the hours and minutes. Rotate clockwise to increase the time and counter-clockwise to decrease it.
- Press the Clock button again or the Start button to lock in the time.
- If the clock continues to drift or reset while plugged in, perform a hard logic reset by unplugging the unit for exactly 60 seconds and then plugging it back in to clear any static charge from the capacitors.
If That Didn't Work
If the clock resets while the microwave remains plugged in, or if it refuses to hold the time for more than a few hours, you are dealing with a hardware failure rather than a lack of a battery.
- Control Board Capacitor Failure: The electrolytic capacitors on the main PCB (Printed Circuit Board) may have leaked or bulged. This prevents the board from holding the minimal charge needed to maintain the clock cycle. This requires a professional replacement of the control board assembly.
- Intermittent Power Supply: Check for a loose connection at the wall outlet. If the prongs on the microwave plug are slightly bent or the outlet is "loose," the unit may be experiencing micro-interruptions in power that trigger the 0:00 reset. Try a different circuit in your kitchen to isolate the issue.
- Firmware Glitch: In rare cases, the internal logic hangs. Unplug the unit and hold the Start button down for 15 seconds while unplugged to discharge any remaining energy from the board, then plug it back in.
- Voltage Fluctuations: If you live in an area with "brownouts" or unstable grids, the BMO850BSS may reset. Installing a small UPS (Uninterruptible Power Supply) or a high-quality surge protector can stabilize the current and prevent the clock from resetting during voltage dips.
Heads Up
- Never attempt to open the chassis of the BMO850BSS to search for a battery. Microwaves contain high-voltage capacitors that can hold a lethal electrical charge even when the unit is unplugged.
- Avoid using "Eco Mode" or power-saving strips that automatically cut power to the appliance when not in use, as this will force you to reset the clock every time you want to use the microwave.
- Keep the ventilation slats at the back of the unit clear of dust; overheating the control board can lead to erratic behavior in the digital display and clock timing.
- If the display shows "Err" alongside the 0:00 reset, this indicates a deeper sensor failure and the unit should be serviced immediately.