I have two domestic Miele ovens, each with a clock.
Recently there was a power cut for about 30 seconds. One clock reset and the other continued as normal?
Why?!
I expect they have batteries in to maintain the time during power outages.
One of them probably has a dead battery. (Some use capacitors to keep
time but that will only work for a very short outage.)
Do you remember if they both used to keep time in power outages ?
Aha - that sounds jolly helpful!
I haven’t had a recent power outage…
If only an oscillator needs to be powered, an appropriately-rated “hold up” capacitor can easily keep it powered for many minutes.
Yeah. Sounds like there should be battery backup, so it’s likely that either one predates that feature or its battery has given up the proverbial ghost.
And even if they’re the exact same model, it’s not at all implausible that the capacitor might have randomly failed in one but not the other. Capacitors do that sometimes, and a cap whose only job is to keep the time intact through a brief power outage isn’t going to be expected to hold to high quality standards.
Neither had I, then 2 in the last couple of months, several hours each.
The whole country is going to the dogs I tell you ! I blame tr%mp.
The ovens do not have batteries (I’ll check if they have capacitors.)
Once again, thanks for joining in! 
If there is a capacitor, it’s probably built into the same module as the clock. It wouldn’t have to be big and obvious.
Or, more specifically, accessible to the user. Almost certainly on the main circuit board behind one of the panels.
“No user accessible parts” territory.
I should have added this: the clock might not use a crystal-based oscillator. It might use the incoming power’s 60 Hz frequency as a time-base. If so, a capacitor backup scheme, assuming one exists, would have to be fairly sophisticated.
Some clock backups don’t bother to advance the clock during a power outage, just hold it. If the outage is brief enough, this doesn’t matter much (and if it’s not brief, the backup won’t last, anyway).
Miele oven clocks can be synched via wifi.
Any modern electronics powered by a wall plug will have capacitors - it’s how they clip and smooth power from AC to DC. Size depends on the amount of power needed, but modern electronics (as separate from the heat elements) does not need a lot of power.
This is why with things like computers or other household electronics, when restarting because of a serious problem, they tell you either turn off the switch or unplug for 30 seconds, then power on. That should give any internal capacitors time to drain.
As I understand - To “reset” chips, there’s a reset line. It’s usually connected to an RC circuit (resistor-capacitor) so it takes a few seconds to get up to voltage on power up. Thus, all the power is running to chips in the device before the critical chips get the reset line toggled on, and begin their boot procedure.
However, I suspect the answer to the OP is that there is a form of small battery sufficient to maintain the clock counter for a while; since not having to reset the clock every power outage is a highly useful feature in some locales.
Also note that you can run a clock for a decade on a battery small enough to fit in a watch. It doesn’t take much.
Yes, and the tolerance on these types of capacitors is very loose (e.g. -20% / +100%), and they also lose capacity as they age. In addition, the current draw of the clock circuit will vary from unit to unit. You could easily have a factor of two difference in ride-thru time during an outage with two of the same microwaves.
I think that a 30 second outage would have reset all my electric clocks, but for what it’s worth, once or twice we’ve had very brief outages of like around one second, and the microwave clock gets reset to zero but the clock on the stove is still showing the right time. The stove of course is on 220V while the microwave is just plugged into a regular 110V outlet. I assume that the stove is benefiting from capacitance somewhere in the system.
Clock backup, if present, could be a battery, but for a few decades now a super-capacitor is a common alternative. Why super? The capacitance is ridiculously high. But they are weird devices and basically don’t work for AC applications. Where AC is essentially anything faster than 1Hz. And voltage rating is miserable, a few volts tops. So they are not useful for the sorts of tasks one sees conventional capacitors generally used for, like power supply filtering. Such super-capacitors are relatively tiny. Of much the same size as the button cell they replace.
Backup clock could be as simple as a resistor and capacitor setting the frequency for the clock chip. If external components are needed at all. Nobody is expecting significant accuracy.
Clock and settings backup is the only place I’ve seen them used. They’re well suited to it, basically little rechargeable batteries that don’t freeze or lose capacity like chemical cells, but surprisingly few other uses. Big 1 Farad caps for power supplies are usually the size of my bicep and I’m not clear on the differences or how they’re able to jam an entire Farad in a component the size of a lentil.
I’ve got an old dashcam with a failed battery and it’s completely useless now: every engine start prompts for time & daye entry. My current cam has a supercap and said to last much longer.
One trend in modern electronics in general is that we’re moving away from things that “wear out”, and towards things that just fail outright, stochastically. With an old battery, if you keep on charging and discharging it, each time it gives you a little less energy back, until it’s mostly good enough but things are performing suboptimally, and the performance gets worse and worse, until eventually you decide it’s bad enough and replace it. With a new capacitor, it might at any time just go completely kaput and instantly become 0% useable, but until and unless it does that, it’s still at 100%, and an old one (that’s survived so far) isn’t appreciably more likely to die in any given timespan than a new one.