I have two clock/radio television units. Both have battery backups. I installed fresh 9 volt batteries and when power was restored last night after a 3 hour outage, the clocks were 8-10 minutes fast. What gives? I thought the batteries were supposed to maintain the time. Just curious.
A lot of the battery back up clocks use the power line frequency for timing. But during power outages they use a simple RC controlled oscillator to keep the time. RC oscillators are very poor at keeping on an exact frequency. So during power outages the clocks that use them can drift quite a bit. The drift in your clocks isn't too far out from what you could expect.
Of course since last year the power line frequency isn't extremely stable. I set mine last January and the time drifted about plus or minus 10 seconds up until Summer. Since then the clock has gained about 30 seconds. That is because the government relaxed their rules on how tightly power sources had to be locked to the national frequency standard.
Plug-in clocks with battery backup usually seem to use a cheap RC timebase when on battery. They're not very good at timekeeping during power failures. Most I have are accurate on AC, fast on battery.
What others have said. In addition, fast is intentional for alarm clocks because fast is better than slow when the goal to make sure you get up on time.
Michael wrote in news:3d0f106b-63b8-46af-bb3c- snipped-for-privacy@10g2000vbu.googlegroups.com:
They did. Just not as accurate as what you expected. Normally the mains frequency determens the clockrate, but when the mains is down/out, a free running oscillater takes over. Sort of. In some clocks.
I have a similar clock. I don't even know if the battery is replaceable. It was also about $20. It has so many buttons, two alarms, etc. Even works off new saving time settings. No touch. It just sits there and works well.
My old clock was a Sony with charging battery. Having to replace a 9 volt battery every year is costly and troublesome.
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