Battery Backup clock radio

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.
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Hecho en Cheena, senor?
I don't know, but my guess is that quality control is suffering, all over.
Christopher A. Young Learn more about Jesus www.lds.org .
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.
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On 10/30/2012 6:12 AM, Michael wrote:

Normal operation uses the typically very accurate 60 Hz line frequency as the time base.
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At on time some clocked synced with the TV vidio. Don't know what happens with the HDTV system.
Might check with you cell phone as those do sync with the system clocks. Another option is one of the Atomic Clocks that sync with the WWVL signal.
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On 10/30/2012 5:12 AM, Michael wrote:

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.
Bill Gill
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On 10/30/2012 05:12 AM, Michael wrote:

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.
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Mark Lloyd
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wrote:

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.
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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.
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On 10/30/2012 6:12 AM, Michael wrote:

Try one of these. They keep near perfect time whether on internal battery, generator or grid power.
(Amazon.com product link shortened)
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On 10/30/2012 05:25 PM, diy savant wrote:

It probably does keep good time, once set. However, according to the reviews, it is NOT self-setting.
BTW, I've had a clock like that too. It even pretended to set itself. It never did
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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.
Greg
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THANKS EVERYONE FOR THE ANSWERS. APPRECIATED.
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On Tue, 30 Oct 2012 03:12:43 -0700 (PDT), Michael

I've noticed the same thing, chalked it up to it not having very good freq control when on batteries.
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