On 10/30/2012 5:12 AM, Michael wrote:
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.

Bill Gill