creating a 9VDC rechargable battery backup for an alarm clock radio

In my community, we have interruptions of electricity more than average. My alarm clock has one of those 9v battery backups, but one 4 hour blackout can significantly drain the battery, requiring me to check and frequently change it. One solution would be to connect my alarm clock to an uninterruptable power supply. But I would need atleast a 500VA, they can be expensive and many of them beep when running on battery power. I was wondering if I could just wire in a 9v rechargable NIMH battery (8.4v) and connect the output of a cheap 9v-style battery charger to this battery (such as

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) Do any of you know what I could expect as far as voltage from the charger across an 8.4v NIMH battery while its charging? As long as the voltage remains below 10v, the clock should tollerate it well. Thanks

Reply to
techman41973
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You can add a diode (a germanium only lowers voltage by 0.2-0.3V), thus stopping current from the charger to the clock, when charging. Add a diode pointing towards the clock connection from the + of the battery. I can draw you a quick schema if you want :)

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Reply to
Vidar Løkken

My cheapo alarm clock 19?, ie 300USD? runs for a year on 2 AA cells.

and its got temp/humidity/logging/remote sensing etc

Get a new alarm clock

martin

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Reply to
martin griffith

Use some voltage from inside the radio to do the charging. FWIW, I've never had a 9V rechargeable battery last. They're tiny cells and seem to short out faster than I can use 'em up. I'd go with bigger, more reliable cells.

Might be better off with a new alarm clock that's always battery powered. mike ps 500VA???? That's gotta be some LOUD alarm.

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Reply to
mike

I would really like to see the "alarm clock" that needs a 500 VA UPS to keep it alive. I would think that one of those ~150 VA "fat power strip" UPSs would keep your alarm clock humming. (But it might be interesting if your clock uses the line frequency for timekeeping and the UPS isn't at exactly 60.0 or 50.0 Hz...) Plus, if you are reasonably careful, it's not too hard to cut the trace to the beeper, or remove it altogether.

Matt Roberds

Reply to
mroberds

500VA ! Must be a really big alarm clock.

If you plan on leaving the charger connected permanantly you should check what the float current is into the fully charged battery. Otherwise you can kill the battery - sometimes quite quickly. Ideally you need just enough current to match the self discharge...

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Reply to
CWatters

4

what kind of alarm clock drains a battery in 4 hours? Big Ben? If this is true, there is something seriously wrong with your alarm clock! Get another.

theres definately something youre not telling us! :)

NT

Reply to
bigcat

A second possibly more effective "solution" would be to modify the clock circuit current drain on battery backup- such as kill the big LED display if it has one....Thanks for the fascinating question and post nonetheless...

Reply to
Fred Bloggs

I read in sci.electronics.design that mike wrote (in ) about 'creating a 9VDC rechargable battery backup for an alarm clock radio', on Tue, 22 Mar 2005:

OTOH, I have a few that have survived for 14 years and still have usable capacity. But then this year is Y MMV!

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John Woodgate

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