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

[[email protected]]

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
http://store.yahoo.com/minilab-supply-store/raynimchar9v.html )
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

 

[Vidar Løkken]

http://store.yahoo.com/minilab-supply-store/raynimchar9v.html )
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

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

 

[[email protected]]

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 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

 

[CWatters]

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.

500VA ! Must be a really big alarm clock.

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
http://store.yahoo.com/minilab-supply-store/raynimchar9v.html )
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

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...

http://store1.yimg.com/I/starbattery_1838_658029

 

[mike]

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
http://store.yahoo.com/minilab-supply-store/raynimchar9v.html )
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

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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[John Woodgate]

FWIW, I've never had a 9V rechargeable battery last.

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

 

[H. Dziardziel]

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
http://store.yahoo.com/minilab-supply-store/raynimchar9v.html )
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

Have you considered just a couple of AA cell holders and use AA
alkalines? Branded AAs have excellent shelf lives and 100 times
the mAh capacity of 9V batteries so will probably last for years
in that application. Total physical size<wallwart size with
about identical cost..

 

[[email protected]]

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.

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.

One solution would be to connect my
alarm clock to an uninterruptable power supply.
But I would need atleast a 500VA,

theres definately something youre not telling us!


NT