Useful alarm clock modification

[James Sweet]

I've got a number of digital clocks around the house that use a 9V battery
to keep time during a power failure. Seems that the batteries only last one
or two power outages and then they're dead which ends up being most of the
time. I found a simple modification to fix this, simply get a 9V NiCd
rechargeable battery, they're actually 7.2v but that doesn't matter. Now
open the clock and follow the wires from the battery snap until you get to
the diode in series with it, this is usually easy to find. Solder a 2.2k
resistor across the diode and measure the voltage at the battery snap,
should be floating somewhere between 10-15V, install the rechargeable
battery and you're good to go. The resistor passes enough current to keep
the battery charged but the diode still allows the full current to pass the
other way during a power outage, no more dead batteries. Depending on the
voltage of the power supply anything from 1.2k to 3.3k or so can be used
just fine, a higher value will result in longer charge time but less risk of
overcharge, too high and it won't fully charge at all. 2.2k has worked in
every clock I've tried so far.

 

[JW]

I've got a number of digital clocks around the house that use a 9V battery
to keep time during a power failure. Seems that the batteries only last one
or two power outages and then they're dead which ends up being most of the
time. I found a simple modification to fix this, simply get a 9V NiCd
rechargeable battery, they're actually 7.2v but that doesn't matter. Now
open the clock and follow the wires from the battery snap until you get to
the diode in series with it, this is usually easy to find. Solder a 2.2k
resistor across the diode and measure the voltage at the battery snap,
should be floating somewhere between 10-15V, install the rechargeable
battery and you're good to go. The resistor passes enough current to keep
the battery charged but the diode still allows the full current to pass the
other way during a power outage, no more dead batteries. Depending on the
voltage of the power supply anything from 1.2k to 3.3k or so can be used
just fine, a higher value will result in longer charge time but less risk of
overcharge, too high and it won't fully charge at all. 2.2k has worked in
every clock I've tried so far.

Assuming there's room, I wonder if a good size capacitor wouldn't do the
trick as well?

 

[mike]

Assuming there's room, I wonder if a good size capacitor wouldn't do the
trick as well?

I'm often tempted to put rechargeable batteries in stuff that wasn't
designed for 'em. The thing I worry about is when I got a new one, I
forgot about the charging, sent the old one to goodwill, someone bought
it for a quarter, put alkalines in it and ends up with caustic goo all
over their
nightstand, or worse an explosion or even worse, the baby touched it and
got it in his eye...When you're dead, that nightstand might be YOUR
grandson's.

If you gotta do stuff like this, at least hot-glue the resistor to the
battery and run a wire inside the box. That way, it will get
disconnected if the battery is replaced.

A lot of tragedy "sounded like a good idea at the time".
Something to think about.
mike

--
Bunch of stuff For Sale and Wanted at the link below.
laptops and parts Test Equipment
Honda CB-125S
TEK Sampling Sweep Plugin and RM564
Tek 2465 $800, ham radio, 30pS pulser
Tektronix Concept Books, spot welding head...
http://www.geocities.com/SiliconValley/Monitor/4710/

 

[James Sweet]

I'm often tempted to put rechargeable batteries in stuff that wasn't
designed for 'em. The thing I worry about is when I got a new one, I
forgot about the charging, sent the old one to goodwill, someone bought
it for a quarter, put alkalines in it and ends up with caustic goo all
over their
nightstand, or worse an explosion or even worse, the baby touched it and
got it in his eye...When you're dead, that nightstand might be YOUR
grandson's.

If you gotta do stuff like this, at least hot-glue the resistor to the
battery and run a wire inside the box. That way, it will get
disconnected if the battery is replaced.

A lot of tragedy "sounded like a good idea at the time".
Something to think about.
mike

I do label the battery compartment, but realistically I'll never get rid of
those clocks until they break, and when that happens I'll remove anything
useful and throw the rest in the trash. A leaky battery won't kill anyone
anyway, it'll just make a mess.