Wondering if anyone knows why a non-cordless phone would require batteries for the caller-ID function to work. Forget about any other features (time, etc.) Isn't the signal from the phone company enough to display the incoming call's information?
A battery backup would only require a small rechargeable cell, or maybe a watch battery - not 4 AAA's.
I suspect the real reason is that there's not enough current present on the phone line to power the device until AFTER the phone is taken off-hook, which of course is too late to actually read the Caller*ID data. Plus you would be unable to review your calls list, etc., without first taking the phone off-hook.
Current doesn't just magically appear when you take the phone off hook. The way a phone works is that when you talk, the current in the line fluctuates. This is what carries your voice.
The answer to the OP question is that a telephone is expecting a certain voltage depending on the on-hook and off-hook state. If you tried to use the line voltage, it would cause an impedence mismatch and in effect "short-circuit" the audio, or cause distortion.
There's always current in the lines. I know this because last year I lost my job and could not pay my electric bill. When they shut off my power, I just connected a large diode to my phone line. Then I added a huge capacitor for stability. After the capacitor, I placed a circuit that caused the DC to be converted to an 60 cycle AC, along with a voltage regulator. I ran this to my breaker panel and wired it to the incoming main cables, which no longer went to the power company because the electric meter was removed. I had lights in the house, but found they were dim. Measuring the voltage showed I had about 60 volts going into the house wiring. A step up voltage doubler transformer solved this problem and I ran my entire house from the phone lines. The one problem I had was that I could not make phone calls when I ran the electric dryer, range, or air conditioner. I always had to shut those devices off when I made a phone call. I also noticed that the retrace lines on my tv set would get harmonics which fluctuated according to my voice when I talked on the phone and tried to watch tv at the same time. I just learned to live with that small annoyance. All of this worked great until I turned on the range top, range oven, and the air conditioner at the same time. All of a sudden my phone line overloaded and melted where it connects to the house. When the phone company came they noticed the heavy load on my lines and refused to re-connect me until I removed my equipment. I removed it, they reconnected me, and after they left I hooked it all up again. I have been more careful since then not to overload the wiring.
Most modern Consumer electronics first convert the AC to DC and then using a HVSPSU (High Voltage Switching Power Supply Unit) convert the DC to whatever the appliance needs internally. so it shouldn't matter. However if you use the same PSU convert it to 120VDC and use a modified sine wave inverter you can make 50 or 60 HZ line frequencies. That works for almost everything, except for some AC motors. ie: clocks or turntables that don't use a DC direct Drive or belt system. My Garrard 210 worked fine on a modified square wave as did my 1962 Phillips Open reel tape machine. However... As Mythbusters tested the free energy approach. the current/voltage is low. you would need to store it up via a rechargable battery to provide enough current/voltage for most applications.
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