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Adding a *rechargeable* battery backup to a small device

Hello,

Imagine you have a small device which is normally plugged into a wall, such as an alarm clock. You want to put a battery back-up into it, so if the wall power should ever fail (unplugged, outtage, etc), your device still functions. But, at the same time, you want the battery to be rechargeable. Ideally, the device will recharge the battery while plugged in, but switch to battery power when necessary.

So, I don't know too much about how a battery charger works, but here's what I was thinking:

Wall power leads into a typical power supply, yielding 9Vdc. Those

9Vdc supply a battery charger which is always charging a rechargeable 9V. The terminals of the 9V go through a 5V regulator to power the rest of the circuit. There is no direct route from the 9Vdc power supply and the 5V regulator.

Does this sound like it would work?

Is there a more common-practice topology for this sort of design?

Anyone have links on how to design a battery charger?

Thanks, Nick

Reply to
nickjohnson
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I bought my Sony clock many years ago because having a rechargable is the only way to go. Changing 9 volt batteries is not what I do. Imagine what that costs after 20 years.

There are various voltages from 9 volt cells, depending on the number of cells used. That critical if driving signifant current into the cell.

Sounds like that 5 volt thing would work, but is 5 volts really used typically?

greg

Reply to
GregS

Design a trickle charger for the optimum trickle charge voltage for your battery plus .7V. Connect it to the battery via a

1N4004 diode, and run your circuit off the battery.

Good Luck! Rich

Reply to
Rich Grise

Thanks Rich, this is exactly what I was looking for.

Why the diode -- just to make sure the charger doesn't draw current from the battery?

Reply to
nickjohnson

You mention clock. Most clocks run off the AC line frequency; so, you need to supply timing to the circuit when the AC goes off. Most commercial digital clocks with battery backup use an RC oscillator to generate 60 (or

50 ) Hz. These are typically set up to gain about 10 minutes/hour.

Years ago I built a digital clock that ran off 11V, with a 9V backup battery The display ran off the 11V, the clock chip off a diode matrix that would use whichever voltage was higher. I included a crystal oscillator and countdown circuit to generate the line frequency.

In the past, PCs used 5V with a 4.5V battery. Crystal was 32.768 KHz, which got counted down to 1 Hz.

Tam

Reply to
Tam/WB2TT

This is interesting; I would have just expected them to use a crystal. Do they do this to reduce product cost, or is the AC line a better clock than a crystal?

Reply to
nickjohnson

Yup.

(BTW, we bottom post here)

Cheers! RIch

Reply to
Rich Grise

AC line is a better clock than a cheap crystal. Power stations vary the frequency slightly to keep clocks accurate. Of course, that only works so long as you don't have a power outage.

Tam

Reply to
Tam/WB2TT

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