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

 

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

 

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

 

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?

 

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

 

You mention clock. Most clocks run off the AC line frequency; so, you need

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?

 

Yup.

(BTW, we bottom post here)

Cheers!
RIch

 

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