Float charging PP3 (as a ups)

[Matthew Greogry]

Hi All,
This is my first post so please be gently with me I have search
google groups for an answer, but nothing came up that could satisfy
any doubts I have.

Quick Question: Can I take 9v DC from a PSU to power a circuit and
also have a rechargeable PP3 permanently in parallel as a backup?

Long Question: We have at work some break-glass fire alarms that go
through batteries very quick! I'm trying to design a circuit that will
allow the units to run off a PSU with battery backup. The units took 2
PP3 batteries original, the second battery was only used to create the
18v needed for the internal sounder. I've replaced the sounder with a
9v one and they can now run off a single PP3.

What I'm after is basically a simple UPS. Will something along the
lines of the following work? http://www.nogsnet.com/personal/images/ups.jpg

The connectors on the left are connected the PSU, the connectors on
the right connected to my load (the fire alarm circuit) and the
battery in the middle is my PP3.

If this does work what chemistry battery should I use and what would
be the float voltage?

Sorry if I have something wrong, I am a computer programmer not an
electrician (only by hobby). Feel free to correct/hurl abuse at me.

--------------------------------------------------------------------------------

 

[TT_Man]

What I'm after is basically a simple UPS. Will something along the

lines of the following work?
http://www.nogsnet.com/personal/images/ups.jpg

The connectors on the left are connected the PSU, the connectors on
the right connected to my load (the fire alarm circuit) and the
battery in the middle is my PP3.

If this does work what chemistry battery should I use and what would
be the float voltage?

Sorry if I have something wrong, I am a computer programmer not an
electrician (only by hobby). Feel free to correct/hurl abuse at me.

--------------------------------------------------------------------------------

Looks ok... Just ensure that the current charging the battery is about 10mA
max

 

[Matthew Greogry]

Looks ok... Just ensure that the current charging the battery is about 10mA
max

Wow! That was a fast reply!

So I am on the right tracks then... just a few quick questions then to
wrap it all up.

1.) So how would I insure 10mA max?
2.) If my maths are correct the values in the PD supplying current to
the battery gives 8.85v but most rechargable PP3 batteries are only
rated at 8.4v... should I change the value of the resistor so the
charging voltage is the same as the battery?
3.) Which would be best for this nickel cadmium or Lithium Ion?

Thanks.

 

[Bob]

Looks ok... Just ensure that the current charging the battery is about 10mA
max

That's too high for permanently charging a nickel metal hydride PP3
but
a nicad would survive. Matthew needs to read some battery datasheets.

Bob

 

[TT_Man]

Wow! That was a fast reply!

So I am on the right tracks then... just a few quick questions then to
wrap it all up.

1.) So how would I insure 10mA max?

Measure the voltage across the 250R resistor( or the current through it).
You don't need the 15K resistor...
The charge current should be no more than 1/10th of the capacity of the
battery in mA/Hrs.If it's only ever going to supply juice once in a blue
moon, you could use 1/20 or less....

2.) If my maths are correct the values in the PD supplying current to
the battery gives 8.85v but most rechargable PP3 batteries are only
rated at 8.4v... should I change the value of the resistor so the
charging voltage is the same as the battery?

You'll need a bit more than 8.85 volts to charge the battery. Don't forget
the V drop across the charging diode.... I'd say better off with 12V. If the
main CCT can't run off 12V, fit 4 diodes in series with the top diode, that
will drop the voltage down without having to use a 9V regulator chip...

3.) Which would be best for this nickel cadmium or Lithium Ion?

Nicad/ NiMh. Don't use Lithium- too risky and needs specialist charging
circuits/protection etc.etc..