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Float charging PP3 (as a ups)

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?

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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.

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Reply to
Matthew Greogry
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Looks ok... Just ensure that the current charging the battery is about 10mA max

Reply to
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? 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.

Reply to
Matthew Greogry

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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

Reply to
Bob

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....

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...

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

Reply to
TT_Man

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