9v battery backup circuit question

Discussion in 'The Projects Forum' started by Mike13, Dec 20, 2009.

  1. Mike13

    Thread Starter New Member

    Oct 29, 2008
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    0
    Hi guys!

    I'm relatively new to designing circuits and I would like to have some input from someone who is knowledgeable about it.

    I'm trying to get a board with some regulators and a PIC on it work even when main power goes out and switches to battery. For that, I'm using an icl7673 chip to perform the automatic switch between main power to battery when needed. The main source would be coming from a 12V adapter and the secondary source (9v rechargeable battery) would be the secondary source. I connected it such as shown in figure 8 of the datasheet attached ..except that I added a 220ohm resistor with an LED between pins 1 and 6 so I could actually see which source it's being used. I checked through the other thread of a person also using this chip in the forum and followed same suggested procedure for calculating what value Rc should be and I ended up using a 680 ohm resistor.

    I've put the chip on the breadboard and I do see that it switches between sources. However I'm having problems now with the battery that I'm wondering if I ruined it or something and would like someone more knowledgeable of this to point out where I went wrong. What I see happening is that after the battery is charged ( I measured it with multimeter and got 8.5 volts or so), I connect it and turn on the main power so the board is being powered by the 12 volt..the program on the PIC works fine and everything is well, but when I turn off the main power, so battery works, the system does not work at all and it all just shuts off despite the battery being "charged". I unplugged the battery and checked it again with multimeter and then I read that it was 1.8 volts. What am I doing wrong? Any suggestions?

    The battery that I am using is this one from Radioshack:
    http://support.radioshack.com/support_tutorials/batteries/bt-nicd-hicap-9V.htm

    Thank you very much in advance for any input. I know you guys are busy I read so many interesting questions some people are having. I'm doing this for a hobby so I don't need immediate response, but I would appreciate some help :)
     
  2. Wendy

    Moderator

    Mar 24, 2008
    20,764
    2,534
    A simple pair of diodes will provide battery backup. The side with the higher voltage will be the source, if the primary goes down the secondary automatically takes over. If put this in front of a regulator then the circuit will probably never react a bit.

    If you can find some Schottky diodes their drop is around 0.2V, which is better than conventional diodes.
     
    Last edited: Dec 20, 2009
  3. Mike13

    Thread Starter New Member

    Oct 29, 2008
    4
    0
    Thanks for replying but I was actually trying to make this circuit work because for me it's just for a learning process. I can make it work with the two diodes, in fact, I already did it. But I'm trying to use the least amount of components as possible because using the two diodes I would have to connect to comparator and there get LED indicator output. I don't want to use a comparator because it's just one more chip and then it would be consuming battery power as well since it would be needed to compare when the battery is being used to when it is not. Also, I know I could connect to a microcontroller and use software, but I don't want to use microcontroller pins for this.

    Thanks
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Looking at the Radio Shack data for the battery, it indicates that there is no limit for the charge time at 1.2mA.

    However, if your battery is at 8.4v, and you have an input of 12v, you have a charge current of (12v-8.4v)/680 = 5.3mA. So, you're over the continuous charge current limit by a fair margin, or about 4.4 times as much as allowed.

    If you left it charging indefinitely like that, you would damage the battery.
     
  5. Mike13

    Thread Starter New Member

    Oct 29, 2008
    4
    0
    a well drats..

    Thank you very much SgtWookie for pointing that out to me! I will change the resistor from 680 to 3.3k..it is the closest value resistor that I got that gets me close to 1.2mA.
     
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