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Thread: Tank Circuit Voltages

  1. #1
    Low Bitrate
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    Oct 2003
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    Tank Circuit Voltages

    I have a small 1.2Ah 12v sealed lead acid battery I plan to use for my car computer. From waht I gathered on the web, the appropriate charging voltage is around 14 to 15v. Unfortunately, my car produces around 14 volts, and after the diode, fuse, and noise filter, my power supply only sees 13v. I assume this is not going to cut it, but maybe I am doing something wrong?

    Also, while searching (this forum), I found several people that recommended a resistor to limit charging current to the extra battery. What I read on google groups suggested that lead acids can be charged with constant-voltage no problem. Also, if you add a charging resistor, isn't that also going to limit the current going to the car computer from the aux battery?

    Thanks in advance,
    --Tom

  2. #2
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    Nov 2003
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    Quote Originally Posted by ciagon
    I have a small 1.2Ah 12v sealed lead acid battery I plan to use for my car computer. From waht I gathered on the web, the appropriate charging voltage is around 14 to 15v. Unfortunately, my car produces around 14 volts, and after the diode, fuse, and noise filter, my power supply only sees 13v. I assume this is not going to cut it, but maybe I am doing something wrong?

    Also, while searching (this forum), I found several people that recommended a resistor to limit charging current to the extra battery. What I read on google groups suggested that lead acids can be charged with constant-voltage no problem. Also, if you add a charging resistor, isn't that also going to limit the current going to the car computer from the aux battery?

    Thanks in advance,
    --Tom
    I am about to try doing the same thing. I have a 1.2Ah SLA battery that I bought, and I plan on mounting it right in the computer case (it is fairly small). Previously, I built a tank circuit using a diode and 22000uF capacitor, but this didn't stop my computer from rebooting on ignition. I have read posts in this forum that suggest just using an SLA battery in place of the capacitor, however, this would connect the battery directly to the car's power supply, and everything I have read about SLA batteries says that this is a bad thing to do. They can be charged at a constant voltage, but it is important to limit the current that is applied to the battery.

    Anyway, I came across this article:

    http://www.emrg.ca/ideas&info/gel_ce...ll_charger.htm

    The circuit that it proposes has a current limiting resistor, but also connects the power supply directly to the radio, placing a diode between it and the battery. In this way the current to the battery is limited, but current to the radio is not. I plan on using a circuit like that.

  3. #3
    Raw Wave
    Join Date
    Jun 2003
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    1,823
    I dont know what PSU you are using but a 0.7V drop is enough to make your PC unstable. You might also have problem in getting your backup battery charged up properly.

    I posted a "zero voltage drop tank circuit" diagram on some thread some months ago. It should avoid any voltage drop apart from split second where the relay changed over. In that split second the 2 diodes still supply power from both batteries.

  4. #4
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    Nov 2003
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    Colorado
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    I am using an EPIA-V 800 motherboard with the iTuner PW-60 power supply. I've been using this successfully for several months now, but I want a functional tank circuit to make everything more stable.

    I'm not an electronics expert, but from what I have read the ideal charging voltage for a SLA battery is around 13.8V. When the car is running, the alternator will put out about 14.4V. With the 0.7V drop from the diode, this will give a charging voltage of about 13.7V, which should be adequate. When the car is off, the voltage drops to around 12V, which is the same or lower than the SLA's voltage (which is about 12.8V). Therfore, no charging will take place. I do have a couple concerns:

    1) With the SLA only charging when the car is running, and at only a very small current (because of the current limiting resistor), I don't know if this will be sufficient to keep it fully charged.

    2) Because the motherboard draws a small current even when the computer is off, I'm concerned that this will drain the SLA when the car is off, because I am using such a small SLA (1.2Ah). I don't think this will happen, because (I think) it would only draw from the SLA if its voltage is higher than the car battery, and then only until the voltages were equal. But like I said, I'm not an electronics expert, so I could be wrong.

    I guess all I can do is test it out. The SLA battery only cost $10, so it's no big deal if I wreck it. I do not, however, want to damage the computer.

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