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Discussion in 'General Electronics Discussion' started by SPANMAN, Feb 18, 2014.

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    Feb 18, 2014
    I have a APC Back-UPS-ES 350 device that charges a back up battery with 13.6 volts going to the battery. Problem is that when the battery is fully charged and the unit switches to battery the system goes into error because the input voltage is to high. For some reason the device doesn't want any more than 12.8 volts to function properly on battery. I mounted a limiting bulb to bleed off the over charge but have to leave it on all the time. I have thought about a diode in line with the source voltage that would limit the voltage from the battery back to the system. From old school and believe that a diode will act like a low resistance path in one direction but will limit the voltage in the reverse direction.

    Am I wrong on this? Maybe its is an old mind dreaming of a simple fix? Any thoughts out there? Thanks
  2. duke37


    Jan 9, 2011
    A silicon diode will drop something over 0.6V when conducting in the forward dirction, it does not act as a resistance. It does not conduct in the reverse direction.
    You do not say what the battery voltage is that you have to reduce to under 12.8V.
  3. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    Yes that sounds workable. Silicon diodes drop between 0.6V and 2V, sometimes more, depending on the amount of current flowing. Schottky diodes have lower voltage drops but are more susceptible to damage. Connect the diode with its anode to the battery and its cathode to the battery input to the backup circuit.

    What's the current drain of the system?

    Have a look on Digikey, Mouser etc for diodes rated for at least twice the maximum expected load current. Download some data sheets and have a look at the typical forward voltage vs. current graphs. Some diodes will give a maximum curve as well as a typical curve. Some diodes have several curves for different temperatures. (Higher temperature reduces the forward voltage.)


    Feb 18, 2014
    I have to reduce 13.5 volts dc down to 12.8 volts dc. Even down to 12.5 volts dc would probably be best. I will get a silicon diode and give it a try. Thanks for your input.


    Feb 18, 2014
    I do not know the current drain but will look at Digikey for a diode that will help me out. Thanks for your help.
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