Battery back up and charging circuit.

Discussion in 'Power Electronics' started by craxe77, Jul 7, 2016.

  1. craxe77

    Thread Starter New Member

    Jun 9, 2016
    Hi all,
    Was wondering if I did my math right here. I have a battery 12v 4ah that I'm including in my chicken door circuit as a back up. I was hoping to charge it as well but don't want to over charge it. I am using a 1k resistor and two 12v 21ma zenar diodes. Here are a few pics of the circuit, battery and charger.

    image.jpeg image.jpeg
  2. craxe77

    Thread Starter New Member

    Jun 9, 2016
    Is there something that I could add that will shut the charging voltage off after the battery is fully charged? Or should it matter?
  3. Colin55

    Active Member

    Aug 27, 2015
    You will only be charging the battery at 3mA and it will eventually go flat.
  4. crutschow


    Mar 14, 2008
    If the DC supply only outputs 12.0V it won't charge the battery.
    A lead-acid battery's fully charged voltage is 12.6V, thus you need more than 13V to trickle-charge it (see here) and a higher voltage (at least 13.8V) to charge it.
    Your best bet is likely to buy a smart charger designed for lead-acid batteries (such as these) and just leave that on the battery while you run the door circuit directly from the battery.
    Last edited: Jul 10, 2016