A backup battery

Discussion in 'General Electronics Chat' started by poofjunior, May 21, 2013.

  1. poofjunior

    Thread Starter New Member

    May 21, 2013
    Hi, everyone,

    I've been considering a circuit that provides both backup power via a battery while charging the battery at the same time.

    My current idea looks like the attached picture, using
    1. a zener diode to keep the voltage capped at the maximum voltage of the battery
    2. a current-limiting resistor to limit the rate of charging the battery.

    I've yet to build up some strong intuition, so I'm hoping to know if there's something obvious that I'm missing, or if I'm completely off, and a project like this isn't feasible. The reason I mention this is because I've poked around through the internet and found many much more complicated solutions to charging a battery.

    Thanks for taking a look, and I really appreciate any tips!
  2. JMW


    Nov 21, 2011
    Are you really asking if you can build a circuit that is powered by a battery and charge it at the same time without connection to an outside power source? Sort of like plugging a UPS into itself. Yes you can do that, but for what purpose? Eventually the UPS battery will run down and shutoff. Sort of like leaving the door to your fridge open and expecting it to cool the room.
  3. GopherT

    AAC Fanatic!

    Nov 23, 2012
    No, it sounds to me like he is asking to run a device and charge the backup battery at the same time!

    Yes it can be done. Your circuit seems to be in the right direction.
  4. timescope


    Dec 14, 2011
    Calculate the power that will be dissipated in the zener diode when the battery is fully charged and select one of adequate wattage.
    Use Schottky diodes to supply the circuit.

  5. poofjunior

    Thread Starter New Member

    May 21, 2013
    Thanks everyone. Aye, GopherT has got it right. The main motivation is a clock with a digital timekeeping chip that can't lose power. I'll give it a shot in hardware some point soon!
  6. Ramussons

    Active Member

    May 3, 2013
    Hi poofjunior,
    Is the diode connected to the output of the power supply required? What's its purpose?

  7. Dodgydave

    Distinguished Member

    Jun 22, 2012
    You need a circuit similar to this , use shottky diodes as they will have a lower voltage drop, alter the regulator to suit the battery voltage .
  8. ramancini8


    Jul 18, 2012
    Buy a UPS.
  9. poofjunior

    Thread Starter New Member

    May 21, 2013
    Thanks everyone; it works! Without zeners, I strung a chain of 1N4001 diodes until I had the correct voltage drop (4.2 [V]) for the single-cell LiPo that I used in my test circuit.

    Thanks, Dodgydave; An interesting note is that the Schottky diodes will drop the voltage (just barely!) below the regulated 9 [V]. I can imagine myself putting a regulator after the two power supplies, rather than before, but the consistency in your circuit makes sense since the battery source is 9 [V].