Supercapacitor, SLA Battery and Solar Cells in parallel

Discussion in 'The Projects Forum' started by PersianEngineer, Jul 20, 2014.

  1. PersianEngineer

    Thread Starter New Member

    Aug 15, 2013
    Hi guys,
    I am trying to put three 10F supercaps in series with active balancer (designed by opamp, mosfet, etc) and a solar panel (10v, 100mA max) and a 6v 3.2Ah SLA Battery and a small system in parallel.
    The solar panel can not supply any current to battery and system and all the currents are going to ground through MOSFETs in balanced state.
    I haven't tested the3 zener diodes in parallel with supercaps for balancing instead of active circuit but they should behave similarly.
    I think in this case we should use a controlling system like switch to removing the battery or supercap from the circuit in a certain time based on the voltage measurement using micro-controller.
    Do you have any other idea?
    When I use 3 LEDs (which have around 2v forward voltage) in parallel with supercaps for balancing, the current can go to battery for charging it and also to the system. In this case after sunset when there is no harvesting, the supercaps don't supply any current to the system or battery although there is self discharge.
    Now with LEDs balancing circuit, if I remove the battery and we have only 3 supercaps in series with 6 voltage voltage across them and a circuit that draws only 1mA, in 30 minutes, the voltage across supercaps drops as 1v. Why does it discharge so fast? according to my calculation, the charge stored in the supecaps is 3.3F x 6v = 19.8 C and the charge drawn in 30 mins is 0.001A x (30x60) = 1.8 C but the voltage drop (1v) is not proportional with that even if we consider the self-discharge!
    Do you have any idea?
    Now I am thinking about using only one bigger supercap (100F) and two DC/DC buck converter for converting 10v of solar panel to 2.5v of supercap and the DC/DC boost converter for converting 2.5v to 3.3 of the system without using any battery and balancing circuit.
    What is your idea about it?
    I'll appreciate for your comments about my problems. Thanks!
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
    Lets have a circuit and we can see whats needed.
    PersianEngineer likes this.
  3. tcmtech

    Well-Known Member

    Nov 4, 2013
    Your 6 volt SLA battery will need around 7.2 volts input to charge properly so if you are limiting your system to 6 volts via the capacitor balancing circuit that could be a large part of your problem.

    On top of that your op amps draw a bit of idle current so they are more than likely where a good deal of your stored capacitor power is being drained out to.

    To me the design sounds overly complicated for what you need to do.
    PersianEngineer likes this.
  4. NorthGuy

    Active Member

    Jun 28, 2014
    Why do you think the capacitors are better than batteries?

    They cost more, last less, and do not provide stable voltage.
    PersianEngineer likes this.