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  • The inside of an EDLC supercapacitor
    Surprised by the damage to the supercapacitor, I described the repair of the fault in the topic: Damage to the EDLC supercapacitor (RTC hold). Reading the answers, I learned from @bestler that this is a standard fault and I overestimated the durability of supercapacitors supporting the operation of RTC systems in relation to traditional batteries and disposable cells, e.g. CR2032.

    @tmf pointed out that the EDLC can spill and destroy the PCB, @RitterX suggested that there are two series capacitors inside. We'll see what's inside the damaged capacitor.

    We remove the plug with the leads, you can see the electrolyte and the contact to the capacitor inserted into the metal housing, it is a "push-in" connection without additional welding or soldering.
    The inside of an EDLC supercapacitor


    The plug on the other side connects to the metal housing.
    The inside of an EDLC supercapacitor


    In the housing there is another element in a plastic cover.
    The inside of an EDLC supercapacitor


    They are two capacitors connected in series. The housing resembles a button cell. There is a spring-loaded conductive pad between the capacitors.
    The inside of an EDLC supercapacitor


    It's exactly as you predicted, inside there are two capacitors that have leaked. It's good that the whole thing is in an additional housing.

    I wonder if spercapacitors are used in a housing such as a button cell?

    If so, you can easily confuse such a capacitor with a disposable cell or a battery (unless it has a meaningful marking on the board or a print on the housing).

    Cool? Ranking DIY
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    TechEkspert
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  • #2
    RitterX
    Level 39  
    Batteries are also made in button housings. Interesting solutions include Seiko Kinetic, where the TiCLi, TC920S battery cell is charged by a rotor-driven generator.
  • #3
    1rycho
    Level 21  
    I have replaced many such capacitors in Mitsubishi motor encoders. After several years of work, the inside of the capacitors looked exactly like the pictures.
  • #4
    austin007
    Level 17  
    They fail from hours worked (under voltage) or it doesn't matter if they are working or a lounger? How is the MTTF defined in the notes? Interesting demolition.
  • #5
    TechEkspert
    Editor
    This capacitor has failed during live operation (buffer power / RTC backup).
  • #6
    RitterX
    Level 39  
    The supercapacitor differs from the classic one in that the electric charge is accumulated not so much on the linings in the area of the electric barrier, but spatially, and thanks to this, additional capacity is gained in terms of dimensions and volume. This means that the electrolyte, unless it is completely polarized as a result of exceeding the operating voltage, which will lead to direct current flow through the electrolyte, and thus damage, is present in it in a large volume. So we have a chemical effect on the casing.
    The formula for the energy stored in the capacitor is E=CU?/2, which is more like capacitance, we care about the maximum operating voltage. To increase them, at the expense of capacitance, because it is more economical, increase the operating voltage by connecting several supercapacitors in series, here two. But here comes the problem of balancing the capacitance of supercapacitors over time. If they were power supercapacitors, no one would bother to add a resistive shunt to equalize the voltages on unequally aging supercapacitors, because no one would make a drama out of a few uA. But here a few uA is already a lot. And so on one supercapacitor, the voltage slowly increases over time until the end of the assembly.
    Fortunately, supercapacitors to support the RTC are usually not killers of printed circuits, destroyed by galloping corrosion, as was the case with NiCd and NiMH cells. These batteries only managed to devastate the laminate.