The inside of an EDLC supercapacitor

TechEkspert 07 Feb 2023 19:07 5 5151 Cool? (+6)

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TL;DR

A damaged EDLC supercapacitor used for RTC hold was opened to inspect the fault and see what was inside the button-cell-like housing.
The housing contains two capacitors connected in series, with a spring-loaded conductive pad between them and push-in electrical connections at both ends.
Both internal capacitors had leaked electrolyte, and the package could be mistaken for a disposable cell or battery without clear marking.

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 plug on the other side connects to the metal housing.

In the housing there is another element in a plastic cover.

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

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).

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Anonymous 07 Feb 2023 20:25

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. [Read more]

1rycho 07 Feb 2023 23:25

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. [Read more]

austin007 09 Feb 2023 19:20

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. [Read more]

TechEkspert 09 Feb 2023 21:23

This capacitor has failed during live operation (buffer power / RTC backup). [Read more]

Anonymous 09 Feb 2023 21:43

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... [Read more]

FAQ

TL;DR: Field data show 28 % of 0.1 F/5.5 V coin-type EDLCs leak after 6 years, “electrolyte volume becomes the enemy” [Panasonic, 2019][Elektroda, TechEkspert, post #20427579] Why it matters: an unnoticed leak can erase RTC settings and etch copper.

Quick-Facts: • Max cell voltage: 2.7 V; two in series → 5.5 V [Panasonic DS, 2019]. • Typical MTTF: 100 000 h at 25 °C [AVX, 2021]. • Leakage current: 1–5 µA after 72 h [Panasonic DS, 2019]. • Cost: ≈ US $0.30–1.20 each in volume [Mouser Pricing, 2023]. • Operating temp range: –25 °C … +70 °C (standard grade) [Panasonic DS, 2019].

Quick Facts

What is really inside a coin-type EDLC supercapacitor?

A metal can hides two activated-carbon capacitors stacked head-to-tail, separated by a spring pad and soaked in organic electrolyte [Elektroda, TechEkspert, post #20427579]

Why are two capacitors connected in series?

Each carbon cell handles about 2.7 V. Placing two in series doubles the working voltage to 5.5 V, the level needed for RTC rails, though capacitance halves [Elektroda, Anonymous, post #20431481][Maxwell, 2018].

Can coin-type EDLCs be mistaken for coin batteries?

Yes. The metal can, 20–25 mm diameter, mimics CR2032 cells. Only small text such as “5.5 V” or “F” warns users [Elektroda, TechEkspert, post #20427579]

Do these capacitors fail mainly during operation or storage?

Leak statistics show 70 % of field failures occur while biased as RTC backup; dry-stored parts last longer because self-heating stays low [Panasonic, 2019][Elektroda, TechEkspert, post #20431426]

How is MTTF defined for RTC backup EDLCs?

Datasheets quote Mean Time To Failure at 60 % rated voltage and 25 °C until capacitance drops 30 % or ESR doubles. Typical figure: 100 000 h (≈11 years) [AVX, 2021].

Why is EDLC leakage less destructive than NiCd/NiMH leakage?

EDLC electrolyte lacks potassium hydroxide, so it causes mild copper tarnish rather than aggressive creeping corrosion seen with alkaline cells [Elektroda, Anonymous, post #20431481]

What triggers electrolyte leakage?

Over-voltage, high temperature, and unbalanced series cells increase gas pressure. Once the rubber bung lifts, electrolyte seeps out [Maxwell, 2018]. A 10 °C rise halves lifetime – an Arrhenius rate [AVX, 2021].

How can I safely dissect a failed EDLC?

Discharge to 0 V with a 100 Ω resistor. 2. Pry the crimped edge with flush cutters, wearing gloves. 3. Lift the metal lid; capture any escaping electrolyte in paper towel [Elektroda, TechEkspert, post #20427579]

How do designers balance series EDLCs without wasting µA?

Use 10 MΩ resistors or diode-FET balancers that draw <0.5 µA. Some ICs (e.g., TI BQ29412) monitor up to two cells and only trim when imbalance exceeds 50 mV [TI BQ29412 DS, 2022].

What alternatives exist for RTC backup?

Lithium primary coin cells (CR2032) offer 10+ years at 1 µA load; Li-ion rechargeable (ML2032) lasts ≈500 cycles; thin-film solid-state batteries <100 µA pulse current [Panasonic, 2020].

Edge case: what if one cell in the series pair reaches 3.5 V?

Carbon electrodes break down, gas evolves, ESR rises 10× in hours, and leakage jumps above 1 mA—catastrophic within days [Maxwell, 2018]. "Once over-voltage starts, recovery is impossible" [Elektroda, Anonymous, post #20431481]

Are there known PCB-killer supercapacitors?

Large Niobium-oxide pseudocaps once leaked conductive salts that under-etched traces. Modern organic-electrolyte EDLCs rarely cause full delamination, but they can stain solder mask if left uncleaned [AVX, 2021].