When a bid sheet lists the same capacitance and voltage, the polymer capacitor may be priced well above the wet electrolytic. That difference is not an arbitrary markup. The two designs handle ripple current, heat, and time in stock in different ways, so a price-per-microfarad comparison will mislead you. Compare them on ripple current per part, lifetime at your operating temperature, and total landed cost including storage and inspection.
What Actually Drives the Price Gap
Wet aluminum electrolytic capacitors have a mature manufacturing base and a lower cost per capacitance-voltage unit. A conductive polymer capacitor uses a more expensive cathode material and a different process, and its voltage range is generally narrower (many polymer series are rated below about 100 V; wet electrolytic types extend to 400–500 V). You are not buying the same part at a different price; you are buying a different set of electrical limits.
Key differences to calculate:
- Ripple current handling. A polymer part typically shows ESR in the tens of milliohms at 100 kHz; a wet electrolytic of similar size and voltage may specify two to five times higher ESR depending on the series. One polymer part often replaces two or three wet electrolytic parts in parallel at high switching frequencies. Compare price per rated ripple ampere, not price per microfarad.
- Lifetime model. Wet electrolytic wear-out is driven by electrolyte evaporation, and life roughly doubles for every 10 °C reduction in core temperature. Polymer capacitors do not dry out, but their life is still a function of applied voltage, ripple current, and ambient temperature, with end-of-life typically defined by a capacitance drop and ESR increase. Both technologies publish lifetime hours at a stated temperature and ripple; use the declared lifetime, not the chemistry, in your bid comparison.
- Shelf life and reforming. A wet electrolytic that sits in inventory for more than about two years should be reformed by applying rated voltage through a current-limiting resistor until leakage current stabilizes. Polymer parts have a longer shelf life, but SMD polymer parts carry a moisture sensitivity level (MSL) and may require baking per J-STD-033 if the moisture indicator card shows high humidity.
- Logistics and storage. The landed cost includes freight, transit time, storage conditions, and the labor for reforming or baking. A lower unit price loses its advantage if a wet-electrolytic batch requires weeks of reforming before release to production.
Putting Competing Bids on the Same Footing
Ask every capacitor supplier for the same rating block, stated at the same temperature and measurement frequency. Many wet electrolytic specifications are given at 120 Hz; polymer specifications are usually given with ESR at 100 kHz. Compare values at the frequency your converter actually switches.
| Parameter to demand in the quote | Liquid electrolytic | Polymer |
|---|---|---|
| Capacitance and tolerance | Measured at 120 Hz, typically ±20 % | Measured at 100 kHz or per series, typically ±20 % |
| ESR / dissipation factor | DF maximum at 120 Hz, 20 °C | ESR maximum at 100 kHz, 20 °C |
| Ripple current rating | Rated RMS at specified frequency and temperature, with derating curve | Same format; typically higher RMS per case size |
| Lifetime | Hours at rated voltage and ripple, e.g., 2,000 to 15,000 h at +105 °C | Hours at rated voltage and ripple, often 2,000 to 20,000 h depending on series |
| Leakage current | Formula-based limit, e.g., k × C × V after 2 min | Confirm the specified limit; the k value differs between series |
| Voltage derating guidance | Typical recommendation 80 % of rated voltage | Typical recommendation 80 % of rated voltage |
| Shelf life / storage | Reform if stored more than about 2 years | Check MSL label and moisture barrier bag |
For a fair unit-price comparison, build a simple figure of merit: rated ripple current at your ambient temperature divided by unit price, then again per expected delivered lifetime hours. If the wet electrolytic must be oversized or placed in parallel to meet the ripple current, the polymer option may come out lower on system cost even though its piece price is higher.
Incoming Inspection Checklist
Set acceptance limits before the lot arrives, and put them in the purchase specification. A quick ESR and capacitance check at incoming quality control catches most chemistry or counterfeit issues:
- Capacitance: measure at the datasheet frequency and temperature. Reject the lot if more than 10 % of the AQL sample is outside the stated tolerance range.
- ESR / DF: any sample part above the datasheet maximum ESR (polymer) or DF (wet electrolytic) fails the lot. Because polymer ESR is so low, use a four-wire Kelvin fixture; even modest test-lead resistance corrupts the reading.
- Leakage current: apply rated voltage through a current-limiting resistor for two minutes at room temperature. A part that exceeds twice the datasheet leakage limit is suspect; for wet electrolytic, leakage that continues to climb during steady-state soak indicates a damaged oxide layer.
- Physical checks: polarity marking, date code, lot traceability, and for wet electrolytic no bulging or vent leakage; for SMD polymer, intact moisture barrier bag and MSL label.
Negotiation and Stocking Decisions
For a production requirement, ask for a blanket order with agreed release schedules rather than one-off spot prices. This makes MOQ flexibility easier to grant: SMD polymer parts often move in full reels, while through-hole wet electrolytics can frequently be split lower. State the annual volume and two delivery windows, then negotiate unit price against those windows, not against a single immediate release.
On payment terms, first orders typically run on letter of credit or cash against documents; once delivery and inspection records are stable, open account terms reduce per-order administrative cost. Do not pay early for a large wet-electrolytic buy unless you can consume it inside its shelf-life window; the reform labor will erase the discount.
Stocking policy should follow the application split. Keep wet electrolytics for high-voltage rails and bulk energy storage where voltage exceeds the polymer range. Use polymer for high-frequency output stages where ESR and ripple current dominate. Hold buffer stock of both types, but rotate wet electrolytic inventory on a first-in, first-out basis, and verify date codes at receipt so the oldest parts go to the next production run.
Document these acceptance criteria in your component specification and review them whenever the supplier changes the series or the manufacturing site. The chemistry matters less than the declared ratings and the inspection discipline you apply at the receiving dock.
