Last Updated: February 2026 | Reading Time: 14 minutes
You've found the exact replacement capacitor for your vintage amplifier, industrial drive, or UPS system. It's new-old-stock (NOS) — genuine, never used, still in the original packaging. There's just one problem: the date code says 2016. That capacitor has been sitting on a shelf for a decade without power.
Can you install it directly? For most electrolytic capacitors stored more than 2-3 years, the answer is no — not without reforming first.
Reforming is the process of gradually reapplying voltage to rebuild the aluminum oxide dielectric layer that partially dissolves during unpowered storage. Skip this step, and you risk immediate high leakage current, overheating, venting, or catastrophic failure when you apply full voltage.
This guide explains why reforming is necessary, how to do it safely, and when you can skip it.
Aluminum electrolytic capacitors store energy using an extremely thin layer of aluminum oxide (Al₂O₃) formed on the surface of an etched aluminum foil. This oxide layer is the dielectric — the insulating barrier between the two conductive elements.
The oxide layer is formed and maintained by a process called anodization: applying voltage causes the electrolyte to react with the aluminum surface, growing the oxide film. Here's the key insight: the oxide layer requires periodic voltage application to remain intact.
When an electrolytic capacitor sits unpowered:
| Time Without Power | What Happens |
|---|
| 0-6 months | Minimal degradation, no reforming needed |
| 6-12 months | Slight oxide thinning begins, usually safe to use directly |
| 1-2 years | Measurable oxide degradation, reforming recommended for critical applications |
| 2-5 years | Significant oxide thinning, reforming required |
| 5-10 years | Substantial oxide degradation, must reform before use |
| 10+ years | Severe degradation possible, reform carefully and test thoroughly |
The oxide layer slowly dissolves into the electrolyte through a chemical process. Thinner oxide means:
- Higher leakage current when voltage is applied
- Reduced dielectric strength (lower effective voltage rating)
- Risk of dielectric breakdown if full voltage is applied suddenly
Reforming reverses the degradation by gradually rebuilding the oxide layer:
- Low voltage is applied through a current-limiting resistor
- Current flows through the electrolyte to the aluminum surface
- The electrochemical reaction reforms (regrows) the aluminum oxide
- As the oxide thickens, leakage current decreases
- Voltage is increased in steps, each time allowing current to stabilize
- At full rated voltage, leakage current should meet the original specification
The process is essentially the same anodization reaction that created the oxide during manufacturing — just done slowly and carefully to avoid damaging the partially degraded capacitor.
- Electrolytic capacitor has been stored more than 2 years without power
- NOS (New Old Stock) parts of any significant age
- Capacitors removed from decommissioned equipment that was powered off for years
- Any electrolytic capacitor where storage conditions are unknown
- Mission-critical applications where failure is unacceptable (UPS, medical, aerospace)
- Capacitor is newly manufactured (within 1-2 years)
- Capacitor was in a powered circuit until recently
- Film, ceramic, or mica capacitors — these don't have electrolyte and don't need reforming
- Solid polymer electrolytic capacitors — these use a solid cathode material that doesn't degrade the same way
- Storage was less than 6 months in reasonable conditions
| Condition | Impact on Shelf Life |
|---|
| Cool and dry (20-25°C, <60% RH) | Best case — extends time before reforming needed |
| Room temperature (25-30°C) | Normal degradation rate |
| Warm (30-40°C) | Accelerated degradation, reform sooner |
| Hot (40°C+) | Rapid degradation, may not be recoverable |
| Sealed original packaging | Better than open storage |
| High humidity | Can cause external corrosion of terminals |
| Item | Specification | Purpose |
|---|
| DC power supply | Variable voltage, 0 to rated voltage of capacitor | Provides controlled voltage |
| Current-limiting resistor | See sizing table below | Limits inrush current to safe level |
| Digital multimeter | DC voltage and DC current measurement | Monitors voltage and leakage |
| Timer or clock | Minutes/hours | Tracks hold time at each step |
| Notebook | — | Records voltage, current, and time at each step |
| Item | Purpose |
|---|
| ESR meter | Before/after ESR comparison |
| Capacitance meter | Verify capacitance after reforming |
| Temperature probe or IR thermometer | Monitor capacitor temperature during reforming |
| Insulated clip leads | Hands-free connections |
| Safety glasses | Protection in case of failure |
| Ventilated workspace | Dissipates any gas venting |
The resistor limits the maximum current that can flow through the capacitor during reforming. Size it based on the capacitor's voltage rating:
| Capacitor Voltage | Resistor Value | Resistor Power Rating |
|---|
| Up to 25V | 100Ω - 470Ω | 5W |
| 25V - 100V | 470Ω - 1KΩ | 10W |
| 100V - 250V | 1KΩ - 4.7KΩ | 10W |
| 250V - 450V | 4.7KΩ - 22KΩ | 10-25W |
| 450V - 500V | 10KΩ - 33KΩ | 25W |
Important: The resistor must handle the power dissipation at full voltage. P = V²/R. For a 450V capacitor with a 22KΩ resistor: P = 450²/22000 = 9.2W. Use a 25W resistor minimum.
Before applying any voltage:
- Inspect the capacitor body — Look for bulging top vent, electrolyte leakage, or corrosion
- Check the terminals — Should be clean and undamaged
- Verify the rubber seal — Should be intact, not cracked or dried out
- Read the date code — Determines how aggressive the reforming needs to be
- Note the rated specifications — Voltage, capacitance, temperature
Stop if: You see visible bulging, leaking, or cracked seals. These capacitors are likely failed and reforming will not help.
Using your multimeter and ESR meter (if available):
- Measure capacitance — Should be within ±20% of rated value. If significantly low, the capacitor may be too far degraded to reform.
- Measure ESR — Note the value for comparison after reforming. Elevated ESR is expected before reforming.
- Check for shorts — Set multimeter to resistance mode. An electrolytic capacitor should show initial low resistance that gradually increases as it charges from the meter's test voltage. Steady zero ohms = shorted, discard.
DC Power Supply (+) ───── Resistor ───── Capacitor (+)
│
DC Power Supply (-) ──────────────────── Capacitor (-)
│
Ammeter ─────────────┘
Connect the multimeter in series to measure leakage current (set to mA or µA range), or connect it across the capacitor to measure voltage.
Double-check polarity. Connecting an electrolytic capacitor in reverse will cause damage regardless of reforming.
Follow this voltage schedule. At each step, hold until leakage current stabilizes (stops decreasing), then move to the next step.
| Step | Voltage (% of rated) | Typical Hold Time | What to Watch |
|---|
| 1 | 10% | 15-30 minutes | Initial leakage may be high — this is normal |
| 2 | 25% | 15-30 minutes | Leakage should be decreasing |
| 3 | 50% | 30-60 minutes | Leakage should be significantly lower than Step 1 |
| 4 | 75% | 30-60 minutes | Leakage should be approaching specification |
| 5 | 90% | 30-60 minutes | Leakage should be near or at specification |
| 6 | 100% | 60+ minutes | Leakage must meet rated specification |
Example for a 450V capacitor:
| Step | Voltage | Hold Until Leakage Stabilizes |
|---|
| 1 | 45V | 30 minutes |
| 2 | 112V | 30 minutes |
| 3 | 225V | 45 minutes |
| 4 | 337V | 45 minutes |
| 5 | 405V | 60 minutes |
| 6 | 450V | 60 minutes |
At each voltage step, record:
- Time: When you set the voltage
- Voltage across capacitor: Should gradually rise to match the supply (minus resistor drop)
- Leakage current: Should decrease over time at each step
- Temperature: Capacitor body should remain near ambient. Warm is OK, hot is a problem.
| Observation | Action |
|---|
| Leakage current doesn't decrease after 30+ minutes | Capacitor may be too degraded. Try holding longer. If still no decrease after 2 hours, discard. |
| Leakage current increases over time | Stop immediately. Dielectric is failing. Discard the capacitor. |
| Capacitor gets hot (>50°C above ambient) | Reduce voltage immediately. Allow to cool. May indicate internal damage. |
| Hissing, bubbling, or venting | Disconnect immediately. Capacitor is failing. Do not continue. Allow to cool in ventilated area. |
| Bulging appears during reforming | Stop immediately. Capacitor is failing internally. Discard safely. |
After holding at full rated voltage for 60+ minutes with stable, low leakage:
- Measure leakage current — Must meet the capacitor's rated specification
- Disconnect power supply — Allow capacitor to remain charged for 10 minutes
- Measure voltage retention — Should retain most of its charge (some drop is normal from leakage)
- Discharge through resistor — Safely discharge before handling
- Measure capacitance — Should be within ±20% of rated value
- Measure ESR — Should be lower than pre-reforming measurement
Most aluminum electrolytic capacitors specify maximum leakage as:
I (leakage) = 0.01 × C × V (result in µA)
Where C is in µF and V is rated voltage.
Example: 6800 µF / 450V capacitor:
- I = 0.01 × 6800 × 450 = 30,600 µA = 30.6 mA
After reforming, measured leakage at rated voltage should be at or below this value. Many well-reformed capacitors will measure 10-50% of the maximum specification.
When reforming a batch of capacitors (common when rebuilding a VFD or UPS capacitor bank):
Connect capacitors in series to reform multiple units at once. Each capacitor sees a fraction of the total voltage.
Advantages: Fewer power supply voltage changes needed
Disadvantages: Voltage distribution may be uneven; weaker capacitors may see more voltage
Only use this method for capacitors of the same value and rating.
Reform all capacitors simultaneously by connecting them in parallel, each through its own current-limiting resistor.
Advantages: Each capacitor is treated identically; can identify weak units easily
Disadvantages: Requires more resistors and a higher-current power supply
Reform each capacitor one at a time.
Advantages: Best control, easy to identify problem units
Disadvantages: Time-consuming for large batches
Capacitors from the 1990s or earlier may have:
- Dried electrolyte (reduced capacitance and high ESR permanently)
- Corroded terminals
- Degraded rubber seals
- Electrolyte chemistry that no longer supports full oxide formation
Approach: Reform very slowly (add intermediate voltage steps at 5-10% increments). If leakage doesn't decrease to specification, the capacitor may have permanent damage that reforming cannot fix. ESR testing after reforming is essential.
Extra caution is required:
- Lethal voltages — A 450V charged capacitor can kill
- Higher stored energy — E = ½CV². A 6800µF/450V capacitor stores 689 joules
- Always use safety glasses and insulated tools
- Never leave a charged capacitor unattended
- Discharge through an appropriate resistor — Not a screwdriver
When you receive NOS capacitors and can't verify storage conditions:
- Assume worst case — reform as if stored for the full time since the date code
- Test thoroughly after reforming
- Consider reforming as a pass/fail test — if a capacitor doesn't reform to specification, don't use it
Once a capacitor passes reforming:
- Install promptly — Ideally within weeks, not months
- The circuit will maintain the oxide — Normal operation keeps the oxide layer healthy
- Document the reforming — Record date, voltage/current readings, final test results
- Mark the capacitor — A small label indicating "Reformed [date]" helps with future maintenance
- If equipment will be powered off for extended periods — Plan to reform again before restart, or keep equipment energized periodically
Typically 3-6 hours for a single capacitor, depending on its voltage rating and how long it was stored. Capacitors stored 2-3 years may take 2-3 hours. Capacitors stored 10+ years may take 6+ hours or require multiple reforming sessions.
You can, but it's more difficult. Some equipment has a "soft start" feature that gradually increases DC bus voltage — this provides partial reforming. For equipment that applies full voltage immediately, you may need to use an external variac (variable transformer) to slowly increase the AC input voltage, which proportionally increases the internal DC bus voltage.
If leakage current remains high after extended reforming (4+ hours at rated voltage), the capacitor has permanent damage. The electrolyte may have dried out, or the aluminum foil may have corroded beyond recovery. Replace the capacitor.
No. Film capacitors (polypropylene, polyester, polycarbonate) don't use a liquid electrolyte or an oxide dielectric layer. They can be stored indefinitely without degradation. Ceramic, mica, and other non-electrolytic types also don't need reforming.
For a $2 capacitor, probably not — the time spent reforming exceeds the replacement cost. But for hard-to-find, obsolete, or expensive computer-grade capacitors where replacements cost $50-200+ each, reforming is absolutely worthwhile.
Yes, a variable DC bench power supply is ideal. Set the current limit to a low value (10-50mA for most capacitors) and slowly increase voltage. The current limit acts as the current-limiting resistor. This is the easiest method if your power supply has accurate current limiting.
- Reform any electrolytic capacitor stored 2+ years — The oxide dielectric degrades without applied voltage
- Use a current-limiting resistor — Never apply full voltage suddenly to an unreformed capacitor
- Increase voltage in steps — 10%, 25%, 50%, 75%, 90%, 100% with a hold at each level
- Monitor leakage current — It should decrease at each voltage step. Increasing current = bad
- Watch for heat and venting — Stop immediately if the capacitor gets hot or makes sounds
- Verify after reforming — Check capacitance, ESR, and leakage against specifications
- Film, ceramic, and mica capacitors don't need reforming — Only aluminum electrolytic types require this
Have NOS capacitors that need reforming before installation? We can advise on reforming procedures for specific capacitor types, or source newly manufactured replacements if reforming isn't practical. Contact us with the part number and date code, and we'll recommend the best approach.
