Reforming capacitors (technical description, not politics)

Eric Smith spacewar at
Wed Jul 29 02:34:07 CDT 2015

Some people seem to think that "reforming" an aluminum electrolytic
capacitor is some kind of cheat, akin to zapping NiCd cells or
rejuvenating CRTs. Actually reforming is the same electrochemical
process that the manufacturer uses to "form" the capacitor in the
first place, building up the aluminum oxide layer, before the sheet is
rolled into cylindrical form. The manufacturer typically uses a
forming voltage higher than the rated voltage, from 135% to 200%, to
provide margin for shelf life.

When the capacitor goes unused for an extended time (shelf life), the
oxide layer gradually breaks down, increasing the capacitor's leakage
current and reducing the effective usable voltage of the capacitor,
which is proportional to the minimum oxide thickness. If the oxide has
developed spots that are too thin for the applied voltage, it may be
damaged ("punch-through") when that voltage is applied. Punch-through
tends to be a runaway process, so even a small amount of punch-through
usually completely ruins the capacitor. Reforming the capacitor by
applying current-limited power rebuilds the oxide layer to prevent
this type of damage, and to reduce the leakage current back to within
the specifications. The current limiting is what prevents the
reformation process from causing punch-through and damaging the
capacitor. Many of the capacitor vendors actually publish
recommendations for reforming their capacitors.

See for example information on manufacture on pages 13-14 and a brief
recommendation of reforming procedure on page of 17 of Kemet
publication F3304 dated June 2009:
Also pages 2-4 on manufacture and page 16 on "recondition" (reform) of
"CDE Aluminum Electrolytic Capacitor Application Guide":
Also pages 1-5 of Nichicon "General Description of Aluminum
Electrolytic Capacitors":

In at least some aluminum electrolytic capacitor manufacturing
processes, there is actually a reforming step done after assembly, in
addition to the initial forming.  See page 9 of the Panasonic
"Aluminum Electrolytic Capacitors Technical Guide", dated April 2013:

The reforming process WILL NOT fix other things that may go wrong with
the capacitor, such as failed seals allowing the electrolyte to dry
out, or corrosion, or punch through which can result if the oxide
layer is degraded and voltage is applied without current-limiting.

The US DoD published a technical handbook detailing their policies and
procedures for reforming aluminum electrolytic capacitors that sit in
inventory for years, MIL-HDBK-1131. As of 1999 this is "for guidance
only and should not be cited as a requirement, but the information in
the handbook may be useful in determining or evaluating requirements."
For non-mil-spec capacitors, it recommends inspection and possible
reformation every 3-6 years of shelf storage. It recommends disposal
after 12 years of shelf storage, but AFAICT they're just being
conservative, possibly due in part to not having enough practical
experience with reforming very old capacitors.

Shelf storage is of course equivalent to having the capacitor
in-circuit but unpowered. Having the capacitor powered in circuit for
any significant length of time will reform the oxide to some extent
based on the applied voltage, though not up to original factory spec.

When I reform capacitors myself, I use a reforming voltage of 135% of
the rated voltage. Since I use a suitably low current limit, this has
no significant probability of damaging the capacitor, but as with the
initial factory forming, provides some margin for further shelf life.
In my experience, aluminum electrolytic capacitors in equipment that
has been unpowered for 30 years or more almost always need
reformation, but they almost always meet factory specs (capacitance,
ESR, and leakage at rated voltage) after reformation.  Since I don't
tend to restore equipment newer than that, I don't have any empirical
data on how much shelf life they can have without needing reformation.

I'm not particularly advocating for or against reformation, as
compared to replacement. Anyone restoring equipment with electrolytic
capacitors is advised to to read the references and decide for


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