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Battery Desulfator Errata
In my battery desulfator article on page 84 of HP77, the value for C2 should have been 0.0022 ìF, not 0.022 ìF. My mistake.
I have put up a Web page that will give more details to help you build and use the desulfator circuit. I will place updates there, and will add a guestbook soon to allow comments and questions to be posted. I encourage a group effort in this, since I don't have all the answers. Thanks.

Alastair Couper
84 Home Power #77 • June / July 2000
Alastair Couper ©2000 Alastair Couper
It was twenty years ago that I left my
on-grid home, and my job as an
electronics engineer, to begin life on
an alternative energy oriented organic
farm. In the intervening years, I have
installed, maintained, and experimented
with numerous RE systems in my area.
What I have come to understand from
this experience is that off-grid life tends
to become very much focused on the
battery bank and its fate.
All power sources and loads breathe through this
crucial pathway. Batteries are heavy, toxic, inefficient,
and—to the amazement of many—electrically very
fragile. Weak or failing batteries are a very likely cause
of breakdown, especially in smaller solar-electric
Most newcomers to renewable energy are quite familiar
with using water tanks or gas tanks, and naturally use
this familiarity in trying to understand their battery
banks. Everyone knows that a bigger water tank is
better than a small one. Unfortunately, batteries are not
like tanks, and the result is trouble.
It is definitely not true that a big battery bank is
necessarily better than a small one. An oversized
battery bank can be almost impossible to charge
properly. Without a minimum daily exercise regimen, it
can become the equivalent of a couch potato. The main
culprit is sulfation, which is a gradual crystallization of
the battery's plate material, rendering it electrically
Some Theory
Past issues of Home Power (see Access) have gone
into the details of keeping lead-acid batteries healthy,
so I will only touch on the main points here. The usual
practice in maintaining a battery in good condition is to
apply a periodic equalization charge over and above
what would be a normal full charge. Unfortunately, this
is an energy-wasting tactic. It ultimately results in clean
battery plates, but at a steep price, especially if the
energy must come from a generator.
I initially went to the Internet to find any available
information on the problem of sulfation. The search
engines turned up several commercial sites that give
useful details on the fine points of battery charging and
equalization. A second resource is the IBM patent
server ( I found relevant patents
there, using keywords like "desulfate" and "rejuvenate."
What this wealth of data shows is that there are
numerous strategies for charging and electrically
desulfating batteries. Most of them were designed or
developed in the last twenty years or so. Considering
that lead-acid batteries have been around for more than
a century, this is a relatively new innovation. Virtually all of the devices and patents I found have in common the
use of some form of pulsing charge current. This is in
contrast to the constant or slowly varying currents
generated by sources like solar-electric panels.
I distilled and simplified these various techniques, and
came up with a basic circuit that will keep small to
medium sized batteries in desulfated condition. It can
even be used to bring old, sulfated units back into
service. Use of the circuit has dramatically reduced the
need for equalization charges in my own home system.
Resonant Frequency
The technique used in this circuit relies on a little known
aspect of lead-acid batteries. They possess what is
called a "resonant frequency," at a surprisingly high...
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