In article ,
Bruce W...1 wrote:

This is not ham radio related but I know you guys have the answer.

I need to store a car unattended for a winter in the midwest. And I
want to put a tricke charge on the battery.

I'd rather not buy a fancy trickle charger because it would probably get
stolen. On the other hand I have a vast array of wall wart
transformers.

If I connected one of these transformers how many milliamps should it
put out at 12V? Or rather what's the least current that would do the
job?

I would guess that 100 milliamperes would be more than sufficient to
overcome any self-discharge of the battery, as long as the vehicle
doesn't have any electronics onboard which are drawing current.

It seems that lead acid batteries can dissipate too much current as
heat. In other words I could probably feed it one amp continuously.

You really don't want to do that.

The reason is this: in order for a car battery to be drawing 1 amp of
charge current on a continuous basis, you're going to have to boost
the voltage up fairly high. You'd probably exceed the electrolysis
voltage, and much of the current would end up breaking down the water
in the electrolyte into oxygen and hydrogen. If your battery's design
and chemistry don't allow the gasses to be recombined into water fast
enough, you'd be at risk of "boiling" the battery dry.

But would 100 mA at 12V do the job? I'd hate to lose a 1-Amp wall wart.

Lead-acid batteries are happiest if you feed them a well-regulated
charge/float voltage, with suitable current limiting. Using an
unregulated or weakly-regulated 12-volt wall-wart is probably not a
good idea - these wall warts tend to deliver a rather high voltage
(often 16-18 volts) under conditions of little or no load. Depending
on the specific wall-wart you choose, and the condition and type of
your battery, you might end up electrolyzing away the water faster
than the battery can recombine the hydrogen and oxygen. Losing a
wall-wart would be annoying; losing the battery would be worse ;-)

The best thing to do is get (or build) yourself some sort of
well-regulated trickle charger. 100-200 mA is probably more than
plenty for this application, as long as it's provided at a
well-regulated voltage. Most battery companies seem to recommend
between 13.6 and 13.8 volts for a "float charging" application - at
this voltage, the battery will self-regulate the amount of current it
takes and will not overcharge or electrolyze itself. One
knowledgeable amateur I know, recommends sticking to a lower 13.5
volts to ensure safe float-charging under a wide range of charge
conditions and temperatures.

About a year ago I put together a simple float charger to keep the
65-amp-hour glassmat battery in our city's RACES ham-shack properly
charged. It's a simple design, based on the jellybean LM317
three-terminal regulator IC and on the schematics in National
Semiconductor's data sheet for this IC. My version includes reverse
polarity and short-circuit protection, a charge float voltage which is
adjustable over a span of a volt or so, and built-in current limiting
to protect the wall wart (a 200 mA 16-volt unit, if I recall
correctly). It's not a bad one-afternoon project and the components
are the sort of thing most homebrewers are likely to have in their
junk-box.

Schematic is at http://www.radagast.org/~dplatt/hamradio/charger.pdf

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
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