Rick Frazier wrote:

Bruce:

As others have indicated, you really want to use a regulated charging
voltage to keep the battery up... The voltage is important, however, more
important to getting the design right is knowing how much of a drain the
automobile puts on the battery when "nothing" is going on... Many cars will
drain a good battery in about a month, just from the residual drain that is
the result of such things as clocks and memories in radios and such. Add
anything else and you've got a good chance of needing significantly more
charging current than you might at first think.

You mention that you are worried about using a fancy trickle charger because
it may get stolen, which implies that the car is outside in the elements,
and not in a protected or secured environment. I'd be willing to bet that
if you have to worry about someone stealing a trickle charger, you should
probably be concerned about just about everything else too, once someone
notices the car isn't moving and has a power cord going to it....

In addition, being concerned about a 1-amp wall wart, which are nearly a
dime a dozen, may be false economy, considering the value of the car and
items built or installed in it. Why wouldn't the tires and wheels
disappear, or the battery itself? Same for radio and/or other
accessories... or even perhaps the car itself.

If you do go and build something to keep the battery up, bear in mind that
you are probably going to need something with a voltage around 16 volts or
so, given that most regulators require a couple of volts of headroom over
the regulated voltage. This may take you into a range of supplies or wall
warts that is higher than you may have on hand anyway.

--Rick

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You guys are getting unnecessarily complex me thinks.

Yes a voltage regulator would be nice but not really essential. And
this might get stolen too. Besides, if I was going to go to that effort
I'd just buy a float charger (about $25).

The car is in a garage in a good neighborhood but the owner of the
garage is on and extended vacation. The theives I'm worried about are
neighborhood children. Actually it's the homeowner's car, I'm just
doing them a favor, and saving myself from having to go there to start
the car once a week. So I'd rather not spend money when I have a
plethora of wall warts.

The car does have an alarm but I don't know how much current it pulls.

Case, I've got one wall wart which has a 17V open voltage. When I pull
it down to 12.0 volts the current is 120 mA. Might this work?

I could increase the size of the transformer if needed. It's just that
I haven't yet figured out what's needed.

Lead-acid batteries have a rated current at which they can be float
charged without causing any damage. This is the number I need, but have
not been able to find it.

Here's a related article:
http://www.4unique.com/battery/battery_tutorial.htm