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

"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?

It seems that lead acid batteries can dissipate too much current as
heat. In other words I could probably feed it one amp continuously.
But would 100 mA at 12V do the job? I'd hate to lose a 1-Amp wall wart.

Thanks for your help.