Car battery trickle charger?
 

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.