I know this might trivial but I need to know...

Is it illegal to keep the engine running in an unmanned parked car in
an Apartment complex here in Maryland? I kept the engine running in my
car for about 20 minutes in the parking lot just to charge the
batteries and the Management Staff in the complex threatened to call
the cops!

Is there a DMV rule that states so?

Hello.

I hate to be the bearer of bad news, but the management staff was
right. It is illegal to leave an unattended vehicle running in
Maryland.

See Maryland Transportation Article § 21-1101:

      "(a)      Except as provided in subsection (c) of this section,
a person driving or otherwise in charge of a motor vehicle may not
leave it unattended until the engine is stopped, the ignition locked,
the key removed, and the brake effectively set.

      (b)      A person driving or otherwise in charge of a motor
vehicle may not leave the motor vehicle unattended until, if the
vehicle is on a grade, the front wheels are turned to the curb or side
of the highway.

      (c)      When a cat or dog is left in the unattended vehicle of
an on-duty law enforcement officer or an animal control officer, the
provisions of subsection (a) of this section do not apply to the law
enforcement officer or the animal control officer."
Source: Maryland Web Statutes
http://mlis.state.md.us/cgi-win/web_statutes.exe


The same provision is quoted elsewhere on the web:

"You are reminded that the Maryland Annotated Code, Transportation
Article, Section 21-1101 (a) makes it a criminal offense for the
driver or person otherwise in charge of a vehicle to leave it
unattended '. . . until the engine is stopped, the ignition locked,
the key removed, and the brake effectively set.'"
Personal Safety Tips, hosted by Stattorney.org:
http://www.stattorney.org/vehicle.htm

"Under Maryland Vehicle Law TR-21-1101, 'a person driving or otherwise
in charge of a motor vehicle may not leave it unattended until the
engine is stopped, the ignition locked, the key removed, and the brake
effectively set.'  It is therefore against the law to leave a key in
the ignition of any motor vehicle . The term 'unattended motor
vehicle' has been held to mean 'without anyone present [in the
vehicle] who is competent to prevent any of the probable dangers to
the public.'"
Source: Charles County Neighborhood Crimewatch Newsletter, January
2002
http://ccso.us/Jan2002.pdf

The same statute is also quoted in two Maryland judicial opinions:

EDDIE MACKEY, JR., et al. v.  MICHAEL R. DORSEY, et al. (1994)
Hosted by findlaw.com:
http://caselaw.findlaw.com/data2/marylandstatecases/cosa/1995/969s94.pdf

FLORENCE MAY v. GIANT FOOD, INC. ET AL. (1997)
Hosted by findlaw.com:
http://caselaw.findlaw.com/data2/marylandstatecases/cosa/1998/1368s97.pdf

Note that certain documents above are in PDF format, so the Adobe
Acrobat Reader is required. If you don't have that, visit Adobe's web
site:
http://www.adobe.com/products/acrobat/readstep2.html    

search strategy: "motor vehicle", unattended, "engine running",
maryland

I hope this helps.

I know it sucks but Hey! you are just doing yr job. Thanx for the quick reply! 
Just curious, but does it apply for California too?

Hi primate...

Well here in brrrrrrrr Canada, thank gawd it is still legal!!!!!!!! I
run out every am 15 minutes before I have to leave for work to start
and warm my car....Never had any complaints here because everyone does
it...no other way except to freeze in car while engine warms up. One
thing though, we do have to put on parking lights so others are warned
that an idling car is nearby.

actually I wonder why u need to keep it running just to keep the
batteries charged? If there is a problem maybe u need new batteries.
or a new alternator.and for 20 Minutes at that !  even to warm up a
car in 20 below 0  I guess 15 minutes is enough . If ure looseing
charge on ur battery then u should change the battery . or change the
alternator. (or other wise have the car looked at by a mechanic)

and IF its an indoor parking lot (which i suspect urs is) I don't
think even in canada u can do that ... for indoor parking lots there
are those plugs which u plug in and it warms up the engine block.