On my 98 Durango I put on a new idle air control and put in new plugs, the battery is new?

Yes the alternator could be responsible for the low rpm. If the regulator is faulty, It would report to the computer that everything is fine, meanwhile the engine is idling low because the voltage from the battery is weak. A weak battery will cause low rpms.

No, typically when the battery is low or the charging system is not enough the vehicles' computer will kick the rpms up instead of down in order the restore charging effficiency. The rpms can be adjusted by the shops who have the diagnostic scanners that allow for changes in the fuel/air mixtures and such. I have two vehicles, both Dodges that are now running low rpms at lights and stuff and so far I've pretty much just ignored it until later.

Low battery voltages or charging voltages can screw up micro-volt sensor readings which in turn can lead to low idle speeds/stalling...IAC/AIS is not sending the correct reading to the PCM. It's normal for the alt to put out lower power at low rpm's but the battery should have enough voltage if it's in good shape to run the electrical systems/computer readings until the car is driven or engine is run for a while and the alt recharges the battery and runs the electrical system. I'd say to load test the battery and also see if it's putting out enough volts...12.65 is fully charged...and test the charging system voltage outputs....should be 13.5-14.5. You might also clean the throttle body and check for any vacuum leaks...make sure the battery cable terminals are clean and tight. If possible, use a scan tool and see what outputs are for the IAC/AIS and if they fall within range for specs. Did you check for proper pintle range extension of the new IAC/AIS?

"When installing a new GM IAC or Chrysler AIS solenoid, the pintle must not extend more than a certain distance from the housing. The specs vary so check the manual or look up the specs in the OEM service literature. Chrysler says one inch (26 mm) is the limit, while some GM allows up to 28 mm on some units and 32 mm on others. If the pintle is overextended, it can be retracted by either pushing it in (GM) or by connecting it to its wiring harness and using actuator test 03 to move it in (Chrysler)."

http://www.aa1car.com/library/idle_speed_control_v...

http://www.justanswer.com/dodge/5znll-dodge-98-dur...

"make sure your battery cables are tight and your battery is fully charged.

Thing to do is just check the voltage at the main engine bay fuse box to make sure you're getting 12.6 volts. These computers don't like the slightest change in voltage at all.

If my battery died and I jump start the truck, I have to feather the throttle awhile until its charged back up and the idle motor "re-learns" how to idle.

And if I'm jumpstarting someone else...if I hold my RPMs steady at 2000rpm...once they hit the key my RPMs drop down some...very weird. But, a tech at a local Dodge dealer said thats normal...and a lot of idling/stalling problems are linked back to the battery cables, as these computers run on 12.6 volts...at 12.5 they start acting weird."

http://www.dakota-durango.com/forum/showthread.php...

"Tech Tips: A weak battery will cause the Powertrain Control Module (PCM) to lose track of the Automatic Idle Speed (AIS) steps. "

http://ramchargercentral.com/vehicle-help/durango-...

Battery tests....

http://carbatteryonline.net/car-battery-tester.htm...

You will have an idle control adjustment on the throttle body. Adjust in very small increments until the idle is in the specified range. This might fix all your problems. You should notice and increases in the voltage output.