[ecomodded]

With my tdi's high idle start up it was due to a near unnoticeable tarnish over the battery terminals.
The alternator belt would squeal after starting as the alternator sucked the life out of the motor to charge the battery.
Cleaned the terminals put a new battery in and the alternator belt has not squealed since.
A fully charged battery in my car makes it run like a top, a weak battery makes it take off like a slug, car is really voltage sensitive or the alternator is sucking the life out of the motor.

* But *

You may have a bad battery and something else wrong, the air conditioning increases the idle when activated for instance, or the voltage regulator is causing issues, You should be able to determine the problem with a multimeter. Since it hasn't thrown a code, it may be charge related.