A new battery can still go bad in very short time if there is something wrong with the charging system. Or at least, the battery runs very low on charge.
Check the battery voltage with the ignition off and on, and maybe even when a helper cranks the motor.
A new fully charged battery should make 13V with no draw on it (ignition off), 12V, perhaps a touch less with the ignition on and engine not running, and it might drop down to 10V while cranking the starter, once the motor is running, the alternator has to make more voltage than the battery for the battery to charge, and the charging voltage will also change with the temperature of the battery, at least 13.5V and going higher to 14.75V the colder the battery. If you're getting much lower voltages something is wrong with the battery, either a malfunction in the battery or the battery is low on charge because of something wrong with the vehicle.
The current needed to light a few small lights compared to the current needed to crank and start the motor is hugely different. Do NOT assume because the lights come on, that the battery is NOT the problem, a bad battery can light the lights but just fall flat on its face when you try to start.
Some of the earlier Commanders did have a problem with corrosion on the Starter Motor Cables, that would short the battery current to the starter motor and thus the starter would NOT crank. Removing and cleaning them, or replacing the cables solved the situation.
As always, the first trouble shooting step is to look for stored fault codes. Search for how to do the key dance, on this site or youtube, to see it explained, you can have the DTC's read out in the EVIC.
Last edited by Mongo; 05-02-2016 at 11:26 AM.