Question regarding a charger modification for a Dell 3100 docking station
These Dell products for the (Latitude series laptops) use power adapters that may look like they use two connections but these connectors are actually using three connections. 

The third connection is a data connection that allows the laptop (and possibly the docking station as well) to recognize the power adapter and to know its power rating.

If you try to charge a Dell Latitude laptop with a non-Dell power adapter the laptop will complain at startup that it doesn't recognize the adapter and that it will not charge its battery due to this.

TLDR: use a Dell adapter with this Dell docking station. As you have found, using a different adapter will give weird and unexpected results.

Note that even the Dell Latitude power adapters from the much older D-series laptops (Like Latitude D620, D830 etc) use the same plug and identification system so usually these older adapters work fine on the newer systems. These power adapters (for Dell Latitude) are very common and easy to find/buy in used condition.