Hi! It's nothing serious, your new power passthrough device is sucking a few watts from your AC charger (even when nothing else is attached), normally up to 5W are lost, if you enter the BIOS menu you'll see exactly how many watts are supplied to your laptop. So it's charging your computer a little slower, and it may need to throttle performance in some cases (maybe when TurboBoost activates, but I doubt it would be a problem even in that case if your battery is charged, it would simply drain more power from your battery). Damaging the battery? I seriously doubt it, I mean perhaps you will be using your battery as buffer a little more often than the usual, but in a certain measure it would happen anyway, but I don't think that bunch of dissipated watts really make a difference. You can buy a 130W USB-C AC adapter from Dell, if you want. That way you'll have all the juice you need and more.
I'm having trouble with power pass through - I probably don't understand it properly. I have an Alogic USB-C Dock UNI which says it supports 4k, 100w power delivery etc.  It has a hdmi plus a few usb connectors plus a single usb C.   I'm not sure I used it correctly - I plugged the 45w dell psu into the hub and then the hub into the xps13's thunderbolt\usbc port.  On the only other usb c on the xps I connected my usb c portable asus monitor. Everything worked, and the battery did very slowly charge with everything on but I got a warning when the xps was powered up saying "You have plugged in a lower wattage power adaptor, usbc charging or power pass-through device.  Your system will continue to work but may not perform at its peak." I don't know why it complains about low power if the hub supports up to 100w pass through.   Am I damaging the battery? Do i need a higher powered psu than the 45W standard one?  
9310 vs ALogic 100W power pass-through hub and a portable usb C screen