Autodrome is correct. If you are charging by micro USB then no risk of ruining the Spark battery. However I think the 45W might be referring to the output of the Power Delivery feature of the USB-C port of your power bank, which I don't think you will connecting to. Additionally the Spark would only charge at a slow rate from a standard USB port unless it supports Quick Charge 2.0 or 3.0.
For instance I do have an 18W power bank which outputs 18watts out the USB-C PD port. However it also has a USB QC 3.0 port that I plug my Spark into at it charges as fast as as plugged into the wall. The power bank has a slower regular USB 2.4amp plug as well which will charge the spark slowly.