Gel
Cells are more sensitive to over voltage than flooded or AGM
batteries, but at 1.5 amps, even they won't be harmed by 15 or 16
volts. This thing is simply not powerful enough to harm a car sized
battery.

Depends what you mean by HARM. Yes it won't explode or catch fire or
damage it in the short term.

But if you continue to charge a fully charged battery so that the
terminal voltage goes much above 14V, it will reduce the long term
life of the battery. By long term I mean the battery may need to be
replaced after 2 years instead of after 7 years.

Mark