Not damage it, just shorten its lifespan. The longest lifetime is if you charge at the 40%-60% point. (Keeping it on charge constantly will give you about 75% of what you could get. The battery won't charge until it discharges a little, then the charger will kick in and keep it fully charged - not the best way to care for a Lithium Ion battery.)
Leave it in the van not plugged in until it gets to thr 4-%-60% point. If it won't last a full day, you should have thought about that before you bought the phone, so you could keep a spare battery and swap them during the day.