2009 Prius 5D red triangle and check engine light on 100 miles after 12v battery change.

Hi folks. First post here!
So my significant other and I just bought. 2009 5D that was sitting for 18 months. We changed the 12v battery and things were great on the ride home until we stopped for lunch after 100 miles of driving. As soon as we hit the road again the red triangle and alarm along with all the other lights popped on. We pulled over to evaluate what we could. After restarting the car the red triangle and the check engine light came on and have remained on the rest of the way home. A few restarts after there is no change. I have been told the red triangle is problematic of the engine battery going bad. Perhaps some corrosion from sitting in a garage for 18 months? Or is the check engine light in combo w the red triangle mean something else? Any insights before I get suckered into a $ trap??? Many humble thanks for any insights and advice!