Question about Battery 2004

Hi I am new here. I am driving a 2004 prius with 200,500 miles. I recently moved to Denver and have had some issues with my car (never had any before). I was in the mountains one day, and we decided that I must not have turned my car off all the way but it was not staying in drive and I realized the battery was down to 1/2 lines in the light purple. I did drive it and the battery eventually charged again and it was fine. Fast forward about three weeks and I was driving on the highway yesterday about fifteen minutes into my drive and noticed the car was driving funny and the battery was at 2/3 lines. I am not sure if it was down the entire time I was driving I didn't look until that point. I'm also not sure if it was driving weird from the get go and it took awhile for me to really notice or of it just started to act weird during my drive. This morning I drove it and the battery was over half charged, it was getting lower than makes me comfortable but the car seemed to be doing fine. I just now made a three mile (ish) trip and the battery went from over half charged down to two charge lines in the light purple and didn't seem to be charging itself back up. I was nervous to turn the car back off with the battery that low but it didn't seem to be charging the battery back up in park so I did turn it off. Do you think my battery is going bad? Is there a way I can be driving it differently that I should be? I am in mountains a lot more than I was before these issues happened and I drive a lot around 40 mph in traffic and think I am using the battery a lot more/in different ways. Could that be affecting it? I'm trying not to get paranoid about it but am not sure what to do right now. I'm hoping someone has had a similar situation or has advice for how I can keep this from happening.

I do usually just hit the off button instead of putting the car into park first. Could that be an issue?

Thanks

 

Also want to mention that I have not gotten any warning codes or anything of the sort.