2005 Ford Taurus Starting Issues

2005 Ford Taurus 4 cyl 75000 miles

Hello everyone. I’m new here.
I’ve had a couple of issues with my Taurus starting. About a month ago, I went to start my car and it wouldn’t start. The fans, radio, and lights came on. I was able to get it jumped. So, I went and got a new battery. I’ve had the car for 4 years, so I figured it was time.
Two weeks to the day, it did the same thing. I was in Wal-Mart for 20 mins, came back out and it wouldn’t start. I was able to get it jumped. Both times, we couldn’t get it jumped right away. I would try “one more time" and it would start.
I’m trying to figure out what things were the same for both two days. I know that I had run the air conditioner both days. Also, I had a cell charger plugged in. But, I’ve had the charger plugged in since Feb. Also, the day that it happened, when I would start it, it would kind of be rough starting.
I had my new battery checked twice, my alternator, and my starter all checked out good. Any ideas on what it could be? My brother-in-law thinks maybe a solenoid is going out. Or perhaps my starting is going out.
If you have any ideas, are these ideas easy to fix? As in having an exp friend change it?

Thanks for your help!