How to use engine mode? Hybrid wont charge battery keeps going to 1 bar occasionally wont start.

My mom has one and it wont start it only has one bar. For some reason it will let the main battery go all the way down to 1 bar and then wont start or go or anything. Idk why it doesn't use the engine to charge it up when it gets low like this.

We know the battery is old but toyota dealership last told us the main battery looks fine. But when he was looking at it, we drove from the high way and it was like 50 to 80% full. This was a month or so ago. It was brought there because it had to be towed, but then started working so we drove it up there but they looked at it and said it was fine.

Anyways it wont start right now. As mentioned, it's done this once before. Then started working like it was totally fine. Not sure why this happens. When possible I will try to have a youtube video made. I'm not there, she just told me about this and so I made an account here to try to figure out what is wrong with it. I think it's a 2008.