u/DS1984

I'm a new first time homeowner dealing with a lawn for the first time in my life. My brother gave me his Dewalt 2 x 20V self drive lawn mower. I bought two batteries (one came with a string trimmer I also bought). they are 20V XR 5AH and the other one is 20V XR power stack 5AH. I used the mower for the first time today and it seems the batteries died after like 20 minutes or so. I charged up the batteries and the same thing happened later in the day. I had let my lawn get pretty bad so maybe the mower is working too hard? Anyway, just checking to make sure I'm using the appropriate batteries.

thanks for any help.