We are mowing two yards every week or so.  My son and I have been mowing about every five to six days.  The first two times we used up both of the 6AH batteries and part of a 5AH battery.  On the third time (#5 and #6) my son ran the mower and he used all of that third 5AH battery.  Last week we shared mowing and we used both of the 6AH batteries and both a 4AH and 5AM batteries.  The grass is not any longer, it may be a bit thicker but I feel the problem is the blade sharpness.  

Before we mow again this week I am going to sharpen the blades to see if I am right!

NOTE:  The mower is still cutting very evenly!

NOTE: With my Honda gas mower I had to do the same thing, sharpen mutiple times over the season.  The Honda did not cut well at all if it was dull.