Charge rate slows the closer you get to a full charge. That is by design to prevent an overcharge. The end result is you will get little benefit if you charge for several hours if the battery is near full after being on charge for a while.
I don't worry about keeping a "full" charge. Instead, I run the generator a short time to make sure I don't go to near empty at night. BTW, 87% was already close enough to full to need only a few minutes or at most an hour. That being said, I just use the 4 led lights, so I don't have a percentage to compare to yours.
