looses charge when sitting for a week LA100

My LA 100 riding tractor looses is charge when sitting for a week. I bought a new battery, and it does the same thing. Now when I jump it, which I have been doing for three seasons now,LOL. It runs fine, and it is charging because when I shut it off after using it, the lights now work and makes an attempt to start. When sitting for a week or two, it will need to be jumped again. I have been a member here for three years , and have not got an answer yet. Some one said that it might be a diode in the charging system? I wish someone had the same problem so I can finally get a fix on this. Thank you all.