Sometimes it is more expensive to fix the problem with the generator charging system than purchase a small 12v battery charger. I would purchase a 12v small battery charger (5A) that you can keep plugged into your house hold power. This will maintain the battery at full charge so it is ready to start when you need it.

First you should check your generator. why it won't start normally. Maybe the battery is used for a long time, maybe other reasons. Hope this article can help you. Why Diesel Generator Fail to Start
Maybe you are interested in this article. Diesel Generator Battery Maintenance