I discharge mine to half capacity when storing it, because I only use it every 2 weeks or so. The guys I buy parts from have links to DJI's forum Guide and DJI's general guide on battery storage on their blog.
So once I read about it, I started discharging the Tello batteries to as close to 50%, the only way to discharge the Tello is by flying it though; my other drones have auto-discharge available in app.