Custom workout has little effect on body battery

I've been using my Venu 3 for cardio and cycling for about 6 months. The body battery feature is life changing. 

I finally figured out how to use it for weight training and I set up a rather intensive 30-minute kettlebell workout. 

This had a body battery impact of negative 4? 

I woke up today pleasantly sore from the workout this morning and had a body battery of 100 with no recovery hours. 

I created and completed an intensive kettlebell weight training workout with 16kg and 20kg kettlebells over 30 minutes.

Why did this have a negligible impact (-4?) on body battery?