The Shimano BT-EN805, BT-EN805-L, and BT-EN806 mainly differ in capacity and intended use.
The BT-EN805 and BT-EN805-L both offer 504 Wh, with the BT-EN805-L being optimized for mountain biking with better weight distribution. The BT-EN806, with its 630 Wh capacity, is designed for longer rides but comes with added weight.
All three are compatible with the same Shimano STEPS systems (EP8, EP6), making upgrades or swaps straightforward.
In short, if range is your priority, go for the BT-EN806. For more balanced or MTB-specific needs, the BT-EN805 models are more suitable.
Buying a second battery as a backup might seem smart, but keep in mind that even if it’s not used, it will still degrade over time. Lithium-ion batteries naturally lose capacity due to chemical aging, regardless of whether they’re actively in use. So, a stored spare battery won’t hold its original capacity forever, meaning you could still see diminished performance when you finally start using it.
I hope that helps clear things up! Happy riding and take care!