Does the J35 serve as a DC-DC charger?

The J35 does not serve as a DC-DC charger.

We offer several kinds of DC-DC chargers:

https://teambmpro.com/product-category/dc-dc-chargers/

The MiniBoost is able to connect directly to the J35:

https://teambmpro.com/products/dc-dc-charger-miniboost/