Can you use a small battery to charge a big one?

Disclaimer:
Do this at your own risk. Improper battery charging can cause batteries to fail or explode.
When charging, do not leave the batteries unattended.
You can use an equal or even lower voltage battery pack to charge the Ecoflow River Max. As an example, I used a 12.3V 15Ah lithium ion battery pack to charge my Ecoflow River Max. I use the XT60 plug from the battery pack to plug into the XT60 jack of the Ecoflow.
Set your Ecoflow (with the App) to car charging to limit charging to 8A so you do not damage your small battery or the Ecoflow. You do not need a voltage booster to enable your 12v battery to effect the charge.