RR 6 Plus ST and a Jackery 290

[pasha]

New here and just got my RR6+ today, which has become inspiration for some crazy ideas I want to vet.

My question is: If I were to take along a portable battery (Jackery 290) that could power the charger for the on-bike battery, would the power draw from the portable battery directly and then the bike battery (when the portable is depleted) or would the portable battery only be used to charge the bike battery (in which case I?m not sure how quickly it will charge based on consumption).

Just trying to see if I can save the $500+ for a second battery.

Thanks!