Hi Craigmck
Not an expert but have bought my first Chargery and will be wiring it up soon.
Yes you can use the internal battery but I believe display etc gets faint when the battery gets low. There is a switch on the back for Internal/External battery.
I believe the relays are for switching when powering high inductive loads - they are not needed during normal operation.
There are a number of videos on YouTube that will help with your understanding of the operation. Also have you looked on the Chargery website.?