[Whaler]

Yes........the electric fan for most of us will be the biggest single load on the running system. Maybe a good set of headlights on high beam are next.

Anyhow your 65 amp charger should take care of all vehicle loads that are pulling off the battery. The initial engine start is one big draw on the battery.

I would check in this order.
Battery load test
Battery voltage running and sitting with engine off
Amperage on the charge wire to the battery or positve side of the solenoid. Use a clamp style/ induction amprobe ( don't have to break the cct. The battery shop may be able to do this for you too.

Tim