The fan comes on automatically in a few seconds after plugging in. The fan will NOT come back on unless it is necessary[3 or 4 batteries in slot, discharging batteries, ect.]
As far as “Null” goes. That can be for a few reasons. Battery is not making Full contact.Battery may be incompatible? For some batteries I had to use magnets to get proper contact.
When you put battery in are you selecting the proper settings? Charge/Charge test/discharge ECT.?
Is the charger new?
If you continue to have problems send it back.
I have had mine Two years with no problems ONCE I put a new fan and More powerful power supply,those were separate issues!