Code -15 is battery volts low,check your battery state of charge and the charger to make sure they are good,sounds like a low/weak battery.
Code 8 for controller box is as follows:

If lower is not requested and the voltage at the
Load Holding Solenoid is less than 1 volt for at
least 1/2 second.
Or
If lower is requested and the Load Holding
Solenoid voltage is greater than 1 volt for 1/2
second.

Corrective Action:

Corrective Actions and Checks:
1. Check the wiring between the Load Holding Solenoid and the
Interface Card (JPS5-2/JPS5-1). Check for both continuity and
positive connections.
With the key switch On, there should always be B+ on pin 2 of
the Load Holding Solenoid.
2. Connect a volt meter between JPS5-1 and TP4 on the contactor
panel. Turn the key switch On. If the voltage is less than 1 volt
without a code on the Interface Card, replace the Interface Card.
3. Check the Load Holding Solenoid for proper resistance.
Remember to run Learn if the firmware or Interface Card is
replaced