Flashing battery light

tested it, working fine, the machine said to change battery... but brand new?... he has alot hooked up to the battery tho, as well as the car, mayb the battery isnt strong enough?
 
If thr battery voltage drops too low, it will not supply sufficent power to the alternator field windings. Then the alternator will not put out the full 13.8-14.2 volts needed to make everything run good.
 
Check voltage of battery when car is idling. Trun all the accessories on and check voltage.

Clean every connection from the batt to the starter solenoid. Replace battery terminals or any cable if questionable. Make sure everything is tight.

Load test the battery.