As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AFTER CLEAN UP ON CLIMB OUT 'BATT DISCHARGE` LIGHT ILLUMINATED, RAN QRH CK LIST, AMP DISCHARGE REGISTERED -4. GROUNDED - YES. AIR TURNBACK - REF B737-900 FIM 24-31 TASK 833 REMOVED AND REPLACED MAIN BATTERY CHARGER AND OPS TEST NORMAL PER B737-900 AMM 24-31-21. ETOPS SIGNIFICANT SYSTEM GROUND VERIFICATION SUCCESSFUL.