What was found
DURING CLIMBOUT, BATTERY DISCHARGE LIGHT ILLUMINATED. FOLLOWED QRH PROCEDURE AND RETURNED TO DEPARTURE. AUX BATTERY SHOWED 0 AMPS AND 28V. MAIN BATTERY SHOWED -6 AMPS AND 27 VOLTS. LIGHT EVENTUALLY WENT OUT AND AMPS SLOWLY WENT TO 0 AMPS OVER 20 MIN. VOLTAGE STABILIZED AT 27. ACFT GROUNDED. PERFORMED A GVI ON MAIN ACFT BATTERIES AND CHARGERS, NO DEFECTS NOTED. REPLACED MAIN ACFT BATTERIES AS A PRECAUTION AND MAIN ACFT BATTERIES OPS CHECKED GOOD FOR MAIN AND AUX. IAW AMM TASK 24-31-11-400-802-002. FOUND MAIN ACFT BATTERY CHARGER GOES INTO A DISCHARGE MODE AFTER COMPLETING A CHARGE MODE. REPLACED MAIN BATTERY CHARGER IAW AMM TASK 24-31-21. MAIN BATTERY CHARGER OPS NORMAL, IAW AMM TASK 24-31-21-710-801.