BATTERY TEMP GAUGE AND OVERTEMP WARNING LIGHTS HOOKED UP IN REVERSE CAUSING WRONG BATTERY TO BE SHUT DOWN WHEN OVERTEMP OCCURRED.

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FLT FROM CRW TO IAD. 68 MI OUT 150 DEG BATTERY OVERTEMP LIGHT ILLUMINATED FOR #1 NICAD BATTERY. BATTERY OVERTEMP SCALE INDICATED MIDDLE OF THE YELLOW BAND FOR #1 BATTERY. EMER PROC FOR OUR LTT; QUICK REF HANDBOOK; CALLED FOR ISOLATING THE #1 BATTERY AND THEN OBSERVING THE OVERTEMP SCALE INDICATOR. IF THE NEEDLE STABILIZED OR RETURNED INTO THE GREEN AREA; WE WERE ALLOWED TO CONTINUE. IF IT CONTINUED TO INDICATE AN INCREASING TEMP; WE WERE TO LAND AT THE NEAREST SUITABLE ARPT. THE TEMP CONTINUED TO STEADILY RISE. WE TOLD CTR WE NEEDED TO RETURN TO CHARLESTON DUE TO A BATTERY PROB; REVERSED COURSE AND DECLARED AN EMER. BY THE TIME WE REACHED CHARLESTON THE TEMP SCALE INDICATOR SHOWED FULL SCALE INTO THE RED; NEEDLE HAD DISAPPEARED ABOVE THE 180 DEG MARK AT THE TOP OF THE RED SCALE. AFTER LNDG AND EVACING THE PAX AT THE GATE; THE FIRE CREWS OPENED THE ELECTRONICS COMPARTMENT DOORS WHERE THE BATTERIES WERE LOCATED. THE #1 BATTERY WAS COLD AND THE #2 HOT. THE #2 BATTERY BOILED; WHISTLED; SPEWED FOR 90 MINS PLUS AFTER BEING REMOVED FROM THE ACFT BY FIRE CREWS (BATTERY COULD BE HEARD WHISTLING OVER 150 FT AWAY). BATTERY OVERTEMP INDICATOR TEMP SCALES AND OVERHEAT LIGHTS WERE FOUND CROSS-WIRED TO THE OPPOSITE BATTERIES. #1 SCALE WAS CONNECTED TO #2 BATTERY AND #2 SCALE CONNECTED TO #1 BATTERY (CANNON PLUGS REVERSED AT BACK OF INDICATOR). SO WE; IN FACT; ISOLATED THE GOOD BATTERY; #1; AND THE HOT BATTERY REMAINED CONNECTED; THUS CONTRIBUTING TO ITS OVERTEMP. HAD WE NOT BEEN SO CLOSE TO CRW; FATALITIES COULD HAVE RESULTED! CANNON PLUGS SHOULD BE CHANGED SO THEY CANNOT BE REVERSED BY AN INATTENTIVE MECH! THE #1 BATTERY IS ON THE R AND #2 ON THE L SIDE OF THE ACFT. PLUGS WERE PROBABLY ATTACHED THINKING #1 L; #2 R. RECOMMEND ENTIRE FLEET OF LTT'S SHOULD BE CHKED AND MURPHY-PROOFED IMMEDIATELY!