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Resistance in the heavy cable between alternator and battery, so charging voltage measures correctly at the alternator and low at the battery. The difference is the voltage lost in the cable, and the battery is undercharged while the alternator appears to be working perfectly.

A meter's continuity test passes the joint that causes it, because resistance only shows as a drop when current flows. Measure voltage drop with the engine running and loads switched on — probe between the alternator output and the battery positive, and anything beyond a couple of tenths of a volt is the fault. Corrosion inside the insulation is common on an older cable and invisible along its whole visible length. Check the earth side as carefully as the supply, since it drops exactly as many volts and is checked half as often.