"Voltage drop" is OFTEN what CAUSES this problem, here is how this works
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FIRST
Depending on the car the wiring, the circuit path for the "ignition run" wire is:
Battery---fuse link---bulkhead--ammeter circuit---welded splice in the under dash harness---ignition switch connector---through the switch--back out the switch connector (on the "ignition run" or "IGN 1" wire) -- back through the bulkhead -- to the
ignition system
alternator field
AND REGULATOR IGN terminal.
Now I intentionally separated "stuff" up above with dashed lines. Every single object separated by dashed lines IS A SUSPECT for "voltage drop":
from the battery ---through all that---to the regulator IGN terminal.
WHY IS THIS IMPORTANT!!!??? You really should actually read a few of my posts, this has been addressed.
IF THIS chain, this circuit path, has a bad switch, loose connections, corrosion, the LOAD current put on this "ignition run" wire causes a VOLTAGE DROP between the BATTERY and the REGULATOR
Now, the regulator IGN terminal does MORE than "just power the regulator," it is also the VOLTAGE SENSING wire.
Every last bit of voltage drop, whether it is .1 (one tenth) or 1 (one volt) is ADDED to the regulator set point. This means that if the regulator is PROPERLY attempting to regulate at 13.8--14.2, and you have X drop in the harness, this voltage drop IS ADDED to the battery charging voltage.
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SECOND
The next part of the equation is (again with the voltage drop) DROP, VOLTAGE between the battery NEG terminal and the REGULATOR GROUND
If the regulator is not truly grounded to the battery, then "it" works just like harness drop, above. ANY drop between the NEG side of the regulator and the battery NEG is ALSO added to the charging voltage, just like above
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To sum this all up. What are the main causes of overcharging?
1----Bad regulator, actually fairly rare. Most regulators either work or they don't
2---Voltage drop in the ignition harness/ charging line
3--Voltage drop in the regulator ground
4--In some cases a bad battery can cause this. Easily checked by simply swapping in a known good battery.