Caltric sells fairly good Chinese products and I have used their products which are available for vintage vehicles that the manufacturer no longer supports, but I gather from your post that there is an underlying problem.
Switch your meter to AC volts and check across the battery - let me know what the reading is.
The alternator produces alternating positive and negative voltage pulses commonly referred to as AC (alternating current) - the rectifier inverts the pulses so they all go the same direction - this creates a pulsating DC (direct current) - the terms are technically incorrect because what is generated is voltage. Voltage is the push and current is flow. "Alternators" create voltage and the voltage is present as long as the magnetic field is rotated around the stator (stationary) winding - the voltage is responsible for current flow when connected to a conductor.
The most common AC Generator is a permanent magnet rotated around a stationary coil (single phase) or 3 coils (three phase). A 3 phase alternator is most commonly constructed with one end of each coil terminated to one end of another coil (referred to as "wound in "wye"). Sometimes the coils are connected in series and referred to as wound in "Delta". Each phase of a "Wye" wound alternator generates voltage across two coils while a "Delta" wound alternator generates across a single coil. Therefore, wye wound stators tend to be more compact to generate the same voltage as a delta wound stator.
Commonly the alternators produce approximately 30 to 50 "peak to peak" AC volts - the rectifier flips half the sine wave which drops the peak voltage approximately in half and the voltage available to charge the battery is the average between zero and peak voltage. So, an alternator producing 30 VAC has approximately 15 volts of pulsating DC to charge the battery. Revving the engine increases the frequency of the AC voltage being produced which raises the average of the rectified voltage available to charge the battery. That's where the "regulator" comes into play. The regulator shunts all voltage over 14.8 to ground so if the regulator is operating properly, you will see no more than 14.8 volts across the battery terminals. As the battery discharges, the regulator will stop shunting voltage to ground (this is usually set at about 13.4 or 13.6 volts) and the voltage to the battery increases. As the battery charges and the voltage increases, at about 14.8 VDC the regulator "turns on" and sends excess voltage to ground. This is one of the reasons most rec/reg units are somewhat large and finned. Changing AC into DC creates heat and shunting excess voltage to ground creates heat too.
That is the short version of how a permanent magnet AC generator (alternator) works.
Now, what can go wrong with this type of charging system...........
A permanent magnet alternator produces AC voltage at the maximum value all the time. The average voltage (read by a common multimeter) will increase with engine speed. The peak voltage will remain constant and must be read with a special peak voltage meter, an adapter that lets an average voltage meter read peak voltage or measured with a calibrated oscilloscope.
If one of the 3 coils opens, the output voltage will drop - this is easy to determine with a continuity test
If the insulation between the winding's of wire fails and the coil shorts out winding to winding or if a wire cracks, the output drops, but only under load. Difficult to determine even with a highly accurate ohm meter
If the insulation of the winding fails on any one coil and the coil wire shorts to ground, the output will drop to near zero, but under certain circumstances, the coil will continue to produce voltage between the lead wire and ground. Under this condition, AC voltage may be present between the terminals of the battery. A short to ground is easily determined with a continuity test. The stator must be replaced when a short to ground is detected.
A simple, single phase full wave rectifier has 2 diodes while a 3 phase has 6. The diodes can open (like a fuse) or short (conduct in both directions). If a diode shorts, you can have AC voltage at the battery, which will kill the battery.
In either case (alternator winding shorted to ground or shorted diode in the rectifier) there can be AC voltage getting to the battery. The regulator may still be doing it's job limiting the voltage to 14.8 volts, but if it's AC voltage the battery will be damaged.
Incandescent lights on the other hand are DUMB - they don't care if they are operating AC or DC and they will operate on both higher and lower than nominal voltage without failure. A typical 12v headlight may operate just fine at 16 volts without burning out prematurely and lowering the voltage almost never damages one. LED and High Intensity lights are a different story.
End of this long story - you could have a failed part of your charging system causing the short life of your battery. The odds of getting one properly activated bad battery is pretty high - getting 2 is like getting struck by lightning - getting 3, there has to be another problem somewhere.
I am not into writing text books and I hope this is not confusing or inaccurate. It is to the best of my knowledge correct and may help you determine the problem with your charging system. There are tests that are not in the book.