Short version: your charging system is undercharging. Also, your battery may be bad (have it
tested!) or it may simply be low on charge.
Long version:
Ideal charging voltage is 13.6 to 14.4. A fully charged lead-acid battery with no load on it will read 12.66V, so anything below this is discharged to some degree. If you read 12.3 at rest, your battery is discharged OR may be bad.
If it helps, you might think of electricity as water, your battery as a bucket, and your alternator as a spigot trying to fill the bucket. Your loads (fan, lights, radio, defroster, starter) are all holes in the bucket that are trying to empty it. The starter takes a big gulp, and then for the next few minutes after you've started the alternator is trying to replace that used energy.
In this analogy, your water level (battery resting voltage or state of charge) is only going to be maintained (or catch up) if the spigot can keep up with the drains. If you have a problem between the spigot and bucket - e.g. Bad battery cables or a skinny charging wire - the bucket will not refill quickly enough to maintain. If your bucket is inherently bad - it loses water or is too small - then the spigot becomes critical. If your spigot can't produce as fast as the loads are drawing on the bucket, then the level in the bucket falls. In your case, it fell to 12.3 volts and there wasn't enough left in the bucket to handle the starter's load.
Since I don't have access to your car, I can't say anything about your specific situation besides the charging voltage is low and your battery became discharged. You should (in this order):
1. Have the battery and alternator tested by a real alternator repair shop or auto electric shop - they will know better than a parts store flunkie if you have an external regulator gone bad, for instance.
2. Replace whichever of those two needs replacement, and BEFORE YOU FIRE IT UP--
3. Fully charge the battery. If you don't, you will let the expensive smoke out of the alternator.
4. This is important for longevity, and it may catch a problem that could kill your new alternator or battery in short order. Start it, and turn on all the electrical accessories. Then hold the revs about 1500-2000 after the glow plugs have finished cycling. Take a voltmeter and perform two tests to tell you if your cables are OK: + probe to output terminal of alternator, - terminal to battery positive (right on the stud) and then + probe to battery negative, - probe to alternator case (scrape to get a good connection.) If you switch +and -, the readings will be negative but no harm will be done.
5. If either of the two readings in step 4 were greater than 0.5V, you have a voltage drop in your wiring that will kill your alternator and eventually your starter. If the first reading failed, check your positive cable, its end at the battery, and the big wire coming off of the alternator. If the second reading failed, check the negative cable and it's end at the battery. If the car has emergency replacement cheesy p.o.s. clamp-to-the-cable style ends where the battery meets the cables, ditch them and get real battery cables of at least 1 gauge thickness if not better. Your car will thank you by starting reliably and not eating its alternator, battery, and starter.
Last thing: between the hole in your car and the hole in your wallet, always fit the biggest battery you can manage. Like a bigger bucket in the earlier analogy, it has further to go before it is drained, and the same size load placed on it (vs. a smaller battery) will make its voltage not fall as far. I live this advice - my 83 Toyota diesel pickup has a battery from an over-the-road truck - group 31 - and I have fitted a Delco AD244 alternator from a newer GMC pickup so I can have 135 amps at my beck and call. It starts immediately even in freezing weather, and my lights don't dim nor fan slow down at idle.
Hope this helps!
TomV
Former auto electric shop proprietor and master tech
