As others have mentioned, you should have the battery removed and capacity tested by a qualified shop. I assume the Meridian has a lead-acid battery. These do have a finite service life, and if it fails the cap check, usually the only option is to replace the battery. Your AMM chapter 5, or the battery manufacturer's CMM should give the required cap check/replacement intervals.
That said, 23.9 volts should have been sufficient, but it is the number of amps that the battery can deliver to the starter than really matter.
Using an external GPU (when available) to assist the battery is always a good idea, though I realize that it may involve extra expense, and smaller airports / FBOs may not have one.