"Might help if you describe yo
"Might help if you describe your system. Charging system etc. You say 24 volt system but are you refering to 2 - 12 volt batteries being charged in parallel or 2 - 12 volt batteries in series .
It would seem if one of your batteries is at 15 volts. This would repersent a battery being charged at full rate by a 12 v. alternator, while the 12 v. battery as one that is not receving any charge at all.
If in fact you do have a 24 v. system with engine running , charging rate should be in the 30 v. range. Another thought. where in system are you taking your measurments ? Poor connections in paticular grounds can lead to much confussion.
Another thing to keep in mind is that to get a accurate battery test , the batterys must be isolated - slowly charged - let sit for 24 hrs. than tested. A good batttery should have a fully charged voltage of 12.6 volts.
Best luck"