The alternator may be failing. Remove it and have it tested at an Auto Parts store.
That means it dosent have to be charged anymoreWhen you start to charge and the red light flashes for a while, then it changes from red to green alternate flashing this means that the actual batteries doing the charging are not fully charged. Either buy new batteries or make sure you charging batteries are fully charged before you attempt to charge it
If they are good batteries fully charged when installed and connected in parellel, yes it should handle them.
New batteries almost always come fully charged. If it was not then charge it with a battery charger and do not use your vehicle alternator to charge a dead battery. You may do harm to the alternator which is not built to charge a dead battery but is instead built to keep a good battery fully charged.
With the batteries in series, the alternator provides the same current, and therefore the same charge to all the batteries. The alternator can take no account of any current being drawn from the 12V take off point. Part of the charge current is being fed to the 12V load and not to the batteries. Therefore, the 12-24V batteries are being fully charged while the 0-12V batteries are being partially charged. Over a period of time, the difference between the charge levels will become more obvious.
A fully charged 12 volt automotive battery should have 12.68 volts.
Not fully, but these days come more than half charged, yes.
12.6 volts when fully charged.
No
It is there to keep the battery fully charged.
As long as the battery is fully charged it will start no matter the condition of the alternator. Eventually the battery will run down if the alternator is not keeping it charged.
12.6 volts when fully charged.
12.8 - 12.9 volts is a fully charged battery