Originally Posted by v8zracer260z

It is normal to see some fluctuation in the voltmeter depending on the condition of he battery and the loads on the system. Typically 13.0 to 14.5 is a normal range. Though low voltage indicates a non charging symptom ie alternator or regulator problem and over charging sytem can cause more problems. Over charging of more than 15-16 volts can blow out lights and fry computors leading to expensive repairs. In addition to checking the level of the fluid I would recommend checking the specific gravity of the battery acid with a hydrometer found at any local auto parts store. This is a better indicator of battery health. Water has a specific Gravity (SG) of 1.000 in a Sulfuric Acid battery A SG of 1.265 is ideal and it can be slightly higher or lower in each cell but if you have one or more cells showing close to 1.000 indicates a dead or shorted cell it's time to replace the battery. Also since you have a wavering idle that in itself may be causing your voltage to drop. Your alternator puts out very little charge if any at idle. Dropping below that rpm may cause a significant drop in charging voltage. You may not have anything wrong with the charging, but have an idle conrol issue. Just a little advice from a prior F/A 18 Avionics Tech Hope this helps you guys out.