The function of the Magic Eye® state of charge battery indicator is based on the specific gravity of the sulfuric acid or electrolyte. When a battery is charging, the change in the density of the acid is a little delayed as compared to the actual state of charge of the battery. However, the battery indicator should show green once the battery is back to 100% state of charge. There may be several reasons as to why it is not showing green:
1. If the battery was allowed to remain in the dis-charged state for a great length of time, the battery may in fact be bad and the battery indicator will not show green because the electrolyte has not reached the proper specific gravity. The specific gravity of the electrolyte is the most accuarate method of measuring state of charge.
2. The acid in the battery is "stratified" meaning that the heavier acid has sunk to the bottom and the lighter water is floating near the top. This happens with batteries that have sat for a long time in the discharged state. Normally, re-charging will stir the electrolyte make it more homogeneous.
3. There may be a small piece of battery separator material preventing the green ball from floating up.
The best thing to do is gently rock the battery back and forth. This will mix the electrolyte and will most likely free the green ball to float up "IF" the specific gravity of the acid is dense enough.
Keep in mind that even though the Magic Eye®is located in only 1 of 6 cells, testing conducted by a major OE car company proved that the battery indicator accurately indicates the state of charge of the entire battery.