best I know is that traditionally a "trickle" charge just trickles a low voltage charge to the battery as long as it's hooked up and left on past whats needed it can eventually cook the battery.
a "tender/minder" type charger floats ... it trickles the charge until the battery reaches the proper charge then it shuts off, then as voltage drops to a certain level it kicks back on and levels things off.
BTW... what color should the eye be? mines sort of burnt orange. not started in a week (tender not on) the voltage shows 11.8v. still started the car fine.
Mine was the same color but voltage was about 12.2 I REALLY don't trust the "eye" site thingie.
THIS IS WHAT IT SAYS ON THE WEB:
Better late than never!!! The Magic Eye® that is used in Motorcraft batteries provides 4 stages of indication, they are:
1. Green: battery is charged and the acid level is good.
2. Yellow: Battery is partially dis-charged and the acid level is good. Re-charge the battery or drive the vehicle.
3. Red: Battery is dis-charged and the acid level is good. Charge the battery.
4. Clear and bright: Acid level is low!!! Add distilled water if possible. If not, change out the battery. Do not charge or jump start the battery!!!
While the Magic Eye® is only located in one cell, it is still quite accurate for indicating the state of charge of all cells as they are within 95% of the same condition. If there is a catastrophic failure such as an internal short circuit, the battery would be bad, but the eye would still show green as the density of the acid is still technically correct. This is very rare.