This is my second time around with this problem. New Battery. Plug in on board charger, get red light, check connection. Water level fine. First time took battery back and told it was bad. Can't believe I have another faulty battery. Wondering if something is on a constant draw??

Did your charger go red immediately after you installed new battery? Is it the cranking battery? What motor do you have?

No, after changing the battery out it was fine. It is the cranking battery. The motor is a Merc 225. Also, I have a MinnKota 3 bank charger and perhaps I'm wrong doing this, but I unplug it a day or so after a trip. It seems leaving it plugged in full time dries the batteries out.

that charger should go to maintenance mode once the batteries hit full charge, you can leave it plugged in.

red is incomplete circuit as well, have you checked the fuse?

check the voltage of the battery, if it is below 6-7 volts the charger will not recognize it.
if it is that low and a start battery then you have engine charge problems as well.

check output voltage while engine is running, should be over 13 volts.