My understanding is that when the green light comes on, the battery is fully charged and the CHARGER is on "standby". When mine has the green lights showing there is only a very faint hum. The unit's red or green lights are always on when the unit is plugged in. I am **** sure NOT an electronics expert, but I would think that the DUAL PRO will need electricity to run the lights and whatever monitors the batteries (thus the hum) and still be in a standby mode without sending any juice to the batteries.
Based on conversations with the guy that sold me my boat, a couple of (long ago) emails with DUAL PRO, and the DUAL PRO web site, my understanding is that after the batteries reach full charge, the unit goes to standby WITHOUT any trickle charge. If and/or when the batteries fall below a certain level of charge (which I don't remember) then the DUAL PRO starts charging again and the red lights come back on until the battery is fully charged. A couple of times over the years, I have checked on the DUAL PRO while everything was still hooked up and the red lights were on. I left it alone and checked back later and the green lights were on, so I assumed that it had been charging and then shut off again.
Hope this helps.



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