Have you had the battery load testing ...??

It can show charged and still be bad to where it discharges over a short time enough to cause a low voltage alarm on the unit ...

And also ... Is the unit wired direct to the battery and not thru the boat wiring...??
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And also ... If you're using the HB TS3 switch... you should know that one side of the selector is 12v hot all the time when that side is selected (and can run the battery down even with the unit powered off) ...

Rickie