To answer the OP's original question:
I have that exact model, and I've used it on the internal AA batteries overnight, as an anchor
alarm, after sailing all day, with no problems.
The caveat: Buy high quality, Lithium AA batteries, and make certain that in the GPS
settings, you configure it for Lithium batteries vs. NiMh (Nickel metal hydride)
The setting doesn't consume more or less power, it simply enables the GPS
to accurately gauge how much power remains in the batteries, and avoid shutting down prematurely.
The next best batteries for this model, are the Energizer rechargeable NiMH batteries. Again, make certain that you select NiMH if you install these batteries. They do last a long time and are reuseable.
The worst batteries are ordinary, alkaline batteries like Duracell. Fine for sailing all day, OR anchor
alarm all night, but probably not both on the same set of batteries.
Hope that helps.