Ship's Power (or portable GPS) Question

I'm an old technology guy, flying comfortably behind my Lowrance 100, but
I've encountered a problem where there shouldn't be one: The internal
batteries still die and take the unit down, even when running on "ship's
power".

Last fall, I added a power outlet in the cockpit to power the Lowrance (as
well as provide an always hot point where I can hook a battery tender during
winter months).

The strange thing is that even using ship's power, the unit still dies from
time to time "wanting" new batteries. Makes no sense to me, since the 4
AA's in it should last a long time given that they shouldn't see any drain..
Besides, shouldn't the unit function just fine on ship's power even if the
internal batteries are dead or nearly so?

By the way, having the always hot circuit for the battery tender is very
handy...

Thoughts on what the GPS/power problem might be?

KB