My previous Elite 5DSI, and now my Hook 7, spend their off hours in a cotton drawstring bag .... and that bag is inside of a waterproof bag (that sits inside a 5gal bucket) ... so, yeah, I "baby" my unit much the same way.
My Lowrance Elite 5DSI screen would darken late in the day, after several hours of trolling (back when I was using a Group 27 battery that was several years old). If I let off the trolling motor, the screen would lighten back up for a few more minutes .... but, if I didn't let off of the trolling motor, the unit would simply shut off. The battery was the problem ... just wasn't big enough to troll hard for 4-5hrs and have enough power to run the depth finder. A Group 29 battery solved that problem.
Maybe your battery isn't holding enough charge to keep the unit operating at full tilt