It could be either of two things or both.
First Lowrance doesn't list power consumption for that unit. If it were drawing 500ma the battery (8amp) should be good for 16 hrs and if it is drawing 1.75 amps it should be good for a little over 4 hrs ... theoretically.
Second you stated you have had and used the battery for several years. With proper maintenance you should get roughly 4-5 years out of the battery.
It could be the unit drawing to much power from the battery. It could be the battery is on its last leg. Or it could be a combination of both.
I would try contacting Lowrance Monday AM and find out the power draw for that unit. My guess would be high because it is color, has a backlit screen and backlit keys.
Also Lowrance lists the LCD on that unit to -4 degrees. Prolonged exposure to that or colder temps will damage the LCD.
Good luck !