These fish finders are designed to operate on a boat with a car type battery.  They are called 12 volt batteries. Ever put a volt meter on a 12 volt battery.  If it is charged and ready to start your car it will be about 14 volts.  If the engine is running and the battery is charged it may be as high as 16 volts.  Alkaline batteries start off at 1.6 Volts and have considerable energy left in them until they go below 1 volt.  When the fish finder voltage goes below 10 volts they start to have trouble finding the bottom and do extra pinging to find the bottom.  This uses more battery. I run with 10 AA in series and that allows the detector to work for about 4 days on a set of batteries. Ya I have a battery hog also.  Others have reported better results on lithium batteries over alkaline.  Lithium batteries are a little higher voltage. My detector could down a set of C batteries in a day before I cut the wires to the lights. Turning the light off did not stop the power hog from running them full brightness during the boot up and until it could find the bottom and by then the batteries were dead.
I am going to be looking for a new detector next year.  What can you see on the Elite-4 with the lights off?
Zski Mentioned Duracell.  I have found that Duracell is one of the better batteries.