Aha......I just went to the user guide and their are 10 backlight levels but they are not in the menu but incorporated into the power button. Interesting.

In any event it would still be interesting to see how she stacks up power wise against some of the more common depth finders in use on the board.

Perhaps all is not lost as I am already invested. Even better is I have a built in excuse as to why I got my as@ kicked by my accomplice on my latest trip  .

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.

MT
I was reading somewhere else where they recommended this battery
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I do not know what you charge it with.  Can you measure the actual current draw on your elite 4x DSI.  Maybe with several backlight settings. That does look like a nice detector.

Measurements I have taken.
Eagle fish mark 320  voltage verses current draw
Wires are cut on backlight. Transducer in bucket of water.      
Voltage      Max      Min      Ave            
8                              
9      0.245      0.135      0.164            
10      0.264      0.136      0.168            
11      0.284      0.138      0.175            
12      0.318      0.14      0.185            
13      0.1996      0.151      0.17            
14      0.355      0.142      0.187            
15      0.376      0.144      0.199            
16      0.398      0.146      0.202            
17                              

I found the manual for your elite 4x DSI at
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On page 23 input voltage range is 10 to 17 volts.   The current draw is 200mA = .2 Amps. We will assume that the detector always draws 200mA.  This means that if you have your detector on for 2 hours it will use ( 2 hours X .2 Amperes = .4 Amp hours) .4ah.
I do not have the lithium data so I will use ultra-alkaline AA.  Data is available at
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You have 8 batteries in series and these batteries put out 1.6V when new. 8X1.6 = 12.8 Volts.  The elite needs minimum on 10 volts to work properly (probably does not shut off till it gets down to 8-9 volts but does not work well below 10 volts).  10 volts divided by 8 batteries ( 10/8 = 1.25) means each battery must supply 1.25 volts for your detector to work well.  Look at the graph on the first page of the above Duracell web site.  The .2A load is closest to the blue line(.25W). Follow the blue line to where it crosses the 1.25 V line.  Ok there is no 1.25 line so go between 1.2 and 1.5 and from that point drop straight down and you will see 4 hours service.  That battery will supply .2A current draw for about 9 hours but in a 8 battery system you do not have enough voltage. 
Lets do that over with 10 batteries in series. 10 batteries in series at 1.6 volts gives 16 volts which is below your manual max voltage. 10 volts divided by 10 batteries means each battery must supply 1 volt for the elite to work properly.  Go back to the chart on the Duracell site.  Follow the blue line down to 1 volt then drop straight down and you get a little over 9 hours.
Lithium batteries are a little higher voltage and you want 9 batteries in series with Lithium.
I do not have discharge curves on the battery that was in my previous link but if it is over 10 volts when it is at it rated ah then you could get 25 hours with your elite.  This battery weighs 18.6 oz. This is a Lithium Polymer ( also called Lipoly) and they require special chargers. 

Db
That is the disadvantage of having one battery that will take you all the way through the trip. Quentin sets his shallow alarm to 1 Ft and when he is getting out of the canoe it goes off reminding him to shut the thing off. 
Shipping charges at the hobby place I posted a link to are HIGH!