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