Most depth finders are designed to operate in a motor boat with a car type battery.  12 volt car batteries are about 14 volts and when charging are about 16 Volts. I have read where others (Jimbo) reported his detector worked better on lithium batteries.  He was using 8 AA batteries.  Lithium batteries are a little higher voltage than alkaline batteries.  I use a power hog detector so I need to use C cells to operate it for a week (C actually last quite long).  I found the detector did not work well and got lower performing as the week went along running on 8 C cells.  I changed to 10 alkaline C cells and the detector worked very well.  When the 10 batteries are fully charged the detector is getting about 16 volts which is not higher than what it could be in a running motor boat.  The detector seems to work quite well ( can pick up a rubber twisty tail) between 12 and 16 volts.