I fish for about 6 hrs at a time and my unit is always on and hooked up to the cranking battery.....no problems with it running the battery down.......YET!.
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I run two depth finders plus my live well with a timer...all day and I can still crank my 90 Yamaha up. The fish finder will not pull the battery down it draws small mili-amps. The TM however draws lots..and even tho the voltage drops as the battery is being used...it will still run a electric motor ( just not as strong) the problem is the complex chips and integrated circuits in todays fish finders...they have to have close to full voltage or they will shut down.