For safety of your fishfinder and the ability to start your engine the fishfinder shuts off automatically when the available voltage falls to about ten volts. Many boats do not have a battery charger that maintains the battery at peak voltage when the boat is stored but the owners depend on the main engine charging system to maintain the battery. Running the bilge pump, night lights, radio transmissions, downriggers and rock and roll music will kill your battery especially when the engine is idling while trolling.