Fish Finders: When to Run Them and How to Save Power

The short version

The screen and the extra sonars use the most power. A basic 4- to 7-inch unit draws under 1 amp. A 12-inch screen or live sonar can each add several times that.
Leave it on while you run and search. That's when depth, mapping and GPS matter most, and it's when the outboard can recharge its battery.
Turn things off when you're sitting still. Stop the sonar or switch off live sonar when you're anchored, eating lunch or fishing a spot you already know.
Dim the screen. The backlight is a big part of the draw. Lowrance rates the Elite FS 7's 2.0 A with the backlight on full. At dawn, dusk and night you rarely need it that bright.
Wire it straight to a 12 V battery with the right fuse, and keep its power wires away from the trolling motor's. That keeps the picture clean.
Size the battery for your real day. A Helix 7 with live sonar running all day needs a battery almost twice as big as the same setup with live sonar on only part of the time.

What a fish finder does for your trip

A fish finder sends sound pulses into the water through a transducer and draws what bounces back. The fish are only part of what it shows you:

Depth and the bottom. You see drop-offs, humps, weed edges and rock, and whether the bottom is hard or soft. That's where fish hold.
Safety. You see shallow water, stumps and rock before you hit them, especially on a lake you don't know.
Water temperature. Units with a temperature-capable transducer show the water temperature, and some can sound an alarm when it changes. Garmin's STRIKER Vivid alarm sounds when it changes more than 2°F, which can point you to a creek mouth, spring or warm bay.
A way back to the fish. GPS models let you save waypoints. Mark a brush pile or a school once and drive straight back to it next time.
Maps of your own. Some units draw contour maps as you drive. Garmin's Quickdraw Contours, for example, builds a map from sonar depth and GPS position as long as you have a memory card with free space. That helps on small lakes that no chart covers.
A path home. A saved track or the launch saved as a waypoint gets you back in fog or after dark.

How much power fish finders use

Every fish finder lists its current draw in its specifications. The bigger the screen and the more sonar types running, the more it uses:

Unit	Draw	8-hour day	12 Ah lithium lasts
Garmin STRIKER Vivid 4cv	0.40 A	3.2 Ah	24 h
Garmin STRIKER Vivid 7cv	0.55 A	4.4 Ah	17.5 h
Humminbird HELIX 5 G3	0.62 A	4.9 Ah	15.6 h
Humminbird HELIX 7 G4	0.80 A	6.4 Ah	12 h
Garmin ECHOMAP UHD2 74sv (typical)	1.52 A	12.2 Ah	6.3 h
Lowrance Elite FS 7 (backlight full)	2.0 A	16 Ah	4.8 h
Lowrance HDS-12 Live (maximum)	3.6 A	28.8 Ah	2.6 h
Add Garmin LiveScope Plus (typical)	about 1.75 A	14 Ah	n/a

Manufacturer specifications at 12 V. Run times use 80% of a 12 Ah lithium battery, the same math as our Fish Finder Battery Life Calculator. Garmin rates the ECHOMAP UHD2 74sv at 1.52 A typical and 3.2 A maximum, so how you set it up matters.

Two things stand out. A small unit sips power: a STRIKER Vivid 4cv uses about as much in a whole day as a 12-inch unit uses in an hour. And live sonar roughly triples what a 7-inch fish finder uses. Garmin rates the LiveScope Plus sonar module at 21 W typical and up to 58 W maximum.

When to run it, and when to switch things off

A fish finder earns its power when you're moving and looking. It wastes power when you're sitting still on water you already know. Here's how a typical day breaks down:

What you're doing	What to run
Running across the lake	Leave it on. Depth and the map keep you off shallows, and a mapping unit keeps building contours. On most boats the outboard is recharging its battery at the same time.
Searching new water	Run everything you have: traditional sonar, down imaging and side imaging. Mark waypoints on anything worth fishing.
Fishing a spot you've marked	Traditional sonar is usually enough to keep you on the edge or brush pile. Use live sonar if you're actually casting to fish you can see, and turn it off between spots.
Anchored or drifting a known flat	Stop the sonar or dim the screen. Keep the unit on if you want GPS and the anchor position.
Lunch, rain delay or a long break	Turn the unit off. Restarting takes a moment while GPS finds your position, which costs far less than an hour of running.
Two units watching the same water	Run one sonar at a time. Humminbird notes that two sonars pointed at the same water interfere with each other, and turning one off is the only cure.

You don't always have to power off to save energy. Most units let you stop the sonar while the screen and GPS stay on. On a Lowrance HDS Live, Stop Sonar stops pinging without powering off the unit. On a Garmin ECHOMAP UHD2, the sonar menu's Transmit option turns off the active sonar, and the power key offers Disable All Sonar Trans. On a STRIKER Vivid, a quick press of the power key turns the sonar on and off.

Settings that save power

Turn the backlight down. Lowrance lists the Elite FS 7's 2.0 A draw "with backlight on full", and Garmin's typical figure for the ECHOMAP UHD2 74sv is less than half its maximum. Use full brightness in bright sun and turn it down at dawn, dusk and night. On a Garmin STRIKER, pressing the power key repeatedly steps through brightness levels.
Run only the views you're using. Live sonar adds a separate sonar module and transducer, so turn it off when you aren't watching it. Garmin rates the LiveScope Plus sonar module at just 24 mW minimum, against 21 W typical while it's working.
Turn off what you don't need. On a kayak or the ice, a basic sonar screen without mapping or networking is all you need.
Use the simulator at home, not on the water. Learn the menus in simulator mode at home on a spare or freshly charged battery, so you don't spend your boat battery learning the buttons.

Example: a kayak day on a 12 Ah battery

Say you run a Humminbird HELIX 7 (0.8 A) and a Garmin LiveScope Plus (about 1.75 A) for a 6-hour day on a 12 Ah lithium battery. Used to 80%, the battery gives you about 9.6 Ah.

How you run it	Uses	Fits in 9.6 Ah?
Everything on for all 6 hours	15.3 Ah	No. The battery runs out after about 3.8 hours.
HELIX 7 all day, live sonar on for 2 hours	8.3 Ah	Yes, with a little to spare.
HELIX 7 alone all day	4.8 Ah	Yes, twice over.

Amps × hours. Live sonar for 2 hours uses 1.75 × 2 = 3.5 Ah. To run everything all day, you'd need about a 20 Ah lithium or 33 Ah AGM battery.

Switching live sonar on only when you're working fish is the difference between a battery that dies at lunch and one that lasts until you load the kayak. Our Fish Finder Battery Life Calculator runs these numbers for your own unit and day.

Sonar types, and when each one earns its power

Most fish finders combine several kinds of sonar:

Traditional sonar sends a cone straight down and draws fish as arches. Garmin explains that lower frequencies use wider beams and reach deeper, while higher frequencies use narrower beams and show more bottom and thermocline detail. It's the workhorse for depth and for fish right under you.
Down imaging (Garmin ClearVü, Humminbird Down Imaging, Lowrance DownScan) uses a thin, high-frequency beam to show a picture-like view of brush, rocks and weeds below the boat. Garmin's ClearVü and SideVü run at 455 and 800 kHz.
Side imaging shows the bottom to both sides of the boat. Use it to scan new water quickly, then mark what you find.
Live sonar (Garmin LiveScope, Humminbird MEGA Live, Lowrance ActiveTarget) shows fish moving in real time, the way a video does. It's the most useful when you're casting to fish you can see, and it's also the biggest extra power draw.

Garmin's transducer guide notes that transducers work best in clean water with no turbulence. Mount it where water flows smoothly past it. A transducer sitting in turbulent water gives a weak, cluttered picture however much power you feed it.

Wiring it for a clean picture

Use 12 V, even on a 24 V or 36 V boat. Humminbird's HELIX installation guide says the control head must be connected to a 12 VDC supply. Never tap one battery of a 24 V or 36 V trolling motor bank for 12 V. It unbalances the bank.
Fuse it at the battery. Use the fuse your manual lists. Garmin's STRIKER Vivid uses a 3 A fuse. Humminbird specifies 3 A for the HELIX 7, 5 A for the HELIX 8 to 12, and 7.5 A for the HELIX 15. Garmin's LiveScope Plus sonar module uses its own 7.5 A fuse.
Keep it away from the trolling motor. Humminbird recommends keeping the fish finder's power cable away from the trolling motor's power cable. If the picture is still noisy, powering the fish finder from a separate small 12 V battery is its next suggestion.
Size the wire for a long run. Most fish finders draw little enough that 16 or 18 AWG keeps voltage drop low on a kayak. A long run to the bow on a bigger boat may need heavier wire. Our Fish Finder Battery Life Calculator picks the size for your run.
Use the noise filter. Humminbird suggests trying each noise filter setting to find the best one for your motor.

Ice fishing and cold weather

Cold cuts what a lead-acid battery can deliver. Battery Tender puts capacity at about 80% of rated at 32°F and 50 to 60% at 0°F. A battery that runs your flasher all day in the fall may quit by early afternoon in January. Keep it in an insulated bag or inside the shanty, and bring it home to charge.

Check your unit's temperature rating too. Garmin rates the STRIKER Vivid for operation from 5°F to 131°F. Lithium batteries handle cold use well, but they shouldn't be charged below freezing, so let them warm up first.

Mistakes to avoid

Running live sonar all day on a small battery. It's the quickest way to end a kayak trip early.
Leaving the screen on full brightness at night. It uses more power and ruins your night vision.
Wiring through the trolling motor's leads. Separate power keeps interference out of the picture.
Skipping the fuse. A short in an unfused wire can start a fire.
Forgetting to charge after the trip. A lead-acid battery left discharged wears out early.

Questions people ask

What size battery do I need for a fish finder?

For a 4- to 7-inch unit on a kayak, a 10 to 12 Ah lithium or a 12 to 20 Ah sealed lead-acid battery usually covers a full day. Add live sonar or a bigger screen and you may need 20 Ah or more. The Fish Finder Battery Life Calculator works it out for your unit and the hours you fish.

Can I run my fish finder off my trolling motor battery?

On a 12 V motor, yes, and many small boats do. The trolling motor can drain it, though, and some motors put noise into the sonar. On a 24 V or 36 V motor, don't tap one battery for 12 V. Use a separate 12 V battery.

Does turning the sonar off really save power?

It saves the power the sonar uses, but the screen keeps drawing. To save the most, dim the screen too, or power the unit off during long breaks. Turning off live sonar saves the most because its sonar module draws more than most 7-inch fish finders.

Will running the outboard recharge my fish finder battery?

Only if the fish finder runs from the battery the outboard charges, such as the cranking battery, or a house battery charged through a relay. A separate fish finder or trolling motor battery doesn't get charged by the outboard unless it's wired to.

Is a fish finder worth it on a small lake or a kayak?

Yes. Even a basic unit shows you depth, drop-offs, weeds and water temperature, which is most of what you need to find fish. A small one costs little to run.