What Size Battery Do I Need for My Fish Finder?

You’re halfway through a promising day on the water when your fish finder screen dims and dies. Not only is your trip cut short, but you’re also now second-guessing every battery purchase you’ve ever made. Choosing the right battery size is not a guess. It’s simple math. With just your amp draw and fishing time, you can avoid shutdowns, overloads, and wasted cash. This guide breaks down everything you need to know about fish finder battery sizing by unit type, chemistry, and use case. So you can power through your day with zero doubts and full confidence.

How Fish Finder Battery Size Works

Most fish finders run on a 12V DC power system, though some higher-end models accept up to 16V for improved efficiency. But here’s the kicker: battery size isn’t about how big the box looks. It’s about amp-hours (Ah). How much current your device pulls is multiplied by how long you plan to fish.

Here’s the formula: Ah = amp draw × hours. A Garmin Echomap 7″ typically draws 0.8A, while a LiveScope combo might pull closer to 3–4A. That’s a huge difference when you’re planning a full tournament day. Underpower your system, and your screen will dim or shut down. Overpower it, and you’re just hauling dead weight especially on a kayak.

If you want less math and more action, LiFePO4 batteries are your best bet. Compared to SLA or AGM, lithium gives you lighter weight, more usable capacity, and no voltage sag. That’s why so many serious anglers, especially kayak fishers, are making the switch.

Battery Size for Small Portable Fish Finders

Running a compact unit like a Garmin Striker 4 or Lowrance Hook2 4x? You’re in luck. These devices sip power, typically drawing just 0.3–0.8A. A 10Ah lithium battery will give you 15 to 20 hours of runtime, easily covering a full weekend trip.

This is where many kayak and ice anglers start—and for good reason. With a 10Ah 12.8V LiFePO4 pack from National Battery Supply, you get a grab-and-go setup that’s ultralight, reliable, and built for cold weather.

If you’re fishing with a Humminbird Helix 5, Garmin Echomap 6 or 7″, or a Lowrance Elite 5, you’re drawing a bit more juice which is usually between 0.8A and 1.5A. These screens are brighter, include GPS mapping, and often run side-imaging sonar.

For this tier, 10Ah gets you by, but 15Ah is the smart play. It adds a safety buffer for brightness, GPS, cold mornings, or even a small nav light. Think about that extra power as peace of mind so you won’t have to squint at a dying screen or bail early.

Many anglers report running 10Ah batteries “for years” on mid-sized units. But those same folks admit they’d size up next time. And with National Battery Supply stocking 12Ah, 15Ah, and 18Ah LiFePO4s, stepping up is easy.

Battery Size for 9″ and Live Sonar Units

Let’s talk about the top players: Garmin GPSMAP 9″, Lowrance Elite FS 9, and live sonar systems like Garmin LiveScope or Lowrance ActiveTarget. These setups can draw 3–5A total, especially if you’re running dual screens.

That means you need serious capacity: 30Ah–50Ah lithium batteries are your go-to. A 30Ah pack typically gives 8–10 hours with a single graph and LiveScope. Add another unit, and you’re looking at 50Ah to stay powered all day.

SLA vs Lithium: What’s Worth It?

Lead-acid (SLA/AGM) batteries are cheap and everywhere. But they’re heavy, offer only about 50% usable capacity, and can suffer from voltage drop under load. Lithium (LiFePO4) costs more upfront but pays off fast with 80–90% usable capacity, better performance in cold weather, and 3–5x the cycle life.

Plus, lithium holds voltage steady. That means no flickering or shutdowns when you’re mid-scan. And if you’re on a kayak or carrying your gear long distances? Dropping 5–10 pounds is no joke.

Choose Your Battery Size Now So Your Screen Never Dies Mid-Trip

The right battery for your fish finder isn’t just about volts. It’s about matching draw, runtime, and real-life conditions. Go too small, and your trip gets cut short. Go too big, and you carry unnecessary weight. But get it just right? You’ll fish longer, charge less, and never stare at a dead screen again.