Tacklebox ’n Tools
Test version

Fish Finder Battery Life Calculator

Find out how long a battery runs your fish finder, and the size to buy for a kayak, a jon boat or the ice.

How to use it

  1. Pick your fish finder. The amp draw fills in from the maker's spec. Type your own number if your manual lists a different one.
  2. Add anything else on that battery. Forward-facing sonar, a phone charger or lights all count.
  3. Enter your battery. Pick the type and the amp-hours on its label. Not bought one yet? Leave the example and see what the results recommend.
  4. Read the results. Run time, the battery size for your day, how common sizes compare, the wire size and what to buy.

Your fish finder

A
units
A total
A phone charger is roughly 1 A. Small LED lights are a few tenths of an amp.

Your day

hours
Count the hours the fish finder is switched on.

Your battery

Type
Most small fish finder batteries are sealed lead-acid or lithium. Lithium weighs much less and can be used more deeply.
Ah

Your wiring

feet, one way
Measure along the path the cable takes.

How the numbers are worked out

Amp draw

Each model's figure comes from the manufacturer's specification, listed under Sources. Some makers publish a typical draw and some publish the maximum with the screen at full brightness, so a few of these numbers lean high. Total draw is fish finder amps × units + sonar + other devices.

Forward-facing sonar uses Garmin's 21 W typical figure for LiveScope Plus, about 1.75 A at 12 V. The display that shows it is counted separately as your fish finder.

Run time

Run time is usable amp-hours ÷ total amps. Lead-acid batteries give fewer amp-hours under heavier draw, so the rating is adjusted with Peukert's law (k = 1.15 for SLA and AGM rated at the 20-hour rate, 1.05 for lithium rated at the 5-hour rate) and never credited above the label.

Wire size

Wire is sized for no more than 3% voltage drop with the ABYC formula circular mils = 10.75 × amps × round-trip feet ÷ allowed volts dropped, the same method as our Trolling Motor Power Calculator. Fuse size comes from your fish finder's manual.

What it leaves out

  • Cold weather, which cuts lead-acid capacity noticeably on the ice. Plan a bigger battery for winter.
  • Battery age. An older battery holds less than its label says.
  • Screen brightness and sonar settings, which change the draw a little.

Planning estimates. Follow your fish finder's manual for wiring and fuse size. Charge lithium batteries only with a charger made for lithium.

Sources