How to Choose the Right Lithium Battery Size for Your Fishfinder
One of the most common questions anglers ask when upgrading electronics is:
“What size lithium battery do I need to run my fishfinder all day?”
To answer that, you’ll need to look at your unit’s power draw in the specifications and calculate how many amp-hours (Ah) are required for your fishing time. Let’s walk through it step by step using a Garmin GPSMAP 1242xsv as the example.
See the typical amp draw for your set up on 12V or 16V.
Step 1: Find the Unit’s Amp Draw
Every Garmin chartplotter lists its power specs under the “Electrical Features” section.
For the GPSMAP 1242xsv, the specs show:
The key number to use is the typical current draw (2.2 amps), that’s how much current the unit pulls under normal operation.
Step 2: Estimate Your Run Time
Decide how many hours you typically fish per day.
Let’s use 8 hours as an example.
Step 3: Calculate Amp-Hours Needed
Multiply your current draw by your expected hours of use:
2.2 amps × 8 hours = 17.6 amp-hours (Ah)
That means the unit will consume roughly 18Ah of battery capacity over an 8-hour day.
If you fish longer, just multiply by your total hours:
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10 hours: 2.2 × 10 = 22Ah
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12 hours: 2.2 × 12 = 26.4Ah
Step 4: Add a Safety Buffer
Electronics rarely run at their exact “typical” draw.
Screen brightness, sonar ping rate, and network accessories can raise power use slightly, and lithium batteries shouldn’t be drained completely.
To ensure reliable operation:
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Add 25–30% extra capacity as a buffer.
So for the GPSMAP 1242xsv:
17.6Ah × 1.3 = ≈23Ah minimum battery size
Step 5: Account for Additional Devices
If you’re also powering a GLS10 Livescope box, network switch, or NMEA accessories from the same battery, you need to add those draws too:
| Device | Typical Draw @12V |
|---|---|
| Garmin GLS10 (Livescope Box) | 3.0 A |
| Garmin Network Switch | 0.5 A |
Combined, that’s 2.2 + 3.0 + 0.5 = 5.7 amps total draw.
Now multiply that by your fishing hours:
5.7A × 8 hrs = 45.6Ah, and then add 30% buffer → ≈60Ah battery recommended.
Step 6: Pick Your Battery Size
Here’s a quick guide based on total load and fishing duration:
| Battery Size | Approx. Run Time for GPSMAP 1242xsv |
|---|---|
| 20Ah | ~8 hours (single unit only) |
| 30Ah | ~12 hours (single unit, safe buffer) |
| 50Ah | ~8 hours (unit + Livescope) |
| 60Ah | ~10 hours (unit + Livescope) |
| 100Ah | Full-day+ power for multi-unit setups |
For most anglers running a single Garmin 12″ display, a 30Ah lithium is plenty.
If you’re running Livescope or multiple graphs, go with 50–60Ah to comfortably last a long day on the water.
Step 7: Verify Voltage Compatibility
Garmin fishfinders run on 10–32 volts, so any 12V lithium (nominal 12.8–13.2V) battery is safe.
Avoid older lead-acid setups that dip below 11V under load, that can cause restarts or screen flickering.
Step 8: Choose a Quality Lithium Battery
When choosing a lithium battery, look for:
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Built-in BMS (Battery Management System)
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Dedicated marine-grade terminals
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Low-temperature cutoff protection if fishing in winter
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Known, tested marine brands rather than budget rewraps
Example Setup
If you run a Garmin GPSMAP 1242xsv + GLS10 Livescope, a 12V 60Ah lithium battery will comfortably power your setup for a full tournament day with room to spare.
