One of the easiest ways to plan a fishing electronics setup is to start with the device's power consumption, not the battery's marketing label. Two batteries with the same amp-hour (Ah) rating can give different results when their voltages, chemistry, and operating conditions differ.
Step 1: Find the real power draw
Look for a figure in watts (W) or current in amps (A) in the device manufacturer's specifications. If the device is rated in amps at a specific voltage, multiply the figures to estimate watts:
Watts = volts × amps. As one real example, the Lowrance Eagle Eye 9 product page lists 650 mA at 12 V DC with the backlight on full. That converts to 0.65 A × 12 V = 7.8 W. The figure is specific to that listing; check your model and revision before relying on it.
Official Lowrance Eagle Eye 9 specifications ↗
Step 2: Translate battery size into watt-hours
A battery rated at 20 Ah and 12.8 V stores approximately 256 watt-hours (Wh) of nominal energy. But usable energy depends on how deeply you plan to discharge the battery. For example, using an illustrative 80% of that energy means 204.8 Wh before other reductions.
Conservative planning assumptions are different from absolute battery limits. Follow the battery manufacturer's published depth-of-discharge guidance. Lead-acid batteries commonly need a more conservative planning allowance to preserve longevity, whereas LiFePO₄ designs can often use more of their nominal capacity. Neither chemistry has a universal usable percentage.
Step 3: Allow for real-world conditions
- Cold weather: battery capacity and voltage performance can change with temperature and load. LiFePO₄ batteries may restrict charging in freezing conditions unless designed for it.
- Other equipment: lights, GPS accessories, wireless modules or a second screen add load.
- Voltage cutoff: your display may switch off before the battery reaches the energy percentage assumed in a simple calculation.
- Wiring and connections: undersized wire, corrosion and loose connections can cause losses and voltage drop.
Example: 20 Ah battery + Eagle Eye 9
| Example input | Value |
|---|---|
| Battery rating | 20 Ah at 12.8 V |
| Planning usable percentage | 80% |
| Additional condition adjustment | 100% |
| Energy-delivery assumption | 95% |
| Fish finder draw | 7.8 W |
| Estimated runtime | 24.9 hours |
The math is (20 × 12.8 × 0.80 × 1.00 × 0.95) / 7.8 = about 24.9 hours. Actual runtime may differ substantially, especially in cold weather or with a battery that limits output.
Step 4: Size for your trip, then add margin
Reverse the equation to estimate how many amp-hours would support your desired number of hours. For a 10-hour trip using the same assumptions, the formula returns about 8.0 Ah. That is a theoretical minimum under those assumptions, not a recommended shopping size. Choose a compatible battery with extra margin, confirm its operating temperature and current limits, and consider the consequences of losing sonar power on the water.
Before connecting any battery
- Confirm the voltage range printed in the device manual and avoid connecting an incompatible power source.
- Use the manufacturer's required fuse and safe wiring practices; place circuit protection as instructed.
- Check polarity twice, protect exposed terminals, and secure the battery against movement and moisture.
- Test the complete system under load before leaving home.
Use the runtime calculator to compare your own devices, batteries and planning assumptions. It runs in your browser and doesn't collect your entries.