Choosing the right battery capacity helps ensure your fish finders, GPS, radios and other marine electronics can operate for the time you need. This guide compares lead-acid and lithium batteries and explains how to estimate the battery capacity and runtime needed for your electronics.
Jump To:
- Lead-Acid Batteries | Cycle Life | Depth of Discharge | Understanding Amp-Hour Capacity | Calculate Lead-Acid Battery Capacity | Lead-Acid Battery Tips
- Lithium Batteries | Understanding Lithium Battery Specifications | Calculate Lithium Battery Runtime
- Battery Runtime Considerations
- Resources
Lead-Acid Batteries
Cycle Life | Depth of Discharge | Understanding Amp-Hour Capacity | Calculate Lead-Acid Battery Capacity | Lead-Acid Battery Tips
Lead-acid batteries have long been used to power marine electronics. When estimating runtime, consider both the battery's rated capacity and how much of that capacity should be used.
Cycle Life
A typical lead-acid marine battery may provide approximately 500 charge-discharge cycles before its performance begins to decline.
Depth of Discharge
To help preserve battery life, the article recommends avoiding discharge below 50% of the battery's rated capacity.
This means the full advertised amp-hour capacity should not be treated as usable capacity when sizing a lead-acid battery.
Understanding Amp-Hour Capacity
Battery capacity is measured in amp-hours (Ah).
For example, a 100Ah battery could theoretically supply:
- 1 amp for 100 hours
- 10 amps for 10 hours
Actual runtime can vary based on operating conditions and battery performance.
Calculate Lead-Acid Battery Capacity
1. Determine Total Current Draw
Find the current draw in amps for each electronic device you plan to operate at the same time. This information can typically be found in the product specifications or power-draw information.
Example:
- Fish finder: 1.5A
- GPS: 0.8A
- VHF radio: 1.2A
- Total current draw: 3.5A
2. Determine Desired Runtime
Decide how long you want to operate the electronics.
Example: 8 hours
3. Calculate Required Capacity
Use this formula:
Battery Capacity (Ah) = Total Current Draw (A) × Runtime (hours)
Using the example:
3.5A × 8 hours = 28Ah
4. Account for Lead-Acid Depth of Discharge
Because the article recommends using no more than approximately 50% of a lead-acid battery's rated capacity, double the calculated requirement:
28Ah × 2 = 56Ah
Adding the article's example 20% safety margin:
56Ah × 1.2 ≈ 67Ah
For this example, the recommended minimum is therefore approximately a 67Ah lead-acid battery.
Important: These calculations are estimates. Actual battery runtime can vary with battery condition, temperature, equipment usage and other operating conditions.
Lead-Acid Battery Tips
- Maintenance: Check electrolyte levels when applicable and keep battery terminals clean and free of corrosion.
- Charging: Use a suitable marine battery charger and follow the battery manufacturer's charging requirements.
- Capacity: Choosing additional capacity can provide more operating margin.
Lithium Batteries
Understanding Lithium Battery Specifications | Calculate Lithium Battery Runtime
Lithium batteries, including LiFePO4 (Lithium Iron Phosphate) batteries, have different usable-capacity and efficiency characteristics than lead-acid batteries.
According to the source material:
- Lithium batteries may provide up to 5,000 charge-discharge cycles.
- They can utilize more of their rated capacity than lead-acid batteries.
- The stated efficiency range is approximately 90% to 95%.
Always follow the battery manufacturer's specifications for allowable depth of discharge and operating limits.
Understanding Lithium Battery Specifications
Three specifications are particularly useful when estimating runtime:
- Capacity (Ah): The amount of charge stored by the battery.
- Voltage (V): The battery's electrical potential.
- Energy (Wh): The total stored energy expressed in watt-hours.
To calculate watt-hours:
Energy (Wh) = Capacity (Ah) × Voltage (V)
For example:
100Ah × 12V = 1,200Wh
Calculate Lithium Battery Runtime
1. Determine Device Power Consumption
If a device's power consumption is provided in amps, convert it to watts:
Power (W) = Current (A) × Voltage (V)
For example, a device drawing 2.5A at 12V uses:
2.5A × 12V = 30W
2. Add the Power Draw of All Devices
Add together the wattage of every device that will operate simultaneously.
Example:
- GPS: 10W
- Fish finder: 15W
- Radio: 5W
- Total: 30W
3. Calculate Battery Energy
For a 12V, 100Ah battery:
12V × 100Ah = 1,200Wh
4. Estimate Runtime
Use:
Runtime (hours) = [Battery Energy (Wh) ÷ Total Power Consumption (W)] × Efficiency
Using the example and 95% efficiency:
[1,200Wh ÷ 30W] × 0.95 = 38 hours
The estimated runtime is approximately 38 hours.
Battery Runtime Considerations
Battery runtime calculations provide an estimate rather than a guaranteed operating time.
Actual runtime may vary based on factors such as:
- Battery type and condition
- Temperature
- Actual equipment power consumption
- Number of devices operating
- Battery capacity
- Battery efficiency
- Depth of discharge
Important: Always use the battery manufacturer's specifications for charging, usable capacity, depth of discharge and other battery-specific requirements.
