In this article, we will explore the fundamental differences in the lifespan of Lead Acid and Lithium batteries, and guide you in calculating the Amp hours needed for your desired runtime.
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Lead Acid Batteries
Lead-acid batteries have been a staple in marine applications for years. However, they have limitations compared to lithium batteries:
Cycle Life:
A typical lead-acid marine battery has a lifespan of about 500 charge-discharge cycles1. These cycles represent the number of times you can fully charge and discharge the battery before its performance starts to degrade.
Depth of Discharge (DoD):
Lead-acid batteries should not be discharged below 50% of their rated capacity. Going beyond this limit can significantly reduce their lifespan. This will be important when selecting the proper capacity for your electronics.
Understanding Lead Acid Battery Capacity:
Battery capacity is typically measured in amp-hours (Ah). This rating indicates how much current a battery can supply over a specific period. For example, a 100Ah battery can theoretically provide 1 amp of current for 100 hours or 10 amps for 10 hours.
Calculating Battery Runtime Step-by-Step:
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Determine the Total Current Draw:
- List all the electronic devices you plan to use and note their current draw in amps (A). This information is usually found in the device’s manual or in our article on power draw.
- Add up the current draw of all devices to get the total current draw.
Example: - Fish Finder: 1.5A
- GPS: 0.8A
- VHF Radio: 1.2A
- Total Current Draw: 1.5A + 0.8A + 1.2A = 3.5A
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Calculate the Desired Runtime:
- Determine how long you want to run your electronics on a single charge. This will be your runtime in hours (h).
Example: 8 hours
- Determine how long you want to run your electronics on a single charge. This will be your runtime in hours (h).
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Calculate the Required Battery Capacity:
- Use the formula: Battery Capacity (Ah) = [Total Current Draw (A) × Desired Runtime (h)]
Example: - Battery Capacity = 3.5A × 8h = 28Ah
- Use the formula: Battery Capacity (Ah) = [Total Current Draw (A) × Desired Runtime (h)]
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Consider Battery Efficiency and Safety Margin
- Lead acid batteries should not be discharged below 50% to prolong their lifespan. Therefore, you need to double the calculated capacity to ensure you don’t over-discharge the battery.
- Also, consider a safety margin to account for inefficiencies and unexpected power needs (this is optional, but generally recommended). Lead Acid batteries have roughly an 80% efficiency, which means only about 80% of the energy stored can be retrieved.
Example: - Adjusted Battery Capacity = 28Ah × 2 = 56Ah
- Adding a 20% safety margin: 56Ah × 1.2 ≈ 67Ah
Choosing the Best Battery
Given the calculations above, a lead acid battery with a minimum capacity of 67Ah is recommended to run your marine electronics for 8 hours without risking over-discharge. Opting for a battery with a larger capacity is advisable to enhance reliability and increase its lifespan.
Practical Tips
- Maintenance: Lead acid batteries require regular maintenance, including checking the electrolyte levels and ensuring the terminals are clean and corrosion-free.
- Charging: Use a quality marine battery charger to ensure your battery is charged correctly and efficiently.
Lithium Batteries
Lithium batteries, especially the LiFePO4 (Lithium Iron Phosphate) type, offer several advantages for marine use:
Longevity:
A lithium-ion battery can last up to ten times longer than a standard lead-acid battery. With as many as 5,000 charge-discharge cycles, lithium batteries are a durable choice for marine applications.
Depth of Discharge (DoD):
Unlike lead-acid batteries, lithium batteries can be safely depleted to 100% of their capacity. This flexibility allows you to utilize more of the stored energy without compromising longevity.
Understanding Lithium Battery Specifications
Before we can jump to calculating our runtime, it is vital to understand the key specifications of lithium batteries:
- Capacity (Ah): The capacity of a battery is measured in ampere-hours (Ah) and indicates how much charge the battery can store.
- Voltage (V): The voltage of a battery is the potential difference between its terminals.
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Energy (Wh): The energy stored in a battery is measured in watt-hours (Wh) and is calculated as:
- Energy (Wh) = Capacity (Ah) × Voltage (V)
Calculating Battery Runtime Step-by-Step:
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Determine the Power Consumption of Your Electronics
- Identify the Current (Amps): Determine the current in amperes (A) that your device or circuit is using. This information is usually provided in the device’s manual or in our article on power draw.
- Determine the Voltage (Volts): Find out the voltage (V) of the power source (battery). This is typically 12V for marine electronics.
- Apply the Formula: Multiply the current (in amps) by the voltage (in volts) to get the power (in watts).
- Example: A SOLIX 10 has a listed power draw of 2.5A. To see this in Watts, we multiply 2.5 × 12 (battery voltage) to get 30W.
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Calculate the Total Power Consumption
- Add up the power consumption of all the devices you plan to run simultaneously. For example, if you have a GPS unit that consumes 10W, a fish finder that consumes 15W, and a radio that consumes 5W, the total power consumption is:
- Total Power Consumption = 10W + 15W + 5W = 30W
- Add up the power consumption of all the devices you plan to run simultaneously. For example, if you have a GPS unit that consumes 10W, a fish finder that consumes 15W, and a radio that consumes 5W, the total power consumption is:
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Efficiency
- Lithium and LiFePO4 batteries have between 90-95% efficiency. This means that up to of 95% of the energy stored in the battery can be retrieved. This is important when calculating a realistic runtime for your battery.
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Calculate the battery Runtime
- To calculate the runtime, you need to know the total energy available in your battery and the total power consumption of your devices. The formula to calculate runtime is:
- Runtime (hours) = [Battery Capacity (Wh) ÷ Total Power Consumption (W)] × Efficiency (%)
Example: If you have a 12V LiFePO4 battery with a capacity of 100Ah, the total energy available is 1200Wh (12V × 100Ah).
Using the total power consumption earlier (30W), let's calculate our runtime hours:
Runtime (hours) = [1200Wh ÷ 30W] × 95% = 38 hours
- Runtime (hours) = [Battery Capacity (Wh) ÷ Total Power Consumption (W)] × Efficiency (%)
- To calculate the runtime, you need to know the total energy available in your battery and the total power consumption of your devices. The formula to calculate runtime is:
Important! Remember that these calculations provide estimates, and actual runtime may vary based on factors like temperature and device usage. Always refer to the manufacturer’s specifications for accurate information.
Resources:
https://www.marineelectricsystems.net/
https://batteryuniversity.com/
