Deep Cycle Battery Size Calculator

Deep cycle batteries are specially designed to provide consistent power over long periods by discharging slowly and deeply, unlike car starting batteries which provide quick bursts. They are commonly used in solar power systems, RVs, boats, off-grid cabins, and backup power systems.

A deep cycle battery’s key specifications include:

• Voltage (V) — commonly 12V, 24V, or 48V systems.
• Amp hour (Ah) capacity — indicates battery storage capacity.
• Depth of discharge (DoD) — the percentage of battery capacity you can safely use without damaging battery life, typically around 50% for lead-acid and up to 80-90% for lithium batteries.

Why Is Generator Size Important for Charging Deep Cycle Batteries?

The right generator size is essential for efficiently charging deep cycle batteries without overloading the generator or harming the batteries.

• If the generator is too small, it cannot supply enough power to recharge the batteries fully or timely.
• Oversizing the generator can lead to inefficiencies and increased costs.

A generator must be sized to:

• Match the voltage system of your battery bank.
• Provide sufficient wattage to power your loads and charge the battery.
• Support the charger’s power requirements that convert generator AC power to DC for the batteries.

Step-by-Step Guide to Sizing a Generator for Deep Cycle Batteries

1. Calculate Your Energy Needs

To size your generator, start by determining the total energy consumption and charging needs:

• Daily energy consumption (Wh or kWh): Identify all devices and loads powered by the battery, and estimate their daily consumption.
• Battery capacity (Ah): The total amp hours in your battery bank at system voltage.
• Days of autonomy: How many days you want to run without recharging (solar/wind or generator).
• Depth of Discharge (DoD): Typically, design for about 50% discharge for lead-acid batteries to increase lifespan.

Example: If you need 6000 Wh/day and have a 48V system, with 50% DoD and 3 days autonomy:Battery capacity needed (Ah)=Daily Watt-hours×Days of autonomySystem Voltage×DoD=6000×348×0.5=750 AhBattery capacity needed (Ah)=System Voltage×DoDDaily Watt-hours×Days of autonomy=48×0.56000×3=750 Ah

This guides the battery bank size.

2. Determine the Charger Size

Your battery charger is an essential element to convert generator power to battery charging current.

• Chargers are rated in amperes (A).
• A common rule: charger current should be 10%-30% of the battery bank’s Ah capacity.

Example: For a 750 Ah battery bank, a charger rated ~75-225 A would be typical.

How to Match Generator Power Output to Charger and Battery Requirements

Generators are rated by their wattage (W) or kilowatts (kW). The generator must power the charger and any additional loads at the same time.

• The charger power in watts = charger voltage × charger current.
• Add any simultaneous loads powered from the generator.

Example: For a 48V charger delivering 100 A:Charger Power=48 V×100 A=4800 WCharger Power=48 V×100 A=4800 W

Add a safety margin (often about 20-30%) to account for efficiency losses and surge capacity; thus, a generator around 6000W (6 kW) would be needed.

General Recommendations for Generator Sizes by Battery System Voltage and Amp Hours

Battery Bank Voltage	Approximate Battery Bank Capacity (Ah)	Generator Size Recommendation (Continuous Watts)	Notes
12V	100-200 Ah	1,200 W – 3,000 W	Smaller, portable use or small solar setups
24V	200-400 Ah	3,000 W – 5,000 W	Larger RV or off-grid cabin setups
48V	400-1000 Ah	5,000 W – 10,000 W	Larger systems, backup power for homes or larger cabins

What Type of Generator is Best for Charging Deep Cycle Batteries?

• Inverter Generators: Provide stable, clean power ideal for battery chargers and sensitive electronics.
• Portable Generators: Common for small systems but can vary in power output stability.
• Standby Generators: Fixed installations, best for home backup with automatic transfer switches.

Ideally, the generator should have voltage regulation and stable output to ensure the battery charger operates correctly without damaging the battery.

Important Charging Tips When Using a Generator with Deep Cycle Batteries

• Never charge batteries directly from generator 12V output: Most generators provide unregulated and insufficient voltage and current.
• Use a compatible battery charger connected to the generator’s AC outlet.
• Allow generator to warm up before connecting charger.
• Match charger voltage and current to battery bank specs.
• Monitor charging voltage and battery temperature for safety.
• Aim to keep battery charging within recommended depth of discharge limits to maximize battery life.
• Consider a smart charger or charge controller compatible with your battery chemistry (AGM, Gel, Lithium, etc.).

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Example Scenario: Sizing a Generator for a 12V Deep Cycle Battery Bank with 200 Ah Capacity

• Batteries: 12V, 200 Ah (usable capacity at 50% DoD = 100 Ah).
• Daily energy use: 1200 Wh.
• Battery charger current: 20% of 200 Ah = 40 A.
• Charger power: 12 V×40 A=480 W12V×40A=480W (plus charging inefficiencies, estimate ~600–800 W).
• Select generator wattage: at least 1000 W (1 kW) to support charger and some margin.

This setup uses a small portable inverter or gas generator rated around 1-2 kW.

Conclusion: Key Takeaways for Generator Size Selection

• The generator size must be sufficient to power the battery charger at a current compatible with your deep cycle battery bank size and voltage.
• Consider the total wattage needed for the charger plus any simultaneous loads.
• Proper battery bank sizing and correct depth of discharge maximize the efficiency of your generator charging.
• Use regulated chargers and inverter-type generators for safer and efficient battery charging.
• Oversizing the generator slightly is preferable to undersizing.
• Factor in the battery type (AGM, Gel, Lithium) and charger technology.