Sizing Your Off-Grid, RV, Caravan or Marine Solar Batteries

Introduction

This knowledge article focuses on battery fundamentals to help inform your battery selection. Battery types divide into two major categories: starting batteries and solar batteries, with the emphasis here on deep cycle solar batteries for off-grid systems.

1. Deep Cycle Battery Types

Deep cycle lead acid batteries come in two primary formats:

  • Flooded lead acid: Dating to the 19th century, these are the most common market option. They use an electrolyte as the conduction medium and require periodic monitoring and top-ups.

  • Sealed lead acid: These are maintenance-free and include gel type and absorbent glass mat (AGM) varieties.

2. Selecting Your Battery

Your first consideration is determining system voltage (Vsystem). Most marine installations use either 12 or 24 volts. Larger systems with inverters may benefit from higher voltage configurations. The advanced battery section covers system voltage selection in greater depth.

3. Sizing Your Battery

Battery sizing depends on multiple factors including solar array size, charge controller size, and appliance requirements. Calculate total power consumption by multiplying appliance voltage and current draw, or reading the wattage label directly. Multiply watts by daily operating hours to determine watt-hours (Wh).

Calculation Examples

100% Discharge (not recommended): Ah = Total power consumption / Vsystem Example: 100W / 12V = 5 Ah

30% Depth of Discharge (recommended): Ah = Total power consumption / (DoD x Vsystem) Example: 100W / (0.3 x 12V) = 28 Ah

A 20-watt fridge running 5 hours daily requires 100 Wh of storage. Never discharge batteries completely, as this significantly reduces lifespan.

4. Preserving Battery Life

Several factors extend battery longevity:

Temperature: Higher ambient temperatures reduce cycle count. Keep batteries in cool, well-ventilated environments when possible.

Depth of Discharge: This dramatically affects cycle count. Example data shows one battery type achieving 1600 cycles at 30 percent discharge and just 200 cycles at 100 percent discharge. Leaving batteries discharged long-term may prevent recharging entirely.

Solar Array Sizing: Panels should easily replenish the battery daily, minimizing depth of discharge for maximum lifetime.

Proper Charging: Select chargers appropriately sized for your battery and type (flooded, gel, or AGM all have different charging characteristics). Slow charging can cause depth of discharge issues; fast charging increases temperature and reduces lifespan.

Contact Information

For questions or feedback, contact: info@luxsolar.co.nz