Battery Size Calculator

Battery capacity calculations revolve around energy consumption over time, typically measured in Ampere-hours (Ah) or Watt-hours (Wh). The foundational relationship between power, voltage, and current is expressed as Application Load = Voltage × Load Current. When determining capacity for a specific duration, we must also factor in the depth of discharge and chemical efficiency. For Lithium-ion batteries, the required capacity formula is:

Remaining Capacity (Li) = (100 × Load Current × Required Duration) / (100 − Remaining Charge)

For Lead-acid (Pb) batteries, the calculation must account for Peukert's law and lower discharge efficiency, represented as:

Remaining Capacity (Pb) = (100 × Load Current × Required Duration) / ((100 − Remaining Charge) × ((0.02 × Required Duration) + 0.6))

Let us walk through a practical scenario: You need to run a 12V direct-current field instrument that draws 5 Amps of load current for a required duration of 10 hours using a Lithium-ion battery system. You want to maintain a safe remaining charge limit (minimum depth of discharge) of 20% to preserve cell health.

Step 1: Calculate total energy demand without safety margins. 5 Amps × 10 hours = 50 Ampere-hours (Ah) of pure consumption.

Step 2: Apply the Lithium-ion capacity formula taking the 20% remaining charge reserve into account. We divide 50 Ah by the usable percentage (100% - 20% = 80%, or 0.80).

Step 3: Compute final capacity: 50 / 0.80 = 62.5 Ah. Therefore, you will need a 12V battery rated at least 62.5 Ampere-hours to safely power your device for 10 hours.

Always factor in temperature fluctuations, as extreme cold reduces available battery capacity significantly. Never drain lead-acid batteries below a 50% depth of discharge regularly if you want to maximize their operational lifespan. Finally, account for inverter efficiency losses (typically 10% to 15%) if you are stepping up DC battery power to AC household voltages.