Battery Run Time Equations

While there are lots of things that can affect the run time of a fridge on a battery, We use a simple equation to provide us with an approximate run time of a fridge in good conditions, and it works reasonably well if you don't happen to know the exact draw of a fridge from having a shunt wired into your battery system.

EXAMPLE

a 40L fridge will use approximately its capacity in amp hours over a 20 hour period. This test becomes more inaccurate as the fridge capacity gets bigger.

40L fridge = 40ah per 24 hours / 100ah battery = 2.5 days of run time.

To estimate how long a fridge or other accessories will run on a 12V lithium battery, you need to consider the wattage of the appliance and the capacity of the battery (in amp-hours, or Ah). Here's a general guide to help you calculate the runtime for different capacities:

Key Information

1. Appliance Power Consumption: Check the wattage (W) of your appliance. For example:

   • A typical portable fridge might use around 50-70W.
   • Other accessories, like lights or fans, vary widely (e.g., 5-20W).

2. Battery Capacity: Common capacities for lithium batteries are:

   • 100Ah
   • 150Ah
   • 200Ah
   • 300Ah

Calculating Runtime

Formula:

1. Convert wattage to amps:
   $\text{Amps} = \frac{\text{Watts}}{12V}$
2. Calculate runtime:
   $\text{Runtime (hours)} = \frac{\text{Battery Capacity (Ah)}}{\text{Amps}}$

Examples

Example 1: Portable Fridge (60W)

1. Power Consumption: 60W
2. Conversion to Amps:
   $\frac{60W}{12V} = 5A$

• 100Ah Battery:
  $\frac{100Ah}{5A} = 20 \text{ hours}$

Example 2: LED Lights (10W)

1. Power Consumption: 10W
2. Conversion to Amps:
   $\frac{10W}{12V} \approx 0.83A$

• 100Ah Battery:
  $\frac{100Ah}{0.83A} \approx 120 \text{ hours}$

Additional Considerations

• Efficiency: Remember that inverter efficiency (if using an inverter) and battery discharge rates can affect runtime. Typically, assume about 80-90% efficiency.
• Depth of Discharge (DoD): Lithium batteries can generally be discharged more deeply than lead-acid, but it's good practice to avoid going below 20% to prolong battery life.

Feel free to adjust the wattage and battery capacity to fit your specific needs!