What size battery do I need?

What size battery do I need?

Choosing the optimal lithium battery for your portable power needs can feel daunting. However, this guide simplifies the process, empowering you to make an informed decision.

Step 1: Identify Your Power Requirements

  • Device Inventory: Start by creating a comprehensive list of all electronic devices you intend to power with the lithium battery. This could include laptops, cameras, medical equipment, or even a portable refrigerator.
  • 12V DC Current: DC appliances are simple, requiring only the current. Typically, the product manual provides the device's amperage per hour (Ah).
  • Wattage Rating for 220v AC appliances: Locate the wattage rating (W) for each device. This information is typically printed on a sticker affixed directly to the device. Wattage represents the device's power consumption rate.

12V DC Current
Wall Fan 0.5A
Ceiling Lights 1.3A
Phone 1A
Water Pump 6A
220V AC Power
Laptop 45W
Blender 200W
Drone Charger 40W


Step 2: Considering Desired Runtime

  • Runtime Needs: Evaluate the desired runtime (in hours) for your devices on a single battery charge. Consider your typical usage scenarios, such as camping trips, extended work sessions, or powering medical devices during travel. The desired runtime will directly influence the battery capacity you necessitate. Generally, longer runtimes necessitate a larger battery capacity, measured in Amp-hours (Ah).
12V DC Current Hours per day
Wall Fan 0.5A 10h
Ceiling Lights 1.3A 10h
Phone 1A 3h
Water Pump 6A 1h
220V AC Power Hours per day
Laptop 45W 1.5h
Blender 200W 2 minute
Drone Charger 40W 0.5h


Step 3: Estimating Battery Capacity

Now calculate the total power needs per appliance per day. With 220v AC appliances you will need to use an inverter which isn't as efficient as DC, for our example the inverter efficiency is rated at 85% (0.85). This needs to be added to your 220v AC calculations. The table below is an example which includes the formula as well:

12V DC Current Hours per day Calculations Total Daily Need
Wall Fan 0.5A 10h 0.5A*10h 5Ah
Ceiling Lights 1.3A 10h 1.3A*10h 13Ah
Phone 1A 3h 1A*3h 3Ah
Water Pump 6A 1h 6A*1h 6Ah
220V AC Power Hours per day Calculations Total Daily Need
Laptop 45W 1.5h 45W x 1.5h/12V/0.85 6.6Ah
Blender 200W 2 minute 200W x 0.03h/12V/0.85 0.5Ah
Drone Charger 40W 0.5h 40W x 0.5h/12V/0.85 6.6Ah
AC+DC Total 40.7Ah

According to this example, the total daily requirement is 40.7Ah. In this case, you could opt for a 100Ah battery, ensuring you have enough power for two days without needing to recharge.

Important Note: This calculation provides a preliminary estimate. Actual runtime may differ based on the devices' actual power consumption patterns.

Step 4: Selecting a Battery with a Buffer

  • Accounting for Surges: It's prudent to choose a battery with a slightly higher capacity (Ah) than your initial estimate. This buffer provides additional runtime and accommodates potential power surges from your devices when they first start up.

Bonus Tip: Different lithium battery chemistries (e.g., LiFePO4) may have varying usable capacities. Research your chosen battery type to understand its typical capacity range.

By following these steps, you'll be equipped to select the ideal lithium battery for your portable power needs, ensuring a reliable and efficient power source.


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