# Battery size calculator

The battery size calculator calculates required the battery rating in ampere hour (Ah) based on load, duration and discharge level.

Parameters:

• Load type (ampere or watt): Select the unit in which the load will be specified, ampere or watt.
• Load (watt): If the load type is watt, specify the load in watt, for example 100 W. Use and average value if its a cyclical load.
• Load (ampere): If the load type is ampere, specify the current in ampere, for example 10 A.
• Voltage (V): Specify the battery voltage, if the load type is watt.
• Required duration (hours): Specify the duration that the load must be supplied for.
• Battery type:  Select the battery type,  lead-acid or lithium-ion.

• Required battery size (Ah): The recommend battery size in Ah.

Current calculation:

• If the load is specified in watt, the current I is calculated as:

$$I=\dfrac{P}{V}$$

where, P is the power in watts, and V is the voltage in volts.

Lithium-ion batteries:

• The required battery size $$B_{li-ion}$$ for litium-ion batteries is calculated by the battery size calculator as:

$$B_{li-ion}=\dfrac {100 \cdot I \cdot t}{100 – Q}$$

where, I is current in ampere, t is the duration in hours and Q is the required remaining charge in percentage.

Lead-acid batteries:

• The battery rating of a lead-acid batteries is proportional to the discharge rate. They are typically rated for a discharge rate over 20 hours. For example, a 20 Ah battery can deliver 1 A for 20 hours, which amounts to 20 Ah.
• The rating of the battery reduces by approximately 62% if it is discharged in 1 hour. For example, if the same 20 Ah battery is discharged in one hour, it can only deliver 12.4 A for one hour, which amounts to 12.4 Ah. The battery size calculator derates the battery linearly based in this characteristic.
• The required battery size $$B_{lead-acid}$$ for lead-acid batteries is calculated by the battery size calculator as:

$$B_{lead-acid}=\dfrac{100 \cdot I \cdot t}{(100-Q) \cdot (0.02 \cdot t+0.6)}$$

where, I is current in ampere, t is the duration in hours, and Q is the required remaining charge in percentage.