# Battery size calculator

The battery size calculator calculates the battery size in ampere-hour (Ah).

## 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 (Vdc): Specify the battery voltage in volts DC, 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.
• Remaining charge (%): Specify the required remaining charged. To prolong the life of a battery, a lead-acid battery should not frequently be discharged below 50 %, and Lithium-ion battery not below 20%. Note that 0% is a flat battery and 100% if a full battery.

## How to calculate battery current?

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

$$I=\dfrac{P}{V_{dc}}$$

Where:

• P is the power in watts.
• Vdc is the voltage in volts DC.
• ## How to calculate the battery size for a Lithium-ion battery?

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

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

Where:

• I is current in ampere.
• t is the duration in hours.
• Q is the required remaining charge in percentage.

## How to calculate the battery size for a Lead-acid battery?

The required battery size $$B_{lead-acid}$$ for lead-acid batteries is calculated 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.
• Q is the required remaining charge in percentage (%).

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.