Question

In battery chemistry I've been reading about specific capacities of various electrochemical cells as mAh/g

. For example, in one article it says the specific capacity of the system is 75 mAh/g at 1.7 V.
Does that mean the a 1 kg
battery would give 75 x 1.7 or 127.5 Watts of energy?

Or is it some electrod-ic cathodic or anodic capacitance. In that case how do we calculate how much Wh/kg of power we would get effectively from a battery?

Answer 1

To calculate the watt-hour per kilogram (Wh/kg) for a specific capacity of a battery, one must convert mAh/g to Ah/kg and then multiply by the voltage, not simply multiply the specific capacity by the voltage. The resulting figure is the energy density of the battery, which is important for comparing storage capabilities but is only one aspect of overall battery performance.

Step-by-step explanation:

Understanding Battery Energy Capacity and Calculation

The specific capacity of an electrochemical cell is measured in milliamp-hours per gram (mAh/g). This metric indicates how much electric charge a battery can store per unit mass. The example you provided mentions a battery system with a specific capacity of 75 mAh/g at 1.7 V. To calculate the total energy storage in watt-hours per kilogram (Wh/kg), we cannot simply multiply the specific capacity by the voltage, but instead, we need a more comprehensive approach.

The proper calculation would involve first converting the specific capacity from mAh/g to Ah/kg (by multiplying by 1000 to account for the kilograms and dividing by 1000 again to switch from milliamp-hours to amp-hours), and then multiplying by the voltage:

(75 mAh/g) * (1 kg / 1000 g) * (1 Ah / 1000 mAh) * 1.7 V = 0.1275 Wh/g, which is equivalent to 127.5 Wh/kg when considering a 1 kg battery.

This calculation gives us the energy density of the battery in watt-hours per kilogram (Wh/kg), a key figure for comparing the storage capabilities of batteries. However, it is important to consider other factors that affect the overall performance and efficiency of a battery, such as charge and discharge rates, temperature, and battery life cycle.

Therefore, while specific capacity is a useful metric, it is one aspect of a battery's performance and is not solely indicative of its overall energy storage capabilities. The energy capacity of batteries is a crucial factor in applications like electric vehicles and portable electronics. When considering the energy density of lithium-ion batteries versus gasoline, the difference in energy content is significant; with gasoline providing much higher energy content per unit mass compared to current battery technologies.