Ampère-hora vs. Watt-hora - Entendendo a diferença entre as baterias

Batteries are one of the most important components in an electrical system. Whether it’s your RV, golf cart, or boat, they all depend on batteries. Therefore, when you are shopping for a battery for any of these situations, you must understand the capacity of a battery. Amp-hours and Watt-hours are units used to measure the capacity of a battery. But, many people need to learn the difference between the two or what they represent. In this article, we will look at Amp hours and Watt hours individually and compare them with one another. 

Understanding Capacity

Capacity is the most important quantifiable characteristic of a battery. It is important that you understand how to measure the capacity of a battery and what its rating means. Capacity is how much power a battery can deliver or the total amount of energy extracted from a battery. This number is represented by Watt hours or Wh, and it is the capacity of a battery. But what are Amp hours, then? Many times, people need clarification on these two: watt hours and Amp hours.

The total capacity of a battery in Watt-hours is calculated by multiplying voltage, ampere, and hours. We can also write it as voltage multiplied by amp-hours. In this equation, there is Amp-hours, or Ah, which is what is often printed on the battery label. The answer from this calculation is theoretical. In reality, the battery’s capacity is less than this number due to several reasons. 

Understanding Watt Hours (Wh)

Watt hours or Wh is the measure of the total energy capacity of a battery, considering both voltage and current. It is a useful measurement tool for comparing different voltage ratings batteries. It is usually written as a “1-watt-hour” battery. For example, suppose a battery is rated as 120Wh. In that case, the battery can transfer 120 Watts of total energy it delivers. When comparing batteries with different voltages, we convert amps to watts for accurate comparison. This is because Watt hours give you a more comprehensive view of a battery’s capacity to deliver energy compared to Amp hours.

Calculation Of Watt Hours

Formula: Wh = Voltage (in volts) x Ah

Example: If a 12-volt battery has the capacity of 100 Ah, then its total energy capacity or Watt hours will be 12×100=1200 Wh.  

Suppose a 24-volt lithium-ion deep cycle battery with a 200 Ah capacity rating. In that case, it will have a total of 4800 watt-hours of energy before reaching its specified endpoint voltage.

Understanding Amp Hours (Ah)

Ampere hours or Ah is the measure of the capacity of a battery to provide a certain amount of current (amperes) over a specific period (usually hours). It is a measurement tool that is used to understand how many amps you can discharge a battery over a certain amount of time. It is usually written in a 1-hour amp rate on a battery. For example, if a battery has a rate of 7Ah, then that means that it can transfer 1 amp of current for 7 hours. After 7 hours, the battery will be at 100% depth of discharge or 0% state of charge. It is the point where the manufacturer has declared that the battery is empty and needs to recharge.

C-Rating

It is important to note that batteries have different C-rates, which tells how a battery runs. Depending on the C-rate, a 7Ah battery could also transfer 7 amps of current for 1 hour instead of 1 amp for 7 hours.

C-rate is the rate at which the battery charges or discharges. This C-rate for a battery differs from manufacturer to manufacturer. A 1-C rating means battery charges and discharges in 1 hour. So if a 20Ah battery has a rate of 1-C, then it charges or discharges at a rate of 20amps for 1 hour. 

Usually, batteries have a rating of 0.5-C or even lower. So, if you want to estimate a device’s run time on battery power, find out the C-rating. 

Calculation Of Amp Hours

Formula: Ah = Current (in amps) x Time (in hours)

Example: If a battery supplies 5 Amps of current for 20 hours, then its Amp hour rating would be 5×20=100 Ah. 

Deep cycle marine batteries have an Ah rating of 50Ah to 300Ah, depending on the size and power needs.        

Key Differences Between Ah and Wh

  • Amp hours is the measure of the battery’s capacity on the basis of how much current it can deliver over time. 
  • Watt hours is the measure of the total energy capacity of the battery, considering both voltage and current. 
  • Amp hour reflects the size of the battery and its capacity. It gives an idea of how long a battery can sustain a load. 
  • Watt hours is a comprehensive understanding of the battery’s energy storage capacity. 
  • Amp hours give us an understanding of the size and capacity of the battery. 
  • In contrast, watt hours give us an accurate measure of the battery’s energy content.

Factors Affecting Ah and Wh

Temperature

Temperature has a significant effect on the capacity of a battery. Therefore, it also affects the Amp hour and Watt hour ratings. High temperature decreases the capacity of a battery. This reduces its Amp hours and Watt hours ratings. Similarly, low temperature also lowers the Ah and Wh ratings. Up to 20% capacity decreases at temperatures below freezing point, affecting the Ah and Wh ratings. 

Discharge rate

The discharge rate is how quickly a battery discharges and this affects both the amp hours and watt hours ratings. A higher discharge rate causes internal resistance in the battery, reducing its efficiency. Therefore, it reduces the Amp hours and Watt hours ratings. Batteries that discharge at a higher rate show lower usable capacity than those discharged at a lower rate. 

Conclusão

Understanding both Amp hours and Watt hours is important to estimate the battery runtime accurately. It gives us an idea of how suitable the battery is for certain applications. By learning the concept of both Ah and Wh, users can make informed decisions when buying batteries according to their specific requirements.

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