How Is Battery Capacity Measured | Understanding Battery Life

When shopping for a new battery it is important to consider how battery capacity is measured. Find out everything you need to know about determining how much energy your batteries can store. 

Did you know that understanding battery capacity is essential to making informed decisions about which batteries to choose for your RV, golf cart or fishing application? You can’t simply count on buying the biggest battery thinking that it has the most capacity because there are a variety of factors that can impact how much energy is actually stored in a particular battery. 

In this post, we’ll delve into the concept of battery capacity, the factors that influence it and how to estimate your battery needs. Discover the importance of battery voltage and amp-hours, learn about how different battery types have different battery capacities and explore the methods for measuring battery capacity – all to help you make the right choice when it comes to powering your devices. 

Let’s get started!

KEY TAKEAWAYS

• Battery capacity is the total energy produced by a battery’s electrochemical reactions, expressed in watt-hours (Wh) or amp-hours (Ah).
• To estimate how much battery capacity you need for your application you need to add up the power draw and expected daily use for every appliance in the circuit

• Temperature, discharge rate and aging all affect a battery’s performance and should be considered when selecting one to ensure best choice for specific needs.

What Is Battery Capacity 

Battery capacity is defined as the total energy produced by a battery’s electrochemical reactions, which is expressed in either watt-hours (Wh) or amp-hours (Ah). This measurement signifies the electrical energy that a battery can supply when it is at full capacity.

How Do You Measure Battery Capacity

To determine a battery’s capacity, it is essential to understand system and matching battery voltage in or to convert between the two units of measurement, Amp hours and Watt hours. 

Voltage, measured in volts (V), is the electrical potential difference between two points in a circuit and plays a pivotal role in determining a battery’s energy provision. Typically you will know the voltage of the electrical system for your application such that, for example, you might have a 36V trolling motor, a 48V golf cart or a 12V recreational vehicle power system.

Your battery voltage must match your application system voltage, which will impact your battery selection because you may have to purchase multiple batteries and connect them in series and/or parallel to achieve the required system voltage and desired battery capacity. 

Ampere hours, more commonly known as Amp-hours and abbreviated Ah, on the other hand, is a unit of measure denoting the amount of electrical charge stored in a battery. It is most commonly understood as the amount of power that can provide 1 amp of current over a certain amount of hours. 

For example, a 100 Ah battery has the ability to send 1 amp per hour for 100 hours, 2 amps of power for 50 hours and so forth. 

Similarly, Watt-hours is a unit of measurement for the amount of energy stored in a battery, but is expressed using the watt, which is a unit of power over time. Watt hours and amp hours measure the same thing - battery capacity. But they are two sides to the same coin in that you may need to convert between the two as you try to determine how much battery capacity your application requires. 

This is derived from the equation, Power = Current x Voltage, where power is measured in watts, current in amps and voltage in volts. 

Thus Watt Hours = Amp Hours x Battery Voltage

Now you can convert between Amp Hours and Watt Hours by knowing the voltage of your electrical system or battery.. 

For example, if you have a 12 volt battery with 100 Ah capacity, you can determine the watt hours by multiplying the 100 Ah capacity x 12 volts = 1,200 Watt Hours. 

How Much Battery Capacity Do You Need?

So, how much battery capacity does your application need? It depends on your specific requirements and usage patterns.

Different battery types, such as lithium battery and lead-acid batteries, have varying capacities. Generally, lithium batteries possess the greatest capacity, which is another reason that we at Enduro Power Batteries have focused on engineering the best lithium battery that will provide the most energy in the smallest and most efficient battery. 

Flooded lead acid and AGM batteries, on the other hand, have less battery amp hours in capacity, but offer other reasons why you may want to look further into them. Understanding the capacities of various battery types can assist in choosing the most suitable battery for your requirements.

All of our lithium batteries will more than meet your battery needs for any application you have in mind and we look forward to helping you find the perfect battery!

Estimating Your Battery Needs

Estimating your battery needs involves calculating appliance power requirements and determining compatible battery specifications. By understanding the power requirements of your devices and the capacities of available batteries, you can select the right battery to meet your needs.

Depending on your application, this is not an easy task. Recreational vehicles, for instance, can have lots of different appliances with lots of different power requirements and none of these appliances are likely to be on all day. 

This section will cover the calculation of appliance power requirements and the determination of compatible battery specifications based on these needs, aiding in a more informed battery selection.

Calculating Appliance Power Requirements

To calculate the power requirements of your appliances, you will need to know their wattage (or amperage) and the number of hours they are used each day. You can calculate the power requirement of an appliance by multiplying its wattage by the number of hours it is in use to determine how many watt hours of battery capacity you need..

For example, if an appliance such as an Instapot has a wattage of 600 watts and is in use for two hours, then the power requirement of the appliance would be 1,200 watts (600 watts multiplied by two hours). 

Follow this process for all of the appliances that you would use on any given day and you can determine the overall amount of battery capacity that you would need to ensure that the devices could be used accordingly. 

Then you will want to match your battery capacity accordingly in watt hours. Or, as most battery’s capacity is listed in Amp Hours, you will need to use the conversion above to convert watt hours to amp hours.

Alternatively, you may not know the watts of each appliance. However, you can determine the amperage draw and follow the same procedure by estimating how long you would use each appliance. 

To determine the amps that each device uses you can either use a clamp-on ammeter or a battery monitor. If using a clamp-on ammeter, clamp the meter around the negative wire leading from your battery. Then turn each device on and measure the change in amperage as each device is turned on and off. 

Similarly, with a battery monitor, simply turn the devices on/off and watch the change in current (amps) on the battery meter for each device.

Again, this can be tedious. But if you want to calculate battery capacity needs accurately, you can use this process as a basic battery capacity calculator to determine approximately how much battery capacity you need, either in amp hours or watt hours.  

Determining Compatible Battery Specifications

Once you have calculated the power requirements of your appliances, you can determine compatible battery specifications by matching the calculated power requirements with available battery options. 

Depending on the type of battery you are using, you will want to be sure that you do not go beyond a certain depth of discharge (DOD) to keep from harming the batteries. These are the recommended depth of discharge, or amount of battery capacity available to use, for the most common battery types:

• Flooded Lead Acid = 50%
• AGM = 60% 
• Lithium = up to 100%

Taking into account the battery’s capacity and your appliances’ power requirements can guide you in choosing the most suitable battery to fulfill your needs.

For example, if you determined that you require 75 amp hours of power throughout the use of a day, then depending on which battery type you select, you may be ok with one battery or you might need two or more to meet your energy needs. 

For example, the battery’s actual capacity will be limited with a 100 Ah battery with the following: 

• Flooded Lead Acid = 50% DOD  x 100 Ah = 50 Ah usable capacity
• AGM = 60% DOD x 100 Ah = 60 Ah usable capacity
• Lithium = 100% DOD x 100 Ah = 100 Ah usable capacity

In this situation, the only single battery that will allow you to use 75 amp hours of energy safely without harming the battery life is a lithium battery. The Flooded Lead Acid battery will only provide 50 of the 75 amp hours required and the AGM will only provide 60 of the 75 required. 

In this case, you would want to have 2 Flooded Lead Acid or AGM batteries wired in parallel in order to have enough battery capacity to meet your power consumption needs. 

Factors Affecting Battery Capacity

Various factors outside of the type of battery can impact battery capacity, including temperature, discharge rate and age. These factors can influence the performance and lifetime of batteries, making it essential to take them into account when selecting a battery for your needs.

Knowing how these factors impact battery capacity can guide you in making well-informed decisions when selecting batteries, guaranteeing the right battery choice for your specific needs and usage conditions.

Temperature's Influence on Capacity

Temperature has a significant impact on battery capacity, as higher temperatures tend to increase capacity. However, intentionally raising the temperature of the battery is not an effective strategy to increase battery capacity, especially in Flooded Lead Acid batteries, as it can cause dangerous battery acid to leak and/or cause battery terminal corrosion, ultimately leading to a decrease in battery lifetime.

On the other hand, cold temperatures can increase the internal resistance of the battery and diminish its capacity. Thus operating batteries within the recommended temperature range is vital for maintaining optimal capacity.

Discharge Rate and Peukert's Law

The discharge rate, the steady current in amps (A) that can be extracted from a battery over a specific period, has a significant influence on battery capacity. High discharge rates diminish the available energy and consequently reduce battery capacity, whereas low discharge rates augment the available energy and thereby increase battery capacity.

In other words, if you constantly use items with a large power draw (amperage), then you will very quickly reduce the battery’s capacity. 

Peukert’s Law states that:

• The capacity of a battery diminishes as the discharge rate increases.
• Batteries with higher discharge rates will have a lower capacity than batteries with lower discharge rates.
• This affects their overall performance and energy storage capabilities.

Aging and Battery History

The age and usage history of a battery can also affect its capacity and the number of available charge and discharge cycles. Aging of a battery leads to a decrease in capacity and an increase in internal resistance.

Usage history can also impact battery capacity, resulting in a decrease in the number of available charge and discharge cycles. Understanding the influence of aging and usage history on battery capacity enables more knowledgeable decisions when choosing a battery to suit your requirements.

At Enduro Power Batteries we understand the importance of having a battery bank that delivers reliable power on demand. Thus we have engineered the best batteries that are Endurance Rated to be able to provide more energy than you can imagine needing.

Summary

Understanding battery capacity is essential for making informed decisions when selecting batteries for your application needs. By learning about the factors that influence battery capacity, such as voltage, temperature, discharge rate and aging, you can estimate your battery needs more accurately. 

Utilizing a battery capacity calculator and calculating appliance power requirements can help you determine the right battery for your needs. Armed with this knowledge, you can make more informed decisions when choosing batteries, ensuring that you select the best battery for your specific requirements and usage conditions.

Frequently Asked Questions

Is battery capacity measured in Ah or Wh?

Battery capacity is measured in amp-hours (abbreviated Ah) or watt-hours (abbreviated Wh), which indicates how many amps or watts the battery can deliver in an hour. There is a way to convert between Amp Hours and Watt Hours if you know the battery voltage. 

What does a battery rated at 150 amp hours mean?

A battery with a rating of 150 amp hours means that it is able to produce 150 amps of current in an hour. Thus if you need 2 amps per hour to power a device, you will have 75 hours (150 Ah / 2 amps) of usable energy (assuming you have a lithium battery with a 100% depth of discharge). Similarly, if you needed 10 amps per hour, you would have 150 Ah / 10 amps = 15 hours of usable energy. 

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