A battery is an important component of a car because it provides the electrical energy needed to power electrical components in the car. This is why it is important to use a battery with the right specification. Although most car batteries come in 12-volts, the ampere ratings differ depending on its storage capacity and the task it is expected to perform. Also, knowing the amp rating of your battery is helpful when its capacity deteriorates with usage and it needs to be charged or replaced. In this article, you will get to understand how a 12-volt car battery is rated, tested and charged. 

What is Ampere Rating?

The ampere rating is a measure of the storage capacity of a battery. This rating differs for most cars but it is typically within the range of 550 – 1000 amperes. The actual capacity of the battery depends on a number of factors including the size of the battery and its chemical make-up. The ampere rating of a car battery is measured in two ways; Cranking Amperes (CA) and Cold Cranking Amperes (CCA). 

Cranking amperes is the current delivered by the battery to start the car engine at 0 oC (32 oF) while the cold cranking amperes is the current needed to start the engine at – 18 oC (O oF). In both cases, the battery delivers current for about 30 seconds at 1.2 volts per battery cell. The major difference between these two ratings is the temperature at which the battery is operating. The higher the CCA of your battery, the better its performance in cold weather. 

Ampere Rating and Ampere Hour (AH)

Amperage and amp-hour ratings present two different approaches of understanding or measuring the capacity of a 12-volt car battery. While the ampere rating is a measure of the highest current the battery can deliver within a short time, the amp-hour measures how much current the battery can supply over a period of time. The difference between the two ratings depends mostly on what the battery will be used for. For instance, a car needs a high current to start the engine and this means the battery must have a high ampere rating. Whereas a solar battery only needs to supply a steady current over a long period of time. Despite this difference, the amp-hour is a practical way of determining the true capacity of the 12-volt battery. You need to know the AH rating in order to determine how long your battery can last when powering the electronics in your car. If this information is not provided by your battery’s manufacturer, then you need to know how to convert from CCA to AH. Knowing the AH rating of the battery will help you make the best decision when buying a new battery for your car. 

This is why on most batteries; you may have both the AH and amps rating on the body. For example; 12V 60AH, 750 CCA or CA. 

Where;

V represents the voltage delivered, 

AH is the ampere-hour which shows the amount of current delivered per hour over a period of time and;

CCA is the peak current which the battery is capable of producing. 

If your battery does not have an AH rating, you don’t have to worry. You can easily measure it with a digital multimeter although it will take a few hours. 

Testing The Amp-hour Rating Of A 12-volt Battery

12V Battery
This is a 12V VRLA Battery.
Credit: wikimedia.org

Step 1 – Preparation

Before you test the amp rating, you will have to prepare the battery. Preparing the battery simply involves charging it with a battery charger until it is fully charged and then let it rest for a few hours to get rid of surface charges. Then measure the voltage across the terminals using the multimeter. Remember to turn the dial to DC before taking the reading, which should be about 12.6 Volts across the terminals when fully charged. If you get this reading, this means your battery is ready to be tested. 

Step 2 – Testing

A multimeter is a necessary tool for testing 12-Volt battery
Connecting a basic resistor in series with a multimeter across the battery terminals is the first thing you need to do when you test the battery.
Image credit: wikipedia.org

Testing the battery basically involves discharging it and monitoring how long it takes until it is about 50% of its original capacity. First, you have to connect a basic resistor in series with a multimeter across the battery terminals. Once this is done, the multimeter will display the current (amps) flowing across the resistor. You will also observe the voltage decrease gradually until it is half of its original value. Take note of the time it takes to reach this value. 

Step 3 – Calculation

Using the equation below, you can calculate the AH rating for your 12-volt battery; 

[AH = battery current (measured by the resistor) x time taken for the voltage to reduce to half its original value x 2]

Let’s say your battery current measured 10 amps and it takes about 8 hours for the battery to discharge up to 12 volts (which is half its value), then; 

AH rating = 10 x 8 x 2 = 160 AH

What this means is that your battery will last for 20 hours when running at 8 amps or 16 hours when running at 10 amps. 

Note that this capacity can either increase or decrease depending on the discharging rate. For better understanding, check out the chart below which shows how long a 100AH battery will last at different discharge rates. 

Battery capacity decreases as the discharge rate increases.
Image credit:caravansplus.com.au

How to convert from CCA to AH

As I mentioned here, your battery may not have the Ah rating written on it. If this is the case, you may have a slight difficulty in determining how long your battery will last when fully charged. This is because the CCA rating only tells you how much power (known as Cracking power) the battery can deliver to your car’s engine during start-up. However, you don’t have to worry because you can easily convert the CCA rating to AH rating by dividing the CCA by 7.25. 

So, for instance, if the CCA rating of your battery is 500, then;

Ah = 500/7.25 = 69 AH. 

How Long Will It Take To Charge Your 12-volt Battery?

As you use your battery, it discharges and soon needs to be recharged. In an automobile, the battery is recharged by the alternator so you may never have to do it manually. But at times, they may be a need to manually charge your 12-volt battery and when this happens, you need to know the AH rating of the battery. This is another situation where knowing the AH rating is very important because it helps you to know the best charger for your battery. Using the wrong battery charger could result in overheating and this could damage the battery. For a 12-volt automobile battery, 10 amps charger is often recommended. Doing the math, you will see that it takes about 16 hours for a 160 AH battery to be fully charged. 

Conclusion

A 12-volt battery is an industry standard for nearly all applications, especially in the automobile sector. Because of its versatility, there are many things you must know about your battery and one of this is the amps. In the article, I have explained what the amp is, why it is important and how you can test for it. I have also explained the different ratings used for a 12-volt battery and what they represent. Hopefully, you’ll find the information in this article to be helpful.   

Read more: About Off-Road International

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