Despite it being one of the most important components in a car’s engine, a lot of people don’t know very much about their battery. We all know car batteries are 12 volts, but how many amps is a car battery – and what does that even mean?
Knowing how your battery works will help you troubleshoot when it has problems. We’ll explain how you can figure out the ampere ratings of a battery, as well as how you should charge it to get the most power.
What Are The Ampere Ratings?
The ampere ratings (or amp ratings) of a battery basically tell you its storage capacity. Typically, the larger the battery, the higher the amp rating. The internal chemistry of the battery will also affect the amp rating. Most car batteries have a capacity between 550 and 1000 amperes.
Matters are further complicated by the fact that you’ll see two-ampere ratings on most batteries: the crank amps (CA) and cold cranking amps (CCA). Usually, the CA rating will be higher than the CCA rating.
Pay more attention to the cold cranking amps (CCA) when you’re figuring out how many amps is a car battery. This is the power you’ll get when the battery is at -4°F and is a truer measure of the battery’s actual power.
What Is The Car Battery Amp Hours Chart?
A car battery amp hours chart shows the ampere-hour (expressed Ah) of the battery. This is something you’ll see most often associated with deep cycle batteries used for recreational and marine vehicles. You can find these charts for car batteries as well, though.
The amp hours allow you to measure car battery amps through the practical lens of how long it will last between charges. If a battery’s ampere-hour is 100Ah, this tells you the battery can deliver 5 amps of power for 20 hours. If you used 10 amps per hour, it would last for 10 hours, and so on.
With the Ah chart, you can see how much of a battery’s capacity it retains after a given amount of time. This chart also shows you the difference the battery’s chemistry makes in its capacity. The dotted line represents the standard flooded battery, while the solid line shows the more efficient AGM style.
How Many Amps To Charge A Car Battery
Most battery chargers send about 2 amps of power to your battery per hour. At this rate, it typically takes over 24 hours to fully charge the battery. The advantage of this relatively low charging rate is that you’re less likely to overcharge the battery, which can reduce its overall lifespan.
You can find faster-charging options, as well. It’s not recommended to charge a car battery at anything over 20 amps. A 10-amp charger will give you a quicker charge and is still relatively safe, though you should take care to remove the charger as soon as the battery is full.
Determining how many amps to charge a car battery is a matter of subtracting the current capacity from the total capacity. Once you know how many amps your battery needs, divide that by the amp rating of the charger. The result is the number of hours it needs to charge.
How To Measure Car Battery Amps
Before you can charge a car battery, you have to know how to measure car battery amps. A designated battery tester can give you this information. You can also use a digital multimeter to check your battery’s current power.
Unlike battery testers, a digital multimeter will tell you the volts of the battery, not the amps. This measurement is more consistent across car batteries, which all should have a 12.6-volt measurement when fully charged. Getting the amps of your battery from this information will require doing some math.
Volts are a measure of voltage, while amps are a measure of current. You can find the amps from the voltage if you know the battery’s resistance, using the formula known as Ohm’s Law: Current equals voltage divided by resistance, or I=V/R. The ohm rating of your battery should be listed on the label.
To measure your battery’s current voltage, turn the multimeter to DC. Position the red lead of the multimeter on the positive battery terminal (usually red and marked with a +). Next, touch the black lead to the negative terminal (often black, marked with a -).
The DC voltage will you what percentage of your battery’s charge remains, based on the chart below:
|<% of charge remaining>|
|11.9 or below||Effectively dead|
If you know the total amp capacity of your battery, you can do some quick math to estimate the remaining amps. For example, a 1000-amp battery that’s at 50% capacity currently holds 500 amps.
How many amps does it take to start the engine?
This depends on the size of the engine. In general, it takes about 300 amps for a small, compact car with a 4-cylinder engine. The larger the car and the more cylinders, the more power it takes to start up.
How quickly does a car battery charge while you’re driving?
The higher your engine’s RPMs, the more power will be sent to the battery from the alternator. In highway conditions, you can fully-charge a healthy battery in about 30 minutes – much faster than it charges using a battery tender.
Does a battery’s amp rating change over time?
Eventually, yes. The stated CCA of a battery tells you its amp capacity when the battery is new. Damage from heat and vibration – along with standard wear and tear – will reduce its total capacity, and in turn its amp hours and amp rating.
How much the battery’s capacity goes down with time depends on how you use your battery. Deep discharges prevent the battery from charging fully and allow it to lose its charge more easily.
Age also negatively impacts a battery’s capacity. Even in ideal circumstances, most batteries die after about 5-7 years and will become gradually weaker until they lose power. Then, you will need to change them.
Bonus: Check out our guide on charging a connected battery!