When people think of how cars work, it’s the engine or transmission that comes to mind, but the battery, the oft-forgotten heart of the car, is rarely spoken of. Sure, we all know what a battery is, but do we actually know how it works?
Unless you’re something of an expert in the area, when we really think about it, the processes of a battery are as mysterious and amazing (if not more so) than the engine itself.
Amps, voltage, current, ohms...we’ve heard the words before, but power is all so confusing that the words as we understand them begin to tangle and become synonymous despite the fact they’re very different things.
Well, we’re going to be honing in, sharpening our focus on ampere ratings to give us a more articulate understanding of batteries and the role they play in our cars.
What Are Amps?
Amps are named for the early French mathematician and physicist, André-Marie Ampère, who was one of the first scientists to start lifting the veil on the mysteries of electromagnetism.
First, let’s look at what amps technically are, then we’ll think about what that means in a simplified way so we don’t have to start taking night classes to understand the physics.
So, what are they? People often mistake amps for a power rating, and that’s understandable because they do contribute towards the overall power of a battery, but power is defined by watts.
Amps are actually units of both volume (as in size), and of flow, of movement (as in speed). They refer to the volume of electrons that can travel through a wire in a second.
One amp = 6.25 x 1018 electrons per second. Sounds overly specific and way too complicated, doesn’t it? But that’s the math behind it.
Let’s look at a commonly used analogy. Picture a garden hose. The pressure of the water running through the hose is like the amps in a battery, the hose itself is akin to volts, and the water released at the end of the hose is what you’d consider the watts.
So, what does this mean in Layman's terms? You’ve probably come across amp-hours, which is what we’ll be using to understand it in the context of a car battery.
You can think of amps and amp-hours specifically, as the capacity of the battery in the sense it describes how long power can be extracted at a certain rate (volume). Still confused? Just focus on the capacity aspect of it.
Generally speaking, the larger the battery, the higher the ampere rating it has, but physical size isn’t the only determining factor. The complex inner workings of the battery also have an impact on the overall amps.
How Many Amps is a Car Battery?
Without further waffling, let’s answer your question. Small cars through to large commercial vehicles use batteries between 35 and 120 amps.
The larger the vehicle, the bigger the battery has to be. To break it down more specifically, small to medium-sized cars will usually run on 36 to 40 amps.
Larger cars and smaller SUVs will likely require between 40 and 60 amps. The goliath vehicles out there require a battery capacity of 60 + amps.
CA and CCA
You may have noticed these two markings on your car battery. Instilling even more confusion, they’re both ampere ratings. CA stands for crank amps, and CCA stands for cold cranking amps.
You shouldn’t worry too much about this distinction. Just know that you’ll mostly be paying attention to the CCA. CCA is a more accurate representation of the battery’s true capacity as it measures it at a temperature of 4 degrees F.
So when you’re trying to work out a car battery’s amps, use the CCA rather than the CA.
Amp Hours (AH) Chart
An amp-hour chart is basically a visual aid to help you work out the capacity of a battery from fully charged to dead.
It will give you the consistent amp rating over a period of time, for example, a battery rated at 100AH will run at 5 amps for 20 hours continually.
From there you can do simple multiplication or division to work out how long the battery will last running at different demands. The same 100AH battery running at 10 amps rather than 5, will work consistently for 10 hours rather than 20.
Charging Your Battery in Amps
Typically, a charge amp will run on 4 to 8 amps and will charge the average car battery over a 10 to 24 hour period, but why don’t they just use a more powerful charge amp?
Well, this is because when it comes to charging car batteries, the slower the better. If batteries are charged gently, it increases their overall lifespan by avoiding overcharge.
Although a full charge takes pretty much an entire day, you’ll probably be able to turn on electrical functions and start the car after around four hours of charging.
If you wanted to charge your battery faster, a 10 amp charger can be used safely as long as you know exactly when to stop the charge. This is why determining the remaining amps is so important.
The math is simple. You subtract the residual capacity from the capacity at full charge, but how the hell do you find out how much juice is left?
Calculating Remaining Amps
The easiest way to measure that juice is by using a standard car battery tester.
It will give you the answer in amps, sorted, but you’re more likely to come across a digital multimeter that gives you the answer in volts.
Measuring voltage is more common because most vehicle batteries are between 12 and 13 volts, whereas amps can differ greatly from vehicle to vehicle.
How to Convert Volts to Amps
To convert volts to amps, you need to know the battery’s resistance.
The equation is known as Ohm’s Law. It states that current (amps) = voltage divided by resistance.
Resistance is measured in ohms which should be labeled on your battery.
Batteries are highly complex things when you’re unsure of the workings.
It can seem like straight-up magic, but breaking down the processes involved, and focusing on one aspect at a time as we have here with amps, can clear things up pretty quickly.