When browsing for lithium batteries, you might see "amp hour rating" (Ah), indicating how much the battery holds and how long it lasts. But what does Ah mean, and how do you use it?
This guide covers amp hours, their relation to lithium batteries, and the difference between amp hours and watt hours. It also explains how to calculate battery life based on Ah and provides real-world examples. By the end, you’ll understand how this measurement affects battery choice and performance.
What Are Amp-Hours?
Amp-hour (Ah) is a way to measure how much electric charge a battery can store. 'Amp' in amp-hour refers to electric current, which is the flow of electrons. 'Hour' refers to how long (in hours) a battery can provide that current.
For example, if a lithium battery is rated for 100 Ah, it can provide a current of 1 amp for 100 hours before being depleted. Or it could provide 10 amps for 10 hours or 20 amps for 5 hours. The total current over time remains 100 amp-hours.
The higher the amp-hour rating, the more energy the battery can hold. Bigger batteries have higher amp-hour ratings. Manufacturers use those ratings to tell buyers how long the batteries will last.
How Amp-Hours Relate to Lithium Batteries
Lithium batteries are a popular type of rechargeable batteries used in many devices today. They're used in phones, power tools, and electric vehicles. Lithium batteries provide portable energy storage.
Amp-hour ratings in lithium batteries show how long they will last. The ratings let you know how much power it can provide before needing a recharge. A high-ampere lithium battery can run devices longer.
For example, if you have two 18-volt lithium batteries for a power drill. One is rated for 1.5 Ah and the other for 3.0 Ah. The 3 Ah battery can provide twice as much current over time. So if the drill uses 1 amp, the 3 Ah battery would last around 3 hours vs. 1.5 hours for the 1.5 Ah battery.
Amp Hours vs. Watt Hours
You may also see lithium batteries listed with watt-hours (Wh) instead of amp hours. So what is the difference between amp hours and watt hours?
Although related, they are not the same. Amp hours measure the electric charge the battery can hold. This refers to how long the battery can supply a constant current. Watt hours measure the total energy stored in the battery. It considers both the voltage and amp hours.
Batteries with higher voltages will have more watt hours than amp hours, even if the amp hours are the same. For example, a 12V battery with 100Ah would provide more energy, 1200Wh, than a 3.7V battery with 100Ah, which would be only 370Wh.
Calculating Battery Life Using Amp-Hours
You can estimate how long a lithium battery will last using it's amp-hour rating. Here is the simple formula:
Battery life = Amp-hour rating / Device current draw
Let's say you have a 100 Ah lithium battery that you are using to power a portable fridge while camping. The fridge requires 8 amps of current to run. To estimate how long the battery will power the fridge, use the simple formula:
Battery life (in hours) = Amp hour rating / Current draw
Battery life (in hours) = 100 Ah / 8 A
Battery life = 12.5 hours
So the 100 Ah battery will power the 8 amp fridge for approximately 12.5 hours before it runs down.
Or let's say you have a drone with a 3 Ah battery that draws 0.5 A when flying.
Battery life = 3 Ah / 0.5 A = 6 hours
The drone can fly for around 6 hours before the battery depletes.
Calculating Battery Life Using Watt-Hours
Watt-hours take voltage into account. To get watt-hours, you multiply amp-hours by voltage:
Watt-hours = Amp-hours x voltage
Now let's say you have a 48-volt 300 Ah lithium battery that you want to use to run a 1200-watt electric skillet. To estimate runtime you first need to convert amp hours to watt-hours:
Watt hours = Amps x Volts
Watt hours = 300 Ah x 48 V
Watt hours = 14,400 Wh
Now that you have the total watt-hour capacity, you can calculate battery life:
Battery life (in hours) = Watt hour capacity / Device power draw
Battery life (in hours) = 14,400 Wh / 1200 W
Battery life = 12 hours
So the 300 Ah (14,400 Wh) battery will power the 1200-watt skillet for approximately 12 hours on a single charge.
Watt-hours tell you about stored energy capacity. Amp-hours tell you about current or charge capacity. Both help characterize lithium battery performance.
Of course, real-world conditions like weather can affect actual performance. But the amp-hour rating gives a ballpark battery life estimate.
Real-World Applications
Knowing the amp-hour rating helps consumers choose the right lithium battery for their needs. Here are some examples:
Smartphones: The sum battery is around 2-3 Ah. Higher-capacity batteries may be 3,000-4,000 mAh (3-4 Ah). This provides enough current for a full day of use.
Electric Bicycles: Ebike batteries range from about 5-20 Ah. The higher amp-hours provide more power and range. A 12 Ah battery can provide about 10-35 miles of assistance.
Electric Vehicles: EV lithium battery packs are very large, often 50-100+ Ah. For example, a Tesla Model 3 comes with a 75 Ah battery, delivering over 200 miles of range.
Cordless power tools: Compact tools like drills use lithium packs of around 1-5 Ah. Higher-capacity batteries provide longer runtime for extended use.
In all these applications, the amp-hour rating correlates with battery life. Checking the Ah lets you match the battery capacity with your energy needs.
FAQ:
Does a higher amp hour rating always mean more power?
Not always. While a higher amp hour rating generally means more battery capacity, the power output also depends on the battery's voltage. Two batteries with the same amp hour rating could have very different power outputs if their voltages are different.
How are amp hours different from milliamp hours (mAh)?
Milliamp hours are a smaller unit that equals one-thousandth of an amp hour. Device makers often use mAh to list the capacity of small batteries like those in phones and laptops. But for larger batteries in electric vehicles or solar storage, they usually specify capacity in amp hours.
Should I get an amp-hour battery rating that exactly matches my device's power needs?
It's better to get a battery with a slightly higher amp hour rating than your device requires. This leaves some extra capacity as a buffer for normal battery wear and other potential factors that affect runtime. Choosing a battery with a higher rating than needed helps ensure you have plenty of power available.
Amp-hour (Ah) indicates the electric charge capacity of a lithium battery, showing how much power it holds and how long it lasts. Higher Ah ratings mean more power and longer runtimes. You can easily calculate battery life using the Ah rating and current draw to estimate how long it will last before needing a recharge.
