What Are These Volts, Amps, and Watt-Hours? How Battery Specifications and Capacity Equate to Capability and Cost
Understanding e-bike batteries can be challenging, even for those of us in the know; the nitty-gritty details are figured out by electrical engineers with years of education and experience under their collective belts – and for good reason, it’s all chemistry and math over there!
You’ll encounter a host of terms when reading about e-bikes or looking at electric bike battery specifications: things like battery size, capacity, voltage, amp hours and watt hours. Some of these words are more-or-less interchangeable, others are related but distinct. All of them can be confusing, but they are also hugely important in understanding electric bikes and their capabilities – most notably when trying to interpret how far they can take you before needing to be recharged.
In this guide to e-bike batteries, the helpful writers at Electric Bike Report will help you to understand the meaning of common battery terms and their relation to the performance of the electric bikes they power.
E-Bike Batteries Explained
Batteries are one of the core elements of electric bikes. They are needed to supply power to the motor, which in turn provides assistance to the rider, and reduces the amount of human effort needed to move the bike.
E-bike batteries come in various sizes, and can be mounted to the frame in different ways. Some are fully internal, and are sealed inside the bike’s frame. As such, they are not removable, except by using special methods and tools available to professional technicians. Others are removable for easier charging and replacement, whether mounted completely externally (outside the frame), partially recessed (sunken into the frame to some degree), or completely recessed (sunken entirely and nearly invisible on the bike).
Regardless of their type, all e-bike batteries are actually battery packs, and are made up of groups of cells, similar to the standard AA or AAA batteries used in everyday applications. The number of cells and the method used to cluster them together determines how quickly they can provide power and how long they can continue to supply it.
In contrast to standard AA or AAA batteries, however, those used in e-bikes are most commonly rechargeable lithium-ion batteries similar to those used inside smartphones and in conjunction with cordless power tools. Lithium-ion batteries are efficient and can be recharged hundreds or even thousands of times if cared for properly. The Light Electric Vehicle Association, or LEVA, has a great article that they allowed us to re-publish regarding proper battery care and safety to ensure maximum life span.
Fully integrated batteries such as the one on the Velotric Nomad 1 can match the bike’s color and disappear into the frame.
Electric Bike Battery Terms and Definitions
Before we dive deeper into the details, let’s consider a couple of examples of e-bike battery specifications in relation to how they usually appear:
36V, 14.4 Ah
48V, 672 Wh
V = Volts and Ah = Amp-hours
V = Volts and Wh = Watt-hours
Both examples convey two basic measurements, albeit a little differently. In both examples, we see volts first; this measurement relates to the availability of the electrical energy the battery can deliver. Next, either amp-hours or watt-hours are shown; these represent a battery’s capacity, or the amount of power it can store.
Let’s define these words (and a few helpful additional terms) a bit more clearly:
Current: the flow of electricity, or transfer of electrons, through a circuit.
Circuit: a closed system of wires and electrical components through which current can travel.
Volts (V): the amount of electrical force or pressure the battery can produce; the speed of the battery’s output of current. This is also sometimes referred to as the electromotive force, and is more specifically the speed at which electrons move through the system.
Note that this is a nominal rating that is used for classification purposes. In reality, a battery’s voltage varies based on the amount of power being drawn from it at a given moment, as well as the battery’s present level of charge. As current is drawn from the battery, its voltage decreases. This can be seen in an e-bike battery voltage chart.
Voltage is determined by the number of battery cells arranged “in series”.
Amps or amperes (A): a measurement of the strength of the battery’s output, or current. More specifically, the volume of electrons passing through the system. This is limited by the size of the wires making up the system. Larger wires allow more current, smaller wires allow less. Generally, systems with higher voltage should use smaller wires (that limit amperage) to prevent overheating.
Amps can also be thought of as the amount of energy being drawn from the battery by what it is powering, and can fluctuate from moment to moment. In the case of e-bike batteries and their motors, a greater number of amps are drawn as the motor works harder (i.e. going uphill or using only the throttle).
Amp-hours (Ah): a measurement of charge; the amount of energy that can be delivered through an electrical system over the course of an hour.
In the case of a 10 Ah battery, it can deliver 10 amps of power in one hour, or 1 amp of power for 10 hours, etc, depending on the needs of the component that is delivering power to.
Amp-hours are determined by the number of clusters of battery cells arranged “in parallel”.
Watts (W): a unit of power, determined by volts and amps; the amount of work that can be done by one amp of current delivered at 1 volt. The amount of work is determined by the rate at which the energy is used.
This measurement is generally applied only to an e-bike’s motor, but its battery must support the motor’s needs.
Watt-hours (Wh): another measurement of capacity. In this case, the amount of work that can be done, or the amount of power that is spent, over the course of an hour. This is a direct result of a battery’s voltage multiplied by its amp-hours.
As such, a 24V, 20 Ah battery and a 48V, 10 Ah battery might look different on paper, but they have about the same amount of energy. This makes watt-hours a more reliable indicator of capacity when comparing different batteries.
Controller: A device that limits the flow of electricity through a circuit, and prevents a battery from discharging its energy all at once. In terms of an electric bike, this is the “brain” that adjusts the pedal assist system, the amount of input the motor contributes, and the e-bike’s speed.
The Electricity-As-Water Concept
All of the above terms describe the strength and total amount of energy in a system, but they can still be difficult to understand in a way that relates to normal, everyday life. For this reason, electrical concepts are commonly related to plumbing and the flow of water.
If we consider a large container filled with water, the container represents the battery itself, and the water inside the container represents current or electrical charge. The container can only hold so much water, so this is likened to a battery’s capacity.
Now, if the container is connected to a pipe or hose, that tube represents the wiring of the electrical system. Voltage can be thought of as the pressure forcing water through the tube. Amps can be thought of as the volume or amount of water flowing through the system, which can change based on the diameter of the pipe and how open the faucet is at the end.
With the faucet, or the flow as governed by the controller, fully open and the water flowing through a connected tube and into a bucket, the amount of water in the bucket after an hour can be compared to an amp-hour.
If the water is instead used to turn a small water wheel, a watt becomes the distance one measure (amp) of water can turn the wheel. And with these combined elements in place, we can determine the distance the wheel can be turned by a full container of water (watt-hours), and with regulation of the water’s flow through the faucet, how long it will take for the container to become empty.
Why Does All of This Matter?
In terms of e-bikes, the above terms affect primarily how fast they can accelerate, how much they cost, and most importantly for many potential buyers, how far they can travel (and for how long). To a lesser degree, other aspects of an e-bike are also influenced, including how confidently an e-bike can power up hills, and how much input its motor can provide.
E-Bike Battery Voltage = Speed and Acceleration
Most e-bikes use batteries that are either 36V or 48V, though some lower- and higher-voltage options exist. Lower-voltage options are usually not able to provide the level of power that most users desire, and in general, anything above 48V has the potential for safety risks from electrical shocks. 52V systems, while above that threshold, do have an advantage over 48V systems in that they are able to deliver a more consistent level of power over longer periods of time, but they are relatively uncommon.
When considering the aforementioned plumbing analogy in relation to the most common battery voltages, think of a 48V system as having greater “water pressure” over a 36V system. In practice, this means that a 48V battery will deliver power to an e-bike’s motor faster than a 36V system.
As such, the motor of a 48V system does not have to wait to access and use the power it receives, and will also get more power in the same amount of time. This means that – provided all other factors are the same – a motor receiving power from a higher voltage battery will accelerate quicker and be able to reach greater speeds than if it were fueled by a battery with lower voltage.
Rad Power Bikes’ RadCity 5 Plus uses a semi-integrated battery that trades visibility for easy separation from the frame.
E-Bike Battery Capacity = Range
While there are many factors that influence an electric bike’s range, battery capacity is one of the primary influences and is generally a good indicator of potential distance. And as we have established, amp-hours and watt-hours are two different means of measuring an e-bike’s battery capacity.
If we consider e-bike battery specs in relation to cars, amp-hours are the equivalent of the gas tank. A greater number of amp-hours means more fuel in the proverbial tank, and therefore a higher watt-hour rating means an e-bike can travel a greater distance (or go faster for a shorter distance). Due to the previously mentioned relationship between volts, amp-hours, and watt-hours, if two batteries have the same amp-hour rating but different voltages, the higher voltage battery will have more watt-hours. For this reason, watt-hours are generally thought of as the most useful battery specification for the consumer.
This is where things start to get a little complicated. Since an e-bike’s battery interacts directly with its motor, the motor’s voltage, type, specs, and efficiency are also part of the range equation (as it turns out, this can be a very long equation).
All of the following factors (and many others) affect e-bike range beyond just its battery’s watt-hour rating:
Motor voltage and watt output
Rider and cargo weight
Weather and wind
Speed (and PAS setting)
Terrain (road surface and hills)
Number of stops/starts
Amount and proper timing of rider input (pedaling)
Tire size and tread type
The more extreme any of these factors are, the less range an e-bike battery will be able to provide, because the motor will need to do more work to compensate. As such, it will need to draw more energy from the battery to compensate, reducing the remaining pool of available charge.
E-Bike Battery Size and Cost
Generally speaking, an e-bike’s battery is its most expensive component. Again, considering how batteries are made up of clusters of individual cells, a greater number of cells equals a greater number of amp-hours and watt-hours, but also an increased cost.
For this reason, it is important to consider a practical range for your own personal needs, to ensure that you are not overpaying for a huge battery you don’t actually need. An extended range might look good on paper, but it might greatly exceed the distance you will regularly travel.
The external batteries on the Evelo Galaxy SL and Galaxy Lux are mounted unobtrusively beneath the cargo rack, and include integrated tail- and brake lights.
E-Bike Battery Amp-hours and Motor Input
An e-bike’s battery voltage and watt-hours must match the needs of its motor. We have discussed how greater voltage in a motor/battery system equals more immediately available power and speed, but volts only go so far. A battery’s capacity also needs to support the nominal wattage of the motor it is paired with. Nominal means that the motor operates at that level MOST of the time, but can peak at higher levels when doing things that require it to work harder, such as traveling on an incline.
We’re crossing more into motor specifications here, but the two are related. A 250W motor will not provide as much power and assistance to an e-bike as a 750W motor. But a larger motor needs to draw more power from the battery – and therefore requires a battery with a greater number of amp-hours (and therefore, watt-hours) to operate effectively and provide a functional range.
Calculating E-Bike Range
Disregarding all factors aside from battery and motor specifications, it is possible to calculate a rough estimate of an e-bike’s range with some basic algebra. We have a separate, more detailed article on how to calculate the range of e-bikes, but a summary of the main points ties in nicely here as well.
A battery’s effectiveness in relation to its motor relates largely to its watt-hour capacity rating. As mentioned in the definitions section above, that can be calculated by multiplying volts and amp-hours as shown below:
Wh (watt-hours) = V (volts) x Ah (Amp-hours)
For example, let us assume that we are working with a 36V, 14 Ah battery. Using the equation above, we can determine that the watt-hour rating for this battery is 504 Wh. Let us also assume that this power source is compatible with three different e-bikes, each with commonly-sized rear-hub motors and controllers designed for a 36V system:
E-Bike 1 has a 250W motor, E-Bike 2 has a 500W motor, and E-Bike 3 has a 750W motor. We can divide a battery’s watt-hour rating by the motor’s watt requirement to find out how long the battery should power each e-bike:
E-Bike 1: 504 Wh / 250 W = 2.02 hours
E-Bike 2: 504 Wh / 500 W = 1.01 hours
E-Bike 3: 504 Wh / 750 W = 0.67 hours or about 40 minutes
Two caveats come into play here. First, as discussed previously, a motor’s wattage rating is nominal, and its actual output varies. The more its output exceeds the nominal rating, the less time it will take for the battery to be depleted.
Secondly, we established that these were rear-hub motors, which tend to be less efficient than mid-drive motors (one that is mounted at the bottom bracket). Mid-drives have a direct connection to the pedal cranks, and subsequently rely much more on rider input, which reduces the amount of work the motor itself has to do, and extends battery life. For this reason, a smaller 250W mid-drive motor can have the efficiency and range of a larger 500W rear-hub motor. Factoring in sub-types of hub motors can affect things further, but for this article, we’ll keep things as simple as possible.
For most folks, a quantity of time is not quite as useful as a unit of distance, so with some additional math, we can calculate a rough idea of an e-bike’s minimum mileage with one more step. Yes, this complicates things further, but the result is ultimately more relevant.
Ideally, you need to know your average travel speed (or velocity) on an e-bike, which can be figured out over time using a fitness tracker app such as Strava, but if you don’t already have that we can estimate roughly 16 mph.
d (distance in miles) = d (time in hours) x v (velocity in mph)
So if we’re considering the same 36V, 14 Wh battery and the same three e-bikes with different motor wattages from before:
E-Bike 1: 2.02 hours x 16 mph = 32.32 miles
E-Bike 2: 1.01 hours x 16 mph = 16.16 miles
E-Bike 3: 0.67 hours x 16 mph = 10.72 miles
Through these calculations, we can see that a motor paired with a battery that has watt-hours exceeding its nominal wattage will function for significantly more mileage than one with watt-hours under its nominal wattage. For this reason, we generally recommend at least a 1-to-1 ratio between a motor’s nominal wattage output and a battery’s watt-hour rating, i.e. a 500W motor and at least a 500 Wh battery.
E-Bike Battery Voltage and Amps = Torque and Uphill Capability
An e-bike’s ability to power up steep inclines is helped by its rider and gearing, but is determined also by torque, or a measurement of rotational power. This dips more into motor, controller, and wiring territory, and therefore more into the engineering aspect of an e-bike’s electrical system, but an increase in watts equals an increase in torque (since watts equal work that can be done). Since watts are directly proportional to volts and amps, increasing either will effectively also increase torque.
For consumers, motor specs are much more relevant to understanding uphill power than battery specs. Every e-bike motor has a torque rating in Newton-meters (Nm), which is the most important number when considering an e-bike’s hill climbing capability. A higher Newton-meter rating equates to better uphill performance.
Understanding E-Bike Batteries
While the topic of batteries and electrical systems is certainly complicated – this article has only scratched the surface, really – we hope that you feel better able to understand e-bike battery specifications with this information.
It can be easy to ignore the meaning of the values included in the specs of an e-bike battery and default to the standard belief that bigger equals better. When you start to consider how larger and smaller battery-related numbers relate to the practical applications of an e-bike, you can better understand how these measurements meet your own needs.
We hope you feel like you’re a bit better armed with knowledge now as you search for the right e-bike for you, and if you need a little more help in that regard, you can always check out our e-bike buyer’s guide for more basics on e-bikes batteries, motors, and more.