Have you ever wondered about the distinction between voltage and amp hours? Maybe you’re new to cordless tools and are unsure where to begin. Well, I’m here to shed some light on this fundamental question. So, let’s dive right in!
Voltage vs. Amp Hours: Decoding the Mystery
With corded tools, power is typically measured in terms of the amperage it draws. However, when it comes to cordless tools, things can get a bit confusing. Many people simplify the concept by equating voltage to power and amp hours to run time. While this is partially true, it’s not the whole story. These two measurements actually stem from the battery’s capacity, known as watt-hours, and can be calculated using the formula:
Ampere Hours x Nominal Voltage = Watt Hours
When you check the label on most batteries, you’ll find the total watt-hours of their capacity. Essentially, the larger the tank (measured in watt hours), the greater energy potential you have—it’s all about how you utilize it.
Wiring to Increase Voltage
If you were to take apart a battery pack (which I strongly advise against), you’d discover separate cells that store and provide power to the tool. Each cell can output a specific amount of voltage, usually around 3.6 volts in the case of lithium-ion cells like the 18650. Need a 12V battery? Connect three of these cells in series. Looking for an 18V battery? Use five cells.
Now, here’s a catch. The voltage within batteries can vary slightly depending on their charge level. When fully charged, a 3.6V battery can actually produce a little over 4V. The math isn’t perfect, but hold on—I’ll explain these exceptions in a future article. For now, let’s focus on voltage as a measure of power.
Adding another battery in series to the existing one will increase the voltage. Each additional battery contributes approximately 4V. So, in theory, you could have 12V, 16V, 20V, 24V, and so on. Thankfully, the tool industry has standardized platforms such as 12V, 18V/20V, and 36V for cordless tools, while other combinations are used for outdoor power equipment.
Wiring to Increase Amp Hours
Amp hours refer to the amount of amperage a battery pack can supply for one hour. If we ignore other factors like temperature and vibration, a 3.0 amp hour battery will provide 3 amps for one hour, while a 5.0 amp hour battery will deliver 5 amps for the same duration. Unlike voltage, this is not a fixed number. You can draw higher amperage from the battery, which will decrease the run time, or draw less current and extend the runtime.
Let’s take an example. Running a 2.5 amp load with a 5.0 amp hour battery will provide 2 hours of run time. On the other hand, if you drain 6 amps from a 3.0 amp hour battery, it will only last for 30 minutes. Here’s a graph that illustrates how current draw affects runtime.
So, how do we increase amp hours? Well, you can connect batteries in parallel. Let’s take a typical 12V battery as an example. Three 18650 Li-ion cells are connected in series, each providing 3.6 volts and 2.0 amp hours. When connected in series, the total voltage remains the same—10.8 volts or 12 volts when fully charged. However, the amp hours do not increase; they remain at 2.0.
To increase amp hours, we need to connect the batteries in parallel. In this configuration, the three cells still produce 3.6 volts, but now we have 6.0 amp hours.
Voltage and Amp Hours: Working in Harmony
In high-capacity batteries, a combination of series and parallel wiring is used. First, you connect multiple batteries in series to reach the desired voltage. Then, another set of batteries is connected in parallel to increase the amp hours. For example, by connecting five batteries in series and then adding another set in parallel, you can achieve 18V with 4.0 amp hours. In theory, you could add another bank to get 6.0 amp hours at 18V.
In some cases, manufacturers repurpose batteries for different tools. For instance, Black & Decker and Craftsman use max 20V batteries for multiple tools. By arranging ten cells in a specific configuration, they can create a max 40V, 2.5 amp hour battery.
Real World Conditions and Their Impact on Runtime
In real-world scenarios, factors like temperature, vibration, and other environmental conditions can affect voltage and amp hours, leading to deviations from ideal values. Nevertheless, these conditions are part of the job site reality. To set more realistic expectations, manufacturers often list lower ratings (e.g., 18V nominal instead of 20V maximum) that better reflect actual operating experiences.
There are various ways to improve batteries. Manufacturers can explore different battery chemistries, such as adjusting the anode, cathode, and electrolyte components. Each cell may have variations in resistance and impedance, which can affect performance. Suddenly, you find that the same number of batteries producing 18 volts and 3 amp hours can now deliver 4 amp hours!
The difference in performance among various brands largely stems from the batteries they utilize and the electronic controls and safety measures they implement. By altering the wiring configuration, manufacturers can enhance the power output, increase the amp hours, or achieve a balance between the two. Ultimately, higher voltage results in more total wattage, while higher amp hours translate to longer run time.
As technology advances, we can expect improvements in battery amp hours through the exploration of different designs and materials. While cordless tool voltages will likely remain stable, outdoor power equipment still has some catching up to do.