Maybe you just want to know more, or see if your idea of volts vs amp hours is correct. Maybe you're new to cordless tools and don't know where to start. So glad you were able to join the party!
Voltage vs. amp hours is one of the most fundamental questions about cordless tools. This can be confusing. With corded tools, we usually describe the amount of power in terms of the amperage it draws. This is great when there is an almost unlimited power supply. Most people boil down cordless tool numbers to the idea that voltage equals power, and amp hours equal run time. Yeah… well, sort of… maybe. These two measurements come from actual battery capacity — a term known as watt-hours. Here is the equation:
Ampere Hours x Nominal Voltage = Watt Hours
If you look at the label on most batteries, it will tell you the total watt-hours of its capacity. Basically, the bigger the tank (in watt hours), the more energy potential you have – it's just a matter of how you use it.
Voltage vs. Amp Hours: Wiring to Increase Voltage
If you were to take apart the battery (please don't do that!), you'll find a separate battery that stores and provides power to the tool. Each cell is capable of supplying a specific amount of voltage, typically 3.6 volts in the 18650 lithium-ion cells used. Need a 12V battery? String 3 of them into a series. Do I need an 18V battery? Use 5.
If you did the math with me, you already knew there was a problem. The voltage within batteries varies slightly depending on the amount of charge they hold. They can produce a higher voltage than low when fully charged. The 3.6V battery actually produces a little over 4V when fully charged. Even then, the math doesn't work out perfectly. However, please don't lose faith in me. I will explain these exceptions in a future article. Now, let's focus on voltage as power.
If you want more power, just add another battery in series to the battery. Each new voltage increases the voltage by about 4V. In theory, you could use 12V, 16V, 20V, 24V, etc. Fortunately, the tool industry has adopted 12V, 18V/20V and 36V tool platforms, while other combinations are used for outdoor power equipment.
Voltage vs. Amp Hours: Wiring to Increase Amp Hours
A simple definition of amp hours is the amount of amperage a battery pack can supply for one hour. Ignoring all other factors such as temperature and vibration, a 3.0 amp hour battery will supply you with 3 amps for one hour. A 5.0 amp hour battery will supply you with 5 amps for one hour. Unlike voltage, this is not a fixed number. You can draw higher amperage from the battery and reduce run time. Jon Bucklew gives an excellent demonstration of this using a Makita 18V LXT brushless angle grinder. You can also draw less current and run longer.
are all linear. Running on 2.5 amps with a 5.0 amp hour battery – you get 2 hours of run time. Drain 6 amps on a 3 amp hour battery – now you only have 30 minutes. Here's a graph showing how the current draw affects runtime.
So how do we get these numbers? Most Li-Ion batteries have a run time of about 2000 mAh, or 2.0 Amp hours. When you connect these cells in series, they still only produce 2.0 amp hours combined. In a series, voltage is incorporated, not amp-hours.
When you need to increase the amp hours, you can connect batteries in parallel. This is an example of a typical 12V battery.
Three 18650 Li-ion cells are connected in series.
Each battery carries 3.6 volts and 2.0 amp hours. Since they are connected in series, we get 10.8 volts (or 12 volts fully charged), but still only 2.0 amp hours.
Different electronic devices take the same three batteries, but connect them in parallel. Right now they only produce 3.6 volts, but 6.0 amp hours.
Voltage vs. Amp Hours: Working Together
What happens in high capacity batteries is a combination of series and parallel wiring. First, you connect 5 batteries in series to get the required 18V. Then, connect another set in parallel to it in the same way. We kept the voltage at 18 but doubled the amp hours to 4.0. In theory we could add another bank to get 6.0 amp hours at 18V.
In a recent best lawn mower article we noted that the Black & Decker and Craftsman basically use repurposed max 20V batteries. They probably are. Take max 20V, 5.0 amp-hour batteries in a 5S2P configuration (5 in series, 2 in parallel = 10 cells arranged in 2 banks of 5) and run them all in series. Now, just change the configuration to 10S (10 Series) and you have a max 40V, 2.5 amp hour battery.
A total of 10 cells arranged in 2 rows, each row of 5 cells in series = 20V Max 5Ah battery pack
A total of 10 cells lined up in 1 continuous row, each row of 10 cells in series = 40V max 2.5Ah battery pack
Now back to the concept of total watt hours… no matter how you connect the cells together, the number of cells determines the watt hours of the pack. The 40V (36V nominal), 2.5 amp-hour Black & Decker battery and its 20V (18V nominal), 5.0 amp-hour cousin have a total of 90 watt-hours.
How Real World Conditions Affect Runtime
In the real world, things start to get crazy. When you talk about temperature (too hot and too cold), vibration and other environmental conditions, voltage and amp-hours start to deviate from ideal values. Still, these conditions are part of life on the job site. In some ways, manufacturers are setting better expectations by simply listing lower ratings (18V nominal rather than 20V maximum) that are more representative of actual operating experience.
There are many ways to make better batteries. You can start by looking at the chemistry inside the battery (anode, cathode, and other components like electrolyte). Each cell varies in resistance, impedance, and other funny words that most normal people can't define. This results in better (or sometimes worse) performance. All of a sudden, the same number of batteries producing 18 volts and 3 amp hours are delivering the same voltage, but 4 amp hours is now 5!
The difference in performance from one company to another has a lot to do with the batteries they use. The electronic controls and safety gear they use also play a role. It simply changes the wiring configuration, allowing you to add more power to the battery pack, increase the amp hours, or both. The net result of the combination can simply be that higher voltage means more total wattage and higher amp hours lead to more total run time.
We will continue to see improvements on the amp-hour end of the watt-hour equation as manufacturers continue to test different battery and case designs. For now, it seems like we'll continue to see cordless tool voltages stay in place while OPE struggles to get into the groove.