Can You Use A 40V Battery In A 20V Black And Decker Tool?

Like many other major power tool brands out there, Black and Decker offers cordless tools to make working outdoors, doing repairs, or cleaning the house a lot more convenient. The brand currently features two primary battery systems that power these tools: the 20V MAX* POWERCONNECT and 40V MAX* batteries. The 20V model works with tools like impact drivers, handheld vacuums, and garden cultivators. Meanwhile, the 40V variant is engineered to power heavier equipment, such as lawn mowers, leaf blowers, and string trimmers. 

If you've been using Black and Decker for some time now, you probably have both systems in your garage. And since the batteries look almost the same, you might also start wondering whether you can start swapping out the 20V battery for the bigger 40V one. 

Well, the short answer is no, you can't use a 40V battery in a 20V Black and Decker tool — the same way you can't insert a 20V battery in a 40V tool. The 40V and 20V Black and Decker batteries simply aren't interchangeable, and for good reason. 

One of the biggest mistakes to avoid when using lithium batteries, like the Black and Decker 40V line, is pairing them with a lower-voltage device. Doing so poses serious safety risks. In the case of the Black and Decker 20V tools, their 20V label clearly indicates that they can only run on 20V batteries — meaning that's the maximum input voltage they can safely handle.

When a tool rated at only 20V gets powered by a 40V battery, you expose the tool to a significant mismatch in the power input. This higher voltage delivers too much power to the device, which could potentially lead to overheating, damage to the internal circuitry, and even a fire or electrical shock. 

Then, there's also the issue of sizing. The Black and Decker 40V and 20V batteries have completely different sizes, with the 40V being larger by a few inches. It measures 5.9 by 3.4 by 7.2 inches, while the 20V variants are only 4.5 by 3.2 by 6.9 inches. Since the 20V tools' compartments are specifically designed to fit the exact size of the 20V battery, a 40V model will most likely not connect properly. You could try forcefully inserting it, but you also risk breaking the battery connectors or the battery itself. 

With all that being said, if you have a 20V Black and Decker tool, make sure to power it with just 20V batteries. Your 40V battery should only go into 40V tools. If you're thinking of increasing the battery voltage from 20V to 40V because you need your device to last longer, what you actually have to do is stay with the 20V line but up the battery capacity (measured in Ah) instead. The higher the Ah figure, the longer it can run.