Along with Milwaukee’s extensive M18 cordless tool system comes a wide range of battery options to power them. Their REDLITHIUM battery packs are some of the most powerful battery solutions on the market, delivering the longest runtime and strongest power output possible. The Milwaukee M18 battery platform is now fully backwards compatible as well, which is great for those who have curated a large collection of Milwaukee tools over the years. However, when choosing the right battery for your tools, it is important to understand the difference between the various battery ranges. We will be taking a closer look at the original M18 REDLITHIUM batteries and the newer M18 High Output batteries to see how each affects the performance of our Milwaukee FUEL tools.

What’s the Difference? 

18650 Battery

The M18 High Output batteries from Milwaukee are designed to provide “50% more power” and “run 50% cooler” than the original REDLITHIUM XC battery packs. The cell technology of each battery is what makes the biggest difference in their performance. Milwaukee uses two types of battery cells: 18650 cells and 21700 cells. The numbers refer to the size of the cell, so 18650 cells measure 18mm by 65mm and 21700 cells measure 21mm by 70 mm. The High Output batteries use 21700 cells, which are newer and now most commonly used to run larger, more powerful tools. The rest of Milwaukee’s traditional battery packs use 18650 cells, which are more compact but don’t have as long of a runtime.  

The High Output battery packs claim to have 50% more peak power and run 50% cooler for sustained use, but that result comes from using them with new tools that are specifically designed to work with the High Output batteries. This made us wonder, do the High Output batteries change the performance of other FUEL tools as well? What about Compact M18 Tools? We recently compared several newer M18 Impact wrenches in various size ranges to test their power output. This test uncovered some interesting results when looking at each tools performance using the traditional REDLITHIUM M18 4.0Ah Battery and a High Output 6.0Ah Battery that we thought might be worth sharing. 

The Battery Test 

Do the new M18 High Output batteries using new 21700 cells actually increase power for M18 tools over traditional M18 Red Lithium batteries with 18650 cells?  

Although we did not originally intend to compare the two battery ranges during this test, we noticed a pretty significant change in the performance of our tools. The original test was just looking at Milwaukee’s 4 most recent M18 Impact additions: the M18 Stubby ½”, M18 Mid-Torque ½”, M18 High Torque Extended Anvil ½”, and the M18 Tire Gun 1” D-Handle. In the shop we have a M18 REDLITHIUM 4.0Ah battery which is several years old but lives in the climate controlled shop with minimal usage as well as a 6.0Ah High Output battery that is brand new out of the packaging. It was clear right out of the gate the tools performed very differently with these two battery packs, so we ran the full test on all 4 impacts with both batteries to see how each might perform.

Torque Output on Skidmore Machine

And the Winner Is!  

We of course were not very surprised to see the M18 High Output batteries, in this case the 6.0Ah battery, delivering a better performance for the 1” High Torque Impact. This tools comes with two 12.0Ah batteries and Milwaukee recommends using High Output batteries with it for best performance. It is very interesting to see all the other tools even down to the compact stubby perform better based solely on the battery.  

We’ve always heard the pitch on Milwaukee FUEL about how REDLINK technology  connects the batteries and brushless motors to optimize performance. While it may seem like a marketing gimmick at first, this test has shown that battery power can make a huge difference on the performance of the motor and overall efficiency of your tool. Other brands are catching on to this trend as well. Dewalt is moving towards the next generation of “Flexvolt Advantage” tools with their 20V brushless tools to basically do a very similar thing with backwards compatible batteries.        

Since completing our test we actually saw another video on the Torque Test Channel, where they were testing Dewalt 4.0Ah vs 5.0Ah vs a Fake Knockoff Battery. We recommend checking it out if you’re interested in seeing how their different battery ranges compare. This makes us really want to go back and test everything Milwaukee and Dewalt from 3.0Ah to 12.0Ah batteries to see how the two rivals hold up against each other.   

Final Thoughts 

As our test proved, moving towards the High Output batteries is a good idea for tool performance even if the tools are more compact. Of course lights, radios and plenty of other M18 tools won’t see any performance changes outside of runtime, but it’s pretty clear all Milwaukee FUEL tools— old and new— will benefit from using the M18 High Output batteries.   

If you have any questions about the M18 REDLITHIUM or High Output battery ranges, the OPT team is here to help! Give us a call at 800-242-4424, email us at, or leave a comment below for assistance. To be the first to know about new products and deals, subscribe to our e-newsletter and follow us on social media!