Milwaukee RedLithium Batteries Power Tool Technology

Milwaukee RedLithium Battery Technology Explained

So last year Milwaukee announced their RedLithium batteries, which replaced their current line of M12 and M18 lithium-ion batteries. The new batteries debuted at the 2010 new product symposium which we were privileged to be a part of. There, the company released new products, announced others, and generally showed off what was in the product chute for the coming year. Of course, one of the stars was the brand new RedLithium battery. Never one to simply accept a new product on face value alone, Pro Tool Reviews vowed to get to the bottom of the technology and find out what made the new battery packs and technology tick… Well, it turns out it’s more than you think – or at least it’s more than I thought.


Milwaukee RedLithium Battery Technology

To get the best information possible, we spoke with Paul Fry, Directory of Cordless at Milwaukee Electric Tool company. Now Paul will be the first person to tell you he’s not an engineer and can’t speak to the circuit board-level of RedLithium, but that’s not really what we were going for. No, we basically were after what makes RedLithium better than their last-gen batteries. In particular, we’ve heard about the advantages, but we wanted to know the “how” and “why” not just the “what”.

So, back in the dawn of lithium-ion, Milwaukee actually brought some guys from MIT on board to get into the business of building actual battery cells. Seriously, they were working with actual cells to adapt them for use with cordless power tools. This is actually pretty inane, because it meant that Milwaukee owned battery production equipment at one point. This is highly unusual to say the least, especially given that before lithium-ion the entire industry purchased their Ni-Cad cells from one of about two places. And for the most part, those manufacturers also built the battery packs (a fact that led to less-than-stellar solutions that couldn’t take much abuse).

Milwaukee RedLithium Battery Technology

How Lithium-ion Batteries Differ from Ni-Cad and NiMH

When Milwaukee moved to lithium-ion as a platform, they changed the way batteries were traditionally constructed (eliminating business as usual). There are way more battery manufacturers now (perhaps a dozen mainstream companies) so there are more options. On top of that, there is more to a battery than ever before – and while this is significant, it also takes the pressure off having to develop actual cells (that part is being done sufficiently well by the key manufacturers). So while Milwaukee (and other major power tool companies) don’t build the actual cell, Milwaukee does build the actual battery pack. Their theory is that you can take best cell in the world, but without electronic protection all that work will result in limited gains.

So in terms of growth, Milwaukee’s transition to lithium ion went from trying to build cells to realizing they could retain these battery-geniuses on staff and redirect their efforts towards driving the partner manufacturers to maintain the best cell construction while adapting the technology and characteristics of the battery packs to meet user expectations. These teams control the production of the battery pack itself and also focus specifically on development of the tool, pack and charger electronics. While the original battery packs had some electronics, Milwaukee deployed additional electronics to monitor and implement new circuits to analyze the signals between the batteries and the tools and from the batteries to the chargers.

So there ends up being three major places to focus when considering battery technology:

  • Battery pack construction
  • The actual lithium-ion cells within the pack
  • The electronics within the battery pack (and consequently, the electronics within the tool and the charger)

Smart Milwaukee RedLithium Batteries

The transition from “dumb” batteries to “smart” batteries seems to have evolved most significantly with the advent of lithium-ion. Batteries have gone from virtually no electronics to systems that monitor all aspects of a battery pack’s charging and discharging functions. During charging, the battery pack must speak to the charger or overcharging can happen. What’s an overcharge? Well, if the battery charges beyond its capacity the cell will break down and cease to work (or worse). Conversely, you could also take a battery pack over its temperature limits during the use of the tool where current draw exceeds limitations. That can melt your motor and also damage the battery. To avoid this, near-constant communication is happening during charging and discharging to monitor and adjust the cells as well as the motor for optimum use and safety.

Editor’s note: Overcharging is when the battery pack is overloaded from the charger (power IN to the battery). Overload is when the tool is being used (power OUT from the battery) and the current draw goes up too high (for example, when the bit is too large or the torque too consistently high). In these cases the tool will turn itself off to protect the tool and battery. A good battery-tool system will monitor both aspects of this energy usage.

The Run-time Equation – Knowing When to Quit

The other side of the equation is framed in terms of run-time. With the advent of lithium-ion, Milwaukee began to provide discharge protection on the battery pack itself. This eliminated the need to discharge the battery fully by holding down the trigger, etc… For anyone who has used Ni-Cad or even NiMH, you are familiar with the memory effect and the tendency for the battery to lose its potential to hold a charge over time. Running it out was a frequent and popular method of regaining the battery’s maximum potential (or at least as much as possible). What users were really doing was “crashing the voltage” of the pack. Eventually, this hurt the battery. Now, electronics will stop the tool from discharging itself to the maximum amount, protecting it from losing its ability to charge fully and completely. When a Milwaukee tool stops – it’s actually got a little charge left in the battery. It’s the electronics that are stepping in to say “it’s time to charge the tool, quit working – you’re done.”

Thermal Protection in Lithium-ion Batteries

So let’s talk about thermal protection, because it’s an equally important aspect of all the electronics that go into the lithium-ion battery packs. Let’s say you are cutting a bunch of lumber – but really we’re talking about any application that engages in high torque or sustained use of a tool. It’s not out of the realm that in cases like this you could take the pack up above its temperature threshold. With Milwaukee’s RedLithium the electronics will cut off the pack. Now what’s interesting to us is the observation that Milwaukee’s original lithium-ion batteries also contained electronic overload and thermal protection, but cut out a lot earlier than the new RedLithium models. According to Paul, the electronics didn’t change all that much, but advancements in cell technology did. So the original lithium-ion batteries didn’t have a different threshold, but rather the new RedLithium cells can simply keep the battery cooler thanks to a change in cell construction and chemistry. Of course, new potential and capabilities in the cells means that the electronics were adjusted to recognize this and respond accordingly. Remember those numbers above? That’s where all this increase came from – it’s real, not just marketing or a different way of measuring performance. The new RedLithium batteries literally build up heat slower and thus run longer before protection kicks in.

Milwaukee RedLithium Battery Technology - thermal protection

Here’s how RedLithium differs in terms of performance, from its predecessors (these are “up-to” numbers):

  • 40% More Run-Time
  • 20% More Speed
  • 20% More Torque
  • Fade-Free Power

And here’s how the actual durability of the RedLithium packs has changed:

  • Operates Down to 0°F/-18°C
  • Runs 20% Cooler
  • Up to 50% More Recharges
  • No Memory Effect

The Intelligence Behind Putting the Brains in the Battery Pack

So the battery pack talks to the tool. The tool talks to the battery pack. Certainly in terms of discharge, or using the power tool, on-tool protection and monitoring is again key. But each tool has to know what’s happening with its battery – what its temperature is, charge level, etc. That way each of Milwaukee’s tools will react and respond appropriately and you don’t get a “one size fits all” approach to battery management and performance. That basically means that different thresholds for different packs are also now possible. It really is a two-way street. If Milwaukee had simply placed overload thresholds only in the pack, then the pack would perform the same whether it was used with a flashlight or a circular saw. This is not the case with RedLithium (or Milwaukee’s original lithium-ion batteries for that matter).

Slim Packs vs. XC Fat Packs?

Milwaukee’s original pack limits – where users could not use a Slim pack with the cordless circular saw, for example, were there for a reason. First and foremost, Milwaukee wanted the ability to provide the user with a satisfying experience – to control the user experience even. If the slim packs were “unlocked” for use with higher-torque tools, they would perform slower, bog down more easily, and have significantly reduced run-time. There is a simple reason for this – the rise to the temperature threshold would simply happen more rapidly. The end result would likely be increased frustration on the part of the user, and Milwaukee simply opted to cut out the whole problem. Currently, RedLithium packs can handle higher temperatures and dissipate heat more easily due to the advanced cell structure and electronics. With this, the “feel” of the tool is sustained, even with less run-time and torque. The tool won’t run out as quickly, and so Milwaukee has opened up all of its tools to use the new batteries. Another feature of RedLithium is that, since the primary gains are in heat dissipation and management, the harder it’s pushed, the more increase in run-time that is noticed. This is a factor of how much better the power curve is realized under load.

Don’t think that this means your Slim pack will result in equal torque for your cordless drill or high power hammer drill. There is still the issue of the power difference between a 5 cell pack vs a 10-cell pack feeding the tool. Some of this is set and managed by electronics, but the tool also has a natural current draw that, while it can be limited by the battery pack electronics in terms of amperes per second, still has an effect on total power. Run-time or torque differences are based on the pack’s ability to feed the motor. What it does is “stiffen” the motor curve (at higher speed under load). This is where more torque and other advances take place with the XC 10-cell packs.

The New Milwaukee Tools

Last Fall, Milwaukee laid out 8 new cordless products with RedLithium. At present, the company is working on re-releasing its line-up of 30+ different tools (in various iterations) on the M12 system, and its 25+ M18 tools as well as the new and improved M28 system (which has around 15 products). Aside from the 5-3/8″ Metal Saw and the M18 Right Angle Drill, I think professional users will have the real eye opening experience when they pick up the new batteries and use them on their existing high torque tools.

What’s In Store for the Future of Lithium-ion

We asked about the future of lithium-ion, and in particular, the potential use of prismatic cells as they advance. The bottom line is that cell technology will continue to progress, albeit slowly. And it’s good to remember that the cell is just 1/3 of the equation. You can have the best cell plus no electronics and literally destroy a tool. Consequently, you can have a poorly constructed battery pack that shatters when dropped, spilling out all the advanced electronics and cell technology onto the ground… no good either. If you lose just one weld on a battery pack, you lose the whole thing – there is no partial credit with lithium-ion.


Increased amp-hour capacity is the likely advance that will come next – plus better cell impedance (which results in faster charging among other things). Given that iPads are really driving more advances in prismatics we may see some movement there, but for now it’s not likely that prismatics will come to power tools anytime soon due to cooling issues and other considerations. So while our batteries may not look different in the near-future, they are bound to last longer and charge faster – and that’s very very cool for professionals and home users alike.

Post Script Editor’s Note #2 – Here are a list of phrases that didn’t make it into this article:

  • “Paul seemed really charged up to talk to us about his new batteries”
  • “We ran our tools through a battery of tests…”
  • “Milwaukee really packed a lot into RedLithium…”
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The RED Lithium packs were released because the 1st generation of M12 Lithium packs were suffering from astoundingly high rates of failure. I have 12 of them, well I had 12 of them, now I only have 3. 9 of the batteries failed a few months past 2 years of age. They worked great up until they ran low, then when put on the charger flashed both lights to indicate a defective pack. No warning at all. 9 batteries just suddenly decided that they would no longer allow the charger to turn on. These were not used hard, they saw… Read more »


excuse me, but what what is the [expletive] chemistry? are they LiFePO4? A123? Nanophosphate?