Power Tool Technology

How It Works: Lithium-ion Batteries Explained


Everyone seems to be on a diet these days. You can’t turn around without someone talking about their latest no-carb or calorie-counting escapades into the world of better living. Me? I’ll take the steak, medium-rare, with a side of onion rings. But one thing does seem to ring true: what you put into your body tends to significantly affect how it performs. If tools have “bodies”, then their “food” would be the amps put out by today’s lithium-ion batteries. As we found out in last month’s Impact Driver Face-off, the Ah rating of a battery has a lot to do with its potential for run-time. But there’s a lot more to the story…a whole lot more.

To help define exactly what’s up, PTR independently interviewed two men we truly respect in the industry: Paul Fry from Milwaukee Electric Tool Corp and Jason Feldner from Bosch Tools. There are other experts, but we know these guys and have been fortunate enough to talk with them regularly over the years about power tools, batteries and lots more. It’s hard to imagine two better sources to deliver the low-down on the mysteries inside those battery packs we use so regularly.

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26700-cellsPro Tool Reviews (PTR): How long have you been working with lithium-ion battery technology?

Jason Feldner (Jason): Well, when you say Bosch you have to include Skill, Rotozip, and Dremel, among others; and Dremel was really our first brand to utilize lithium-ion batteries. We made that battery pack in 2003, and it really met our purpose of achieving a smaller more compact solution that could hold the same power. Or, it could stay the same size and offer more power. In 2005, Bosch launched their 12V Max (10.8V) tools. For the first time, lithium-ion batteries enabled smaller, more compact, tools to actually do like 80-90% of what was needed. A lot of driving applications simply didn’t require 18V tools…and this was a real wake-up call. The “pocket driver” was born. It really went from “Oh how cute” to “Wow, amazing!” To put the size in perspective, a 12V battery was essentially the size of the stem on the old stem packs. In 2008, Bosch introduced 18V lithium-ion battery packs, and 36V came out as well, but the new power density really made 18V the key size.

Paul Fry (Paul): Milwaukee began researching alternative battery pack designs and chemistries, including lithium-ion, in the mid-90s. Years of development led up to the V28 platform in 2005. One of the key elements we found was that you can take best lithium cell in the world, but if you don’t protect it, you can destroy it the first time you use it. V28 helped work through that process, and it was the first pack to carry its own electronics, which extended the life of the pack. The biggest challenge for us was delivering lithium-ion batteries that could sustain the high current required throughout the discharge cycle.

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PTR: Can you tell me how lithium-ion battery technology has changed from your 1st-gen batteries until now?

Paul: Well, the batteries changed, but also the industry changed. We had 2 production generations of V28 packs that used the 26700 cell—these big paper-wrapped cells. Then, we went through 4 generations of the 18650 variety of smaller cells in the M series beginning in late 2007. Overall, we’ve been through 6 generations. The first 4V 3Ah cells were developed by Milwaukee with E-One Moli in Canada. We quickly recognized that we needed a battery manufacturer to handle the volume. After that, we realized that our best position became to use our knowledge of lithium-ion to drive manufacturers to make better solutions for power tools. The 18650’s in the market today began because battery manufacturers started in energy cells (the batteries used in laptops) and were looking to expand their businesses. These same companies that were so successful in the electronics market realized they weren’t really equipped to make a power cell. They were in a high-volume, low-margin business, and the power cell market needed smaller quantities, but provided higher margins. Our industry ended up diversifying their businesses.

Editor’s Note: Check out the new Milwaukee 9.0 Ah batteries for the latest innovation and highest 18V power pack currently on the market

Jason: Lithium-ion technology has changed with regards to the role the battery cells play in the system. There are three components to a lithium battery pack: the battery cells, the electronics, and of course, the tool. For lithium-ion to function well, the electronics are essential. They control the battery and the movement of energy. The very nature of the the chemistry requires this. Ni-Cad cells didn’t need electronics; however, lithium-ion requires both the tool and the battery to communicate with each other. How that system works and communicates determines how the tool will function at its optimal level. These are the areas of advancement that are really changing the playing field. For Bosch, we’ve got the electronics in the tool, and there are minor electronics in the battery (mostly for identification). Some companies have batteries that could be placed in existing tools—placing the critical electronics in the battery itself. For Bosch, it made sense to come out with whole new line of tools. The theory is that both the tool and the battery must be designed together in order to work best.

PTR: How do you approach battery design—from cell selection to electronics and the actual packs themselves?

Jason: Certainly, you choose cells wisely, but to us, heat is the number one issue. Whether it’s your car, the human body, etc., heat is a big deal. For tools, the same is true, particularly with respect to batteries. The contacts will deteriorate under extreme heat, so Bosch uses a heatsink that’s built right in. If you look at the bottom of our packs, it has ribs on it that create more surface area for heat to leave the battery. On top of that, we engineered our packs to be durable and to survive drop tests.

Paul: Not all lithium cells are created equal. Battery manufacturers know how to make batteries, but they aren’t end users. They may not understand the high current needs of those tools. So a lithium-ion manufacturer is capable of delivering a cell emphasizing certain traits. High capacity Ah or Watt hours—they can do that. But just because it has the capacity doesn’t mean it can sustain the current.

Paul Fry on Energy Cells vs. Power Cells: A Cell is a Cell, Right…? Wrong.

Lots of energy cells are designed for consumer electronics, which is a priority of capacity over current capability—and there is very little focus on how the life of a cell depreciates over extended loads. In consumer electronics, there’s just not as much current, and it’s not used as often or as hard. Cell specification can be radically different than what power tools might require. While the external ratings of a pack might look identical (4.0Ah, lithium-ion, etc.) the lifespan, power delivery, and current handling capabilities can be wildly different. To put it in perspective, a typical 18V lithium-ion pack may be called upon to sustain in excess of 20 amps throughout its discharge. This is the kind of high current required to drive a power tool. Compare that to a computer or other consumer electronics device whose energy cells demand only 3-4 amps!

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PTR: How important are the electronics in modern Li-ion batteries?

Paul: Electronics really play a role in protecting the battery packs thermally. Take a lithium-ion cell in excess of 75ºC (167ºF) for a sustained time, and you will do permanent damage. Electronics regulate the system and protect your investment. They also handle discharge control—a new concept that came along with this technology. For Milwaukee, up until V28 you could discharge the pack as far as you wanted, but with V28 you couldn’t flatten the voltage in the pack. Milwaukee’s V28 shut off the power right before you would cross that critical voltage line. Even in late stages of Ni-Cad, when there were no real memory issues, people would still act as if there was, and they would run it all the way down. Because of the way the “death curve” worked on Ni-Cad that wasn’t good for the tool, but people still did it.

So now the electronics also handle cell balancing, controlling how voltage goes in and out of the cell. If the cells get unbalanced you can get poor performance and then eventually do damage to the pack. To most people, the electronics are most evident with the new fuel gauges on the battery packs and tools, but it’s really in the areas of performance and durability that you see the big benefits.

Jason: Essentially the battery communicates with the tool at all times, and therefore, the tool knows exactly what it can do. We’re constantly monitoring temperature on both the battery and the tool along with energy flow and the load. A difficult load means that the battery will step in and grant more power output. It also monitors for too much load, and it knows when damage or overheating will occur so that it can react accordingly to protect both the battery and the tool. The charger also analyzes the battery, so that it knows what to do and when to stop. It will be able to engage in “front-loading” the charge and then topping it off. This is sort of like how you fill your car up with gas or how you might fill your glass at the sink. You slow down at the end to avoid over-filling.

Jason Feldner on How a Cordless Power Tool Works

From the time the trigger is pulled, to engaging a load, there is a lot that happens in a power tool. This is particularly true when the tool encounters a load that is beyond the capabilities of the motor, causing it to begin to overload or overheat. Then, there’s the battery discharging and charging process. Combine this with how and where the electronics are located, and you can see that these next generation lithium-ion tools are not your father’s cordless tools! If you want to try and understand, here’s how we break down what’s really happening:

1. Slap the battery on the tool. The battery and tool say, “Hello,” and the electronics in the tool analyze the battery to determine how the tool can work with it. It knows whether this is a Slim or Fat pack and how much reserve it has in order to get the work accomplished.

2. Pull the trigger. The temperature is checked at battery. The tool asks, “How much energy can you give me?” while the battery queries, “How much do I need to do what you are demanding of me?” The tool continues analyzing the load, and the battery gives more (or not) based on heat and energy flow.

3. Rinse. Repeat. This analysis of what can be safely delivered occurs over and over at the millisecond level to ensure the tool’s safe operation. Simultaneously, the electronics make sure not to discharge the battery too far.

4. Charging. When a battery is placed on the charger, it’s analyzed. (Now, the charger, in effect, says, “Hello.”) The battery temperature is checked, and the pack is allowed to cool down before charging if needed. Charge time on a completely discharged battery may differ from a partially depleted battery due to the front-loading effect.

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PTR: Can you tell us anything about where you see the future of lithium-ion battery technology moving?

Jason: Well, lithium-ion batteries are a technology that is constantly being watched. There is “what’s possible” vs. “what users need”. What was nice about our new 4.0Ah lithium-ion batteries is that, thanks to the sophisticated work we did on the electronics, the new battery didn’t require a complete redesign. Rather, it just analyzed the new available power schedule and adapted tool and battery accordingly. Basically, the electronics will understand that they can push harder or run longer. So the future is definitely improvements in the cell chemistry and perhaps even gradual improvements to the way the tools and batteries communicate.

Paul: The challenge for manufacturers is that in the marketplace you’ll continue to see people pushing the far edges of what can be done. Car manufacturers are all over lithium-ion battery technology right now, and certainly, it plays some part with the green energy people. Take it all into account and the maturing of lithium-ion batteries has a lot of potential. We’ll most certainly see a progression of capacity, but there will also be trade-offs that can be made with positive effects. Currently, you don’t see power tools using prismatic cells (the non-cylindrical form factors), but there is probably room to go down that path. Three years ago, people would have said these new 12V and 18V 2.0Ah and 4.0Ah battery packs were a pipe dream. Now, gains in the manufacturing process and the new chemistries involved have allowed advanced packs like this to be made.

We want to thank both Jason and Paul for agreeing to be interviewed about battery technology…Um, particularly since neither was aware we were interviewing the other for this article. (Don’t worry, we let them both check it out to make sure we got all our details correct.) It’s hard work and innovation from guys like this (and their teams) that help bring the tools we love to market.

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If you are a product manager and have an interesting story to share about how a particular tool or technology works, please feel free to send us a note at [email protected], and we can consider using it in a future article or contacting you to get more information.

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