You're halfway through a cross-cut on a chunky piece of pressure-treated 4x4 and the saw just... stops. That familiar, frustrating silence. You look at the fuel gauge on your DeWalt lithium ion battery and it’s flashing a single, mocking red bar. We’ve all been there. It’s tempting to blame the tool or the charger, but the reality of lithium-ion technology is a bit more nuanced—and honestly, a lot of what people think they know about these yellow-and-black power bricks is just plain wrong.
Batteries aren't just plastic boxes full of "juice." They are complex chemical reactors. Inside that casing, lithium ions are physically migrating back and forth between an anode and a cathode. When you’re pushing a 20V Max circular saw through wet lumber, you’re asking those ions to move at a violent pace.
The Voltage Myth and the 20V Max vs 18V Reality
Let's clear the air on the "20V Max" branding immediately because it causes more arguments on job sites than it should. If you put a multimeter on a fully charged DeWalt 20V pack, you’ll see 20 volts. That’s the "maximum" initial battery voltage measured without a workload. However, the nominal voltage—the steady state it actually runs at once you pull the trigger—is 18 volts.
It’s the same thing as Milwaukee’s M18 or Makita’s LXT systems. DeWalt just marketed the peak surge rather than the sustained average. Does it matter for performance? Not really. What actually matters is the Amp-hour (Ah) rating and the "C-rate," which is how fast the battery can discharge its energy without overheating.
If you're using a 2.0Ah compact battery on a high-draw tool like a grinder, you’re basically trying to suck a milkshake through a cocktail straw. The battery gets hot. Heat is the absolute, number-one killer of lithium cells. When the internal temperature spikes, it triggers a chemical degradation called "capacity fade." You can't undo it. Once that capacity is gone, it’s gone for good.
Why 21700 Cells Changed the Game
For a long time, every DeWalt lithium ion battery used 18650 cells. These are about the size of a standard AA battery but slightly larger. They were the industry standard. But then DeWalt introduced the "21700" cells in their larger packs, like the DCB206 6.0Ah and the FlexVolt lines.
These newer cells are physically larger, sure, but they have significantly lower internal resistance. Lower resistance means less heat. Less heat means you can draw more current for longer periods without the battery's internal protection circuit (the BMS) shutting you down. If you’ve ever noticed that a 6.0Ah battery makes your drill feel "punchier" than a 2.0Ah battery, it’s not your imagination. The larger cells can literally deliver more "oomph" to the motor.
FlexVolt: The Engineering Trick Most People Miss
The FlexVolt system is arguably the most clever bit of electrical engineering in the power tool world. It’s a DeWalt lithium ion battery that can change its voltage depending on what tool you plug it into. How? It's all about series versus parallel circuits.
Inside a FlexVolt pack, there are 15 cells. When you slide it onto a 20V Max tool, the battery connects those cells in three parallel strings of five. This gives you a massive amount of runtime at 20 volts. But when you slide it into a 60V Max miter saw, the mechanical "key" in the tool flips a switch inside the battery. This reconfigures the cells into a single series of 15.
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Basically, it's a transformer you carry in your pocket.
The downside? Weight. Carrying a 9.0Ah or 12.0Ah FlexVolt on a drill-driver all day is a great way to end up at a physical therapist's office with tennis elbow. Use the right tool for the job. Use the right battery for the tool.
Cold Weather is a Silent Killer
Most guys know not to leave their batteries in a hot truck in July. But winter is just as bad, though for different reasons. Lithium-ion batteries rely on a liquid electrolyte. When it gets freezing, that liquid becomes more viscous. The ions can't move.
If you try to fast-charge a frozen DeWalt lithium ion battery, you risk "lithium plating." This is where the lithium ions, unable to enter the anode quickly enough, coat the surface in metallic form. This can cause a short circuit or even a fire. If your batteries have been in the garage at 20 degrees all night, bring them inside for an hour before you slap them on the charger. Your wallet will thank you.
The "Fake" Battery Epidemic on Amazon and eBay
You've seen them. Those generic, unbranded black batteries that claim to be "6.0Ah for DeWalt 20V" and cost $25. It’s tempting. Don’t do it.
Genuine DeWalt packs use high-quality cells from manufacturers like Samsung, LG, or Sanyo. These cells are rated for high discharge. The "knock-off" brands often use "B-grade" cells or, worse, reclaimed cells from old laptops. They might work for a month, but they lack the sophisticated Battery Management System (BMS) that communicates with the tool.
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A real DeWalt pack talks to the tool. It tells the tool, "Hey, I'm getting too hot, slow down," or "I'm empty, cut the power." Cheap clones often lack these thermal sensors. You aren't just risking a dead battery; you're risking a melted tool or a garage fire. Stick to the OEM stuff or reputable third-party brands that actually provide a warranty you can track down.
Maintenance Habits That Actually Work
Stop "topping off" your batteries every ten minutes. While lithium-ion doesn't have a "memory effect" like the old NiCad batteries did, it does have a limited number of charge cycles. Usually around 800 to 1,000. Every time you put it on the charger, you’re using a bit of that life.
Conversely, don't run them until the tool literally won't move. Modern DeWalt tools have a low-voltage cutoff, but if you leave a completely "dead" battery on the shelf for six months, it will naturally self-discharge. If it drops below a certain voltage threshold, the charger will see it as "defective" for safety reasons and refuse to charge it.
If you're storing batteries for the winter, leave them at about 2-bars (roughly 50-60% charge). This is the "Goldilocks" zone for chemical stability.
Troubleshooting the "Fast Blink"
We've all seen the charger light go into a seizure—that rapid red blinking. Usually, this means "Hot/Cold Delay." It just needs to reach room temperature. However, if it happens with a room-temp battery, check the terminals. Construction sites are dirty. Sawdust, drywall dust, and moisture can create a thin film on the copper contacts.
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Take a Q-tip with some high-percentage isopropyl alcohol and clean the contacts on both the battery and the charger. You'd be surprised how many "dead" batteries are actually just dirty. If it still won't charge, the cells might be "unbalanced." This happens when one group of cells inside the pack has a different voltage than the others. Some high-end repair shops can jump-start these, but for most people, that's when the battery becomes a paperweight.
Actionable Steps for Battery Longevity
To get the most out of your investment, you need a strategy. Stop treating these things like disposable AA batteries. They are the most expensive part of your kit.
- Match the Ah to the Task: Use 2.0Ah or 1.7Ah PowerStack batteries for overhead drilling or light assembly. Use 5.0Ah or 6.0Ah for high-torque tasks like lag bolts or circular saws.
- The 20-80 Rule: If you can, avoid draining the battery to 0% and avoid leaving it at 100% on the charger for weeks. Staying in that middle 60% margin significantly extends the total lifespan of the chemistry.
- Label Your Packs: Use a silver Sharpie to write the purchase date on the bottom of your batteries. When one fails, you’ll know if it was a premature death or if it simply put in its four years of hard labor.
- Invest in a Fast Charger for Big Packs: If you're running 9.0Ah FlexVolt batteries, the standard small DCB107 charger will take half a day. Get the DCB118 or the newer four-port chargers that can push 8 amps. Faster charging isn't always better for the cells, but DeWalt's fan-cooled chargers manage the heat well enough to make it worth the time savings.
- Register for the Warranty: DeWalt usually offers a 3-year limited warranty on their batteries. Most people lose the receipt. Take a photo of it and save it in a "Tools" folder on your phone. If a $150 battery dies at month 14, you'll be glad you have it.
Modern cordless power is incredible, but it’s still bound by the laws of physics. Treat your DeWalt lithium ion battery with a little respect—keep it dry, keep it moderately cool, and keep the contacts clean—and it will likely outlast the tool it's attached to.