That battery says 20V Max, 5.0Ah. Do you know what any of that actually means? Most people don’t, and the tool companies are counting on it. Those numbers on the label are a mix of useful specs and marketing spin, and telling the difference can save you real money.
This guide breaks down what volts, amp hours, and watt hours actually mean in plain language. No electrical engineering degree required. By the end, you’ll know exactly which battery specs matter for your work and which ones are just noise.
Voltage: The Push Behind the Power
Voltage (V) is the force that pushes electricity through the motor. Higher voltage means the tool can deliver more raw power to the business end of the bit, blade, or disc.
Think of it like water pressure. A garden hose at low pressure can water your flowers. A pressure washer at high pressure can strip paint off a deck. Same water, different force behind it. Voltage works the same way for electric motors.
Here’s how voltage tiers break down in practice:
- 12V — Compact tools for light-duty work. Drill/drivers for furniture assembly, impact drivers for cabinet screws, oscillating tools for detail work. Small, light, easy to use overhead or in tight spaces.
- 18V / 20V Max — The mainstream sweet spot. Handles 90% of what DIYers and most pros need. Drills, impact drivers, circular saws, reciprocating saws, and more.
- 36V / 40V Max — Heavy-duty cordless. Miter saws, table saws, large angle grinders. Makita’s 40V Max XGT line lives here.
- 54V / 60V Max — The top tier. DeWalt’s FlexVolt system. Powers tools that traditionally required a cord or gas engine.
The practical takeaway: For most DIY work and general contracting, 18V/20V Max tools are all you need. Only step up to higher voltage if you’re regularly running large saws, grinders, or outdoor power equipment.
The “20V Max” Marketing Trick
This is the single most important thing to understand about power tool batteries, and the brands would prefer you didn’t know it.
DeWalt’s “20V Max” batteries are 18V batteries. They are electrically identical to Milwaukee M18, Makita 18V LXT, and Bosch 18V batteries in terms of cell configuration. Same cell count, same nominal voltage, same actual power output.
Here’s how the trick works: A lithium-ion cell has a nominal voltage of 3.6V (its average working voltage) but a peak voltage of 4.0V when fully charged. Power tool batteries use 5 cells in series:
- 5 cells x 3.6V nominal = 18V (what Milwaukee and Makita report)
- 5 cells x 4.0V peak = 20V (what DeWalt reports as “20V Max”)
Same battery architecture. Same cells. Same real-world power. Different marketing. DeWalt started labeling theirs “20V Max” when they switched from NiCad to lithium-ion, and the bigger number looked better on the shelf. Other brands followed with their own “Max” labeling.
This pattern repeats at every voltage tier. Makita’s “40V Max” is really 36V nominal. DeWalt’s “60V Max” FlexVolt is really 54V nominal. Whenever you see “Max” after a voltage, subtract about 10% to get the real working voltage.
Does this mean DeWalt is lying? Not exactly — the battery does hit 20V for a brief moment right off the charger. But it spends the vast majority of its life operating at 18V, just like every other 18V battery on the market.
Amp Hours: Runtime, Not Power
This is the number one misconception in power tools: “I bought a 5.0Ah battery so my drill is more powerful now.” No. Your drill makes exactly the same power. It just runs longer before needing a charge.
Amp hours (Ah) measure capacity — how much energy the battery can store. It has nothing to do with how much power the tool produces at any given moment.
The gas tank analogy works perfectly here. Putting a bigger gas tank on your truck doesn’t give it more horsepower. It just means you can drive farther between fill-ups. A 5.0Ah battery is a bigger gas tank than a 2.0Ah battery, but the engine (your tool’s motor) runs exactly the same.
So when you swap from a 2.0Ah compact battery to a 5.0Ah battery on your impact driver:
- Same torque — the motor hasn’t changed
- Same speed — the voltage is identical
- Longer runtime — roughly 2.5x more work before recharging
- More weight — bigger batteries have more cells
That weight trade-off is real and worth thinking about. A compact 2.0Ah battery on a drill weighs noticeably less than a 5.0Ah pack. If you’re driving screws overhead all day, you’ll feel the difference in your shoulders by lunch.
Watt Hours: The Number That Actually Matters
If you want one number that tells you the total energy stored in a battery, watt hours (Wh) is the one. The formula is simple:
Volts x Amp Hours = Watt Hours
Watt hours let you make apples-to-apples comparisons across different voltage platforms. Here’s why that matters:
| Battery | Voltage (Nominal) | Amp Hours | Watt Hours |
|---|---|---|---|
| DeWalt 20V Max 2.0Ah | 18V | 2.0Ah | 36 Wh |
| Makita 18V LXT 3.0Ah | 18V | 3.0Ah | 54 Wh |
| DeWalt 20V Max 5.0Ah | 18V | 5.0Ah | 90 Wh |
| Bosch 18V CORE18V 8.0Ah | 18V | 8.0Ah | 144 Wh |
| DeWalt FlexVolt 60V Max 9.0Ah* | 54V | 3.0Ah (at 60V) | 162 Wh |
| Makita 40V Max XGT 5.0Ah | 36V | 5.0Ah | 180 Wh |
*FlexVolt 9.0Ah rating is in 20V mode. In 60V mode it delivers 3.0Ah — same total energy (162 Wh) either way.
This table reveals something useful: a Makita 40V Max 5.0Ah battery stores more total energy than a DeWalt FlexVolt despite the “lower” voltage label. Watt hours cut through the marketing noise and show you what you’re actually getting.
What Amp Hour Rating Do You Actually Need?
Bigger isn’t always better. The right battery depends on what you’re doing with it.
| Ah Rating | Best For | Trade-Off |
|---|---|---|
| 1.5 – 2.0 Ah | Precision work, overhead tasks, light fastening. Cabinet installation, furniture assembly, electrical work. | Light weight is a genuine advantage here. Charges fast (under 30 min). Limited runtime on power-hungry tools. |
| 3.0 – 4.0 Ah | General-purpose DIY. Drilling, driving, light cutting. Weekend projects. | Good balance of runtime and weight. The “Goldilocks” size for most homeowners. |
| 5.0 – 6.0 Ah | Extended work sessions. Deck building, framing, continuous drilling. Pro daily drivers. | Noticeably heavier. Worth it if you hate stopping to swap batteries mid-task. |
| 8.0 – 9.0 Ah | High-drain tools: circular saws, reciprocating saws, large angle grinders, demolition hammers. | Heavy. Best paired with two-handed tools where the weight sits low. Overkill for a drill or impact driver. |
| 12.0+ Ah | Cordless table saws, miter saws, outdoor power equipment. DeWalt FlexVolt territory. | Premium price. Maximum runtime for the most demanding cordless applications. |
Pro tip: Most professionals carry a mix. Compact 2.0Ah packs on drills and impact drivers for the weight savings, and 5.0Ah or larger packs on saws and grinders that chew through energy. You don’t need the same battery on every tool — match the battery to the job.
Battery Compatibility: Will Any Battery Work on Any Tool?
“Can I put a 5.0Ah battery on a tool that came with a 2.0Ah?” Yes. Always.
Within a single battery platform, every battery works on every tool at that voltage level. A DeWalt 20V Max 2.0Ah battery and a DeWalt 20V Max 9.0Ah battery use the exact same mounting interface. Snap either one onto any DeWalt 20V Max tool and it works perfectly.
The same is true for Makita 18V LXT, Bosch 18V, and every other major platform. The battery doesn’t control the tool’s power output — the motor and electronics inside the tool handle that. The battery just supplies electricity.
There are two exceptions to be aware of:
- Different voltage = different platform. A 12V battery won’t fit a 20V tool. The physical mount is different by design, so you literally cannot make this mistake.
- Some high-draw tools require a minimum Ah. A few tools (usually large saws) may specify a minimum battery size in the manual because the tool draws more current than a small battery can safely deliver.
This backward compatibility is one of the best things about modern battery platforms. Buy into a system, and every battery you own works across your entire tool collection. If you’re just getting started, our best cordless combo kits guide can help you pick the right platform from day one.
DeWalt FlexVolt: How One Battery Does Two Voltages
DeWalt’s FlexVolt batteries are genuinely clever engineering. A single battery automatically switches between 20V and 60V depending on which tool you slide it into. Here’s how it works.
A FlexVolt battery contains 15 lithium-ion cells arranged in three groups of five. When you mount it on a tool, the battery mount’s physical design routes the electrical connections differently:
- On a 20V Max tool: The three groups of 5 cells connect in parallel — 5 cells x 3.6V = 18V nominal (20V Max), with 3x the amp hours because three groups feed the tool simultaneously.
- On a 60V Max tool: All 15 cells connect in series — 15 cells x 3.6V = 54V nominal (60V Max), with the base amp hour rating.
The switching happens mechanically through the battery receptacle design. No electronics, no buttons, no software. The tool’s mount physically routes the connections differently based on its voltage requirement.
This is why a FlexVolt “6.0Ah” battery is labeled 6.0Ah at 20V but only 2.0Ah at 60V. The total energy (watt hours) is exactly the same either way — the battery just delivers it differently. It’s like having one hose that can deliver a wide, low-pressure stream or a narrow, high-pressure stream from the same amount of water.
For a deeper look at how cell configurations, battery management systems, and thermal protection work under the hood, read our complete guide to power tool battery technology.
Maximizing Your Battery Life
Lithium-ion batteries don’t last forever, but proper care can significantly extend their useful life. Here’s what actually matters and what doesn’t.
Charging Best Practices
- Use the manufacturer’s charger. Third-party chargers may lack proper cell balancing, which can shorten battery life or create safety concerns.
- Don’t worry about leaving batteries on the charger. Modern smart chargers stop charging when the battery is full. This isn’t the NiCad era — you won’t damage lithium-ion batteries by leaving them on the charger overnight.
- Let hot batteries cool before charging. If you just ran a battery hard and it’s warm to the touch, give it 10-15 minutes before plugging it in. Most quality chargers will delay charging a hot battery automatically.
Temperature Matters More Than Anything Else
- Cold reduces performance temporarily. Below 40 degrees F (4 degrees C), lithium-ion batteries deliver noticeably less runtime. The chemistry slows down. This isn’t permanent damage — they’ll return to normal performance once warmed up.
- Heat causes permanent damage. Leaving batteries in a hot vehicle (above 140 degrees F / 60 degrees C) degrades the cells permanently. Summer truck beds and dashboards are battery killers.
- Store at room temperature. A climate-controlled space is ideal. Garage shelves are fine in moderate climates, but avoid uninsulated sheds that get extreme temperatures.
Storage Tips
- Store at 30-50% charge if you won’t use them for a few months. Fully charged or fully drained batteries degrade faster in long-term storage.
- Use them regularly. Batteries that sit unused for a year or more may not recover full capacity. If you have seasonal tools, put the batteries on the charger at least once every few months.
When to Replace a Battery
A battery is done when you notice a significant drop in runtime doing the same type of work. Most quality lithium-ion tool batteries last 500-1,000 charge cycles before capacity drops noticeably. For a typical DIYer using them on weekends, that’s 5-8 years. Professionals who charge daily may see 2-3 years.
If a battery won’t hold a charge at all, won’t charge, or gets unusually hot during normal use, it needs to be replaced. Check our best power tool batteries guide for current replacement recommendations.
Frequently Asked Questions
Can I use a Milwaukee battery on a DeWalt tool?
No. Each brand uses a proprietary battery mount. Milwaukee M18 batteries only fit Milwaukee M18 tools. DeWalt 20V Max only fits DeWalt 20V Max. Third-party adapters exist, but they can introduce electrical issues, void your warranty, and bypass the communication between the battery’s management system and the tool’s electronics. Stick with matching brands.
Is it bad to leave batteries on the charger?
No. Modern lithium-ion chargers from the major brands have smart charging circuits that stop when the battery is full and trickle as needed to maintain charge. This isn’t like the old NiCad days where overcharging caused memory effect and reduced capacity.
Why does my battery die faster in cold weather?
Lithium-ion chemistry relies on the movement of lithium ions through an electrolyte solution. Cold temperatures thicken the electrolyte and slow ion movement, reducing the battery’s ability to deliver current efficiently. It’s temporary — warm the battery up (keep spares in your truck cab or an insulated bag during winter) and it’ll perform normally again.
Does fast charging damage batteries?
Marginally, over time. Fast chargers generate more heat, and heat is the primary enemy of lithium-ion longevity. But the difference is small with quality chargers that manage temperature properly. If you need fast turnarounds on the job, use the fast charger without guilt. If you have the luxury of charging overnight, a standard charger is slightly gentler on the cells.
Are higher voltage tools always better?
No. Higher voltage tools are more powerful but also larger and heavier. A 12V drill is genuinely better than an 18V drill for light tasks like furniture assembly or installing cabinet hardware — it’s smaller, lighter, and far easier to handle in tight spaces. Match the voltage to the job, not to your ego.
What’s the difference between “20V” and “20V Max”?
In practice, nothing. “20V Max” is the label DeWalt uses to indicate the battery’s peak voltage (4.0V per cell x 5 cells = 20V). The “Max” acknowledges that this is the ceiling, not the sustained working voltage. If you see “20V” without “Max,” it’s the same thing — manufacturers just sometimes drop the “Max” in casual copy. The nominal voltage is 18V either way.
The Bottom Line
Voltage determines what class of work the tool can handle. Amp hours determine how long it runs before recharging. Watt hours (volts x Ah) is the real measure of total energy. And “20V Max” is just 18V wearing a fancier label.
For most people, an 18V/20V Max platform with a mix of compact (2.0Ah) and standard (5.0Ah) batteries covers everything from light assembly to serious cutting. Don’t buy the biggest battery for every tool — match the battery to the work and your arms will thank you.
Ready to go deeper? Our full technical guide on battery cell chemistry, BMS electronics, and thermal management covers the engineering behind everything discussed here. And if you’re choosing between platforms, our battery platforms comparison breaks down DeWalt vs Makita vs Milwaukee vs Bosch in detail.
