You own one battery platform, and you want to know which one will keep a train horn blasting longer at the tailgate, on the boat ramp, or out at the farm. The honest answer surprises most people: a Milwaukee M18 pack and a DeWalt 20V MAX pack of the same amp-hour rating give you the same number of blasts. The real difference isn't the brand on the battery — it's how big a single pack each platform lets you carry.
The short answer
For matching battery sizes, M18 and DeWalt 20V MAX are a dead tie. A 6.0Ah pack on either platform runs a BossHorn unit for the same 500+ short blasts or about 200 sustained two-second blasts. Where DeWalt pulls slightly ahead is at the very top end: its FlexVolt 15.0Ah pack is the single largest battery on either system, so if you load the biggest pack each brand makes, DeWalt squeaks out more total blasts. If you run identical mid-size packs, pick whichever batteries you already own.
Both platforms actually run at 18 volts
This is the part that clears up most of the confusion. "20V MAX" is a marketing number — it's the peak open-circuit voltage of a DeWalt pack before you put any load on it. The moment you pull current, the pack settles to its nominal 18 volts. Milwaukee simply labels its packs by that nominal figure and calls it M18.
So under load — the only condition that matters when a compressor is running — a DeWalt 20V MAX battery and a Milwaukee M18 battery sit at the exact same voltage. That means a 5.0Ah pack from either brand stores the same energy and drives the horn's compressor for the same amount of time. The "20V vs 18V" debate is a labeling difference, not a power difference.
Why watt-hours decide the count, not amp-hours
Amp-hours (Ah) only tell half the story. What actually drains while you fire the horn is energy, measured in watt-hours (Wh): voltage multiplied by amp-hours. Because both platforms run at 18V nominal, the math is identical for matching sizes:
| Pack size | Energy at 18V nominal | Available on M18? | Available on 20V MAX? |
|---|---|---|---|
| 2.0Ah | 36 Wh | Yes | Yes |
| 5.0Ah | 90 Wh | Yes | Yes |
| 6.0Ah | 108 Wh | Yes | Yes |
| 8.0Ah | 144 Wh | Yes | Yes (incl. PowerStack 8Ah) |
| 12.0Ah | 216 Wh | Yes (High Output) | Yes (XR / FlexVolt) |
| 15.0Ah | 270 Wh | No | Yes (FlexVolt only) |
One note on label-reading: DeWalt's spec sheets sometimes print watt-hours using the 20V max figure instead of the 18V nominal one, which inflates the number by about 11 percent. DeWalt rates the FlexVolt 15.0Ah at 300Wh (15Ah x 20V); at the apples-to-apples 18V nominal it's 270Wh. Milwaukee lists its 12.0Ah High Output at 216Wh the same nominal way. Compare nominal-to-nominal and you're comparing fairly.
Blasts per charge: head-to-head
BossHorn rates both the Milwaukee and the DeWalt Extreme Series horns identically — 500+ short blasts or roughly 200 sustained two-second blasts on a 6.0Ah pack — because the compressor and trumpets are the same; only the battery foot changes. Using that 6.0Ah figure as the anchor and scaling by watt-hours, here's what each platform delivers:
| Battery | Energy (18V nominal) | Short blasts (est.) | Sustained 2-sec blasts (est.) |
|---|---|---|---|
| 5.0Ah (either brand) | 90 Wh | ~415 | ~165 |
| 6.0Ah (either brand) | 108 Wh | 500+ (rated) | ~200 (rated) |
| M18 12.0Ah High Output | 216 Wh | ~1,000 | ~400 |
| DeWalt FlexVolt 15.0Ah | 270 Wh | ~1,250 | ~500 |
Only the 6.0Ah row is a manufacturer-stated number. The rest are straight-line estimates scaled by watt-hours — real counts shift with temperature, how long you lean on the button, and cell age. The takeaway: matched packs tie, and DeWalt's exclusive 15.0Ah pack is the only thing that breaks the tie, giving roughly 25 percent more blasts than M18's biggest 12.0Ah.
These are the two horns those numbers come from — the Extreme Series Train Horn for Milwaukee 18v Battery and its DeWalt twin:
Where DeWalt pulls ahead
DeWalt's FlexVolt 15.0Ah (model DCB615) is the highest-capacity pack on either cordless system. In a 20V MAX tool it runs as a 15.0Ah battery, and that single pack carries more energy than anything Milwaukee offers in the M18 line. If your goal is the most blasts from one battery without a swap — say you're running a horn all day at an event — the FlexVolt pack is the reason to lean DeWalt.
FlexVolt packs are heavier and pricier, so this advantage only matters if you genuinely need marathon runtime. For a few hundred blasts at a tailgate, it's overkill.
If you're on the DeWalt platform, here's the full lineup of horns that fit your 20V MAX packs:
Where Milwaukee pulls ahead
Milwaukee's edge isn't watt-hours — it's the ecosystem. The M18 12.0Ah High Output is extremely common, and most M18 owners already have 5.0Ah or 6.0Ah packs in the truck. The platform's strength is that you almost certainly have a spare pack on the charger, and swapping a fresh M18 in takes seconds. With one spare you effectively have unlimited runtime regardless of which brand technically holds the single-pack record.
For a deeper breakdown of exactly how the M18 numbers play out across pack sizes, see our guide on how many blasts a Milwaukee M18 train horn does per charge.
If you run M18, these are the horns built for your packs:
So which should you buy?
Buy the horn that matches the batteries already in your garage. That's the whole decision for 90 percent of people, because the per-blast performance is identical at matching pack sizes. A few tiebreakers:
- You're brand-new to both and want max single-pack runtime: DeWalt, for the FlexVolt 15.0Ah.
- You already own either platform: stay put — the horn performs the same, and your existing packs and chargers carry over.
- You want the loudest, longest-running setup regardless of brand: pair an Extreme Series horn (up to 150 dB) with the biggest pack you own and keep one spare charged.
FAQ
Is a DeWalt 20V battery more powerful than a Milwaukee 18V?
No. "20V MAX" is the peak voltage with no load; under load both packs run at 18 volts nominal. A 5.0Ah DeWalt and a 5.0Ah Milwaukee store the same energy and run the horn for the same time.
Which battery gives a train horn the most blasts?
At equal amp-hours, it's a tie. The single pack that delivers the most blasts is DeWalt's FlexVolt 15.0Ah, because it's the largest-capacity battery either brand makes — roughly 25 percent more than Milwaukee's 12.0Ah High Output.
Does a bigger battery make the horn louder?
No. Loudness comes from the compressor and trumpets, not the battery. Both Extreme Series horns hit up to 150 dB across three volume levels no matter which pack you fit. A bigger battery only changes how many blasts you get.
Can I use aftermarket batteries on either horn?
Yes. Both horns accept any M18-style or 20V MAX-style pack, genuine or aftermarket, from 2.0Ah up. The watt-hour math is the same — an aftermarket 18V 5.0Ah pack still stores about 90 Wh and gives a similar blast count.
What happens when the battery runs low mid-event?
Both horns have an auto-cutoff around 15 percent that warns you via the LED before shutting off, so you're never caught flat. Keep a spare pack on the charger and swap to keep going — that matters more than which brand you picked.