If you already own Ryobi tools, you own the hard part: a shelf of 18V ONE+ batteries. Slide one onto a BossHorn unit and you have a 130 to 150 dB train horn with no wiring, no air tank, and no separate power source to babysit. The only real questions are which ONE+ packs actually fit, and how long each one keeps the horn blasting. Here's the straight answer.
The short version
Every 18V ONE+ lithium battery Ryobi has made fits a BossHorn Ryobi horn — from the little 1.5Ah compact up to the 12Ah HP brick. They're all the same 18-volt platform, so compatibility is never the issue. The only thing that changes from pack to pack is runtime: a bigger amp-hour rating means more blasts before you swap. A mid-size 4Ah pack is the sweet spot for most people, a 6Ah pack is what we'd hand a tailgater or a farmer, and the 1.5–2Ah compacts are fine for a glovebox backup.
Every ONE+ battery fits — that's the whole point of the system
Ryobi has built the ONE+ line around one promise since 1996: any 18V ONE+ battery works with any 18V ONE+ tool and charger. That same promise is why the horn is so easy to feed. Whether your pack is a years-old blue lithium unit or the latest green HP brick, the slide-rail interface and the 18-volt output are identical, so it locks onto the horn the same way it locks onto your drill or blower.
A few specifics worth knowing:
"18V ONE+" and "20V MAX" are the same voltage
This trips people up constantly, so it's worth clearing up. A DeWalt or Craftsman "20V MAX" pack and a Ryobi "18V ONE+" pack put out the same working voltage. "20V MAX" is just the peak reading taken with no load on the battery; the second you actually draw current, the pack settles to its nominal 18 volts. Ryobi labels its packs by that nominal figure. So a Ryobi ONE+ horn isn't "weaker" than a 20V-badged one — same volts under load, same compressor, same sound.
That matters when you compare runtime across brands, because the real measure of how long a pack lasts isn't amp-hours alone — it's watt-hours (volts multiplied by amp-hours). Since every ONE+ pack runs at 18 volts nominal, the math is clean and you can read runtime straight off the amp-hour rating.
Battery size vs. blasts per charge
Here's how the common ONE+ packs stack up. Watt-hours are the honest capacity number; the blast estimates assume a quad-trumpet BossHorn unit firing short signaling blasts, with sustained two-second blasts draining things faster.
| ONE+ pack | Amp-hours | Energy (18V nominal) | Best for |
|---|---|---|---|
| Compact (P102/P190) | 1.5–2.0Ah | 27–36 Wh | Glovebox backup, occasional use |
| Standard | 3.0–4.0Ah | 54–72 Wh | Everyday carry, best all-rounder |
| High capacity (P193) | 6.0Ah | 108 Wh | Tailgates, farm, boat ramp |
| HP high-output | 9.0–12.0Ah | 162–216 Wh | All-day events, leave-it-mounted setups |
The pattern is simple: double the amp-hours, roughly double the blasts. A 4Ah pack gives most owners hundreds of short blasts between charges, and a 6Ah pack pushes that well past 500 short blasts or around 200 sustained two-second blasts — the same energy a 6Ah pack delivers on any 18V platform. If you only ever need a few attention-getting honks at a time, even a 2Ah compact will outlast your patience before it dies.
Standard vs. HP — which pack should you actually grab?
For a horn, you don't need the high-output HP packs the way a circular saw does, because the horn's compressor pulls far less current than a saw under load. What HP packs buy you is sheer capacity. So pick by how you use it:
- Daily driver / quick honks: a 4Ah standard pack. Light, cheap, and good for hundreds of blasts.
- Events, farm, marine: a 6Ah or 9Ah pack so you're not thinking about charge level all weekend.
- Cold-weather use: Ryobi's lithium packs are rated to hold output in frigid conditions, and many carry an onboard LED fuel gauge so you can check remaining charge at a glance before you head out. Handy when the horn lives in a truck bed in January.
If you're shopping the whole Ryobi-compatible range — dual, quad, and the louder premium tiers — it's worth seeing them side by side rather than guessing from spec lists.
Thinking about jumping platforms?
Some folks run Ryobi for yard work but keep a second platform for the truck. If you're weighing where to put your loudest setup, our flagship lives on the Milwaukee side: the Extreme Series Train Horn for Milwaukee® 18v Battery is the benchmark for raw output in the lineup. The good news is the battery rules are identical across platforms — 18 volts nominal, fits the packs you already own — so whichever ecosystem you're invested in, the horn meets it where it is.
FAQ
Do all Ryobi ONE+ batteries really fit the horn?
Yes — every 18V ONE+ lithium pack, compact through HP, uses the same mount and the same 18-volt output, so they all fit. We'd just steer you toward lithium packs over the old NiCad blue ones for cleaner power to the compressor.
What size battery should I buy if I'm starting fresh?
A single 4Ah pack covers most people for everyday use, and a 6Ah pack is the move if you want a full day of tailgating or farm work without swapping. Buy the horn as a kit if you don't already own ONE+ batteries; add a spare if you'll run it hard.
Will a 20V MAX battery work instead?
Not physically — 20V MAX packs from DeWalt, Craftsman, or others use a different mount, even though the voltage is the same. Match the horn to your battery brand. We build a separate version for each major platform, so pick the one that matches the packs on your shelf.
How many blasts will I get on a charge?
It scales with amp-hours. A 6Ah pack delivers well over 500 short blasts or roughly 200 sustained two-second blasts; a 2Ah compact gives you a smaller but still generous count. Short signaling honks stretch a charge much further than long held blasts.
Can I leave the battery on the horn?
You can, but for storage it's better to pop the pack off and keep it indoors at a moderate charge. Lithium cells last longer when they're not sitting fully drained or baking in a hot truck bed for weeks.