Short answer: no. A Ryobi® 40V battery will not run a train horn gun — it won't even click onto the mount. Ryobi builds two completely separate battery systems, and the horn runs on the other one: ONE+™ 18V.
The Short Answer: Wrong Platform, Wrong Plug
Ryobi's 40V line and its ONE+ 18V line are two independent battery platforms. They run at different voltages, use different pack shapes, and connect through different interfaces, and Ryobi does not make them cross-compatible in either direction. A 40V pack won't attach to anything built for the ONE+ 18V mount — including a battery-powered train horn gun — and an 18V pack won't attach to a 40V mower or trimmer.
The difference is obvious the moment you hold both packs side by side. A 40V battery is a slide-on pack with its contacts exposed on the surface, sized for the wide docks on outdoor equipment. A ONE+ 18V pack uses Ryobi's signature stem design — a post that inserts up into the base of the tool. No rail, notch, or contact lines up between the two. You physically cannot make the connection, and that's intentional on Ryobi's part.
So if you have a shelf of 40V packs from your mower and trimmer and you were hoping they'd double as horn power, they won't. The good news: the packs that do work are among the cheapest and most common cordless batteries in America, and there's a real chance you already own one.
Why Ryobi Runs Two Separate Battery Systems
The split is deliberate, and it explains the whole compatibility picture. ONE+ 18V is Ryobi's tool platform: drills, drivers, saws, lights, inflators — more than 300 products that all share the same 18V battery. Because those packs are compact, affordable, and everywhere, this is the platform that battery-powered train horn guns for Ryobi batteries are built around.
The 40V line is Ryobi's outdoor power equipment platform: lawn mowers, string trimmers, leaf blowers, chainsaws, snow blowers. Ryobi puts the count at more than 85 products in the 40V system, and essentially all of them are yard machines that need sustained high wattage for long stretches. That workload justifies a physically bigger, higher-voltage pack — and a completely different mount to keep the two systems from ever being mixed up.
| Platform | Built for | Nominal voltage | Runs a train horn gun? |
|---|---|---|---|
| ONE+ 18V | Power tools (300+ products) | 18V | Yes — this is the one |
| 40V | Outdoor equipment (85+ products) | 36V | No — different mount, different voltage |
What "40V" Actually Means on the Label
Here's the number that surprises people: a Ryobi 40V battery isn't really a 40-volt battery. Inside the pack are lithium-ion cells rated at 3.6V nominal each, wired ten in series. That works out to 36V nominal, climbing to roughly 42V fresh off the charger. "40V" is the between-the-two marketing figure — the same labeling convention that lets other brands call an 18V-nominal pack "20V MAX."
A ONE+ 18V pack is the same chemistry at half the scale: five cells in series, 18V nominal, right around 20V straight off the charger. Electrically, it sits in the same class as every other major brand's compact tool pack — which is exactly the voltage a horn gun's compressor motor is designed around.
That gap matters for more than fitment. A motor and trigger switch engineered for 18V nominal would see double their design voltage on a 36V pack. Overvolting a DC motor that far is a fast route to overheated windings and a dead compressor — which is one more reason the mounts are keyed so the mistake is impossible to make by accident.
What About a 40V-to-18V Adapter?
Ryobi doesn't make one, in either direction. The adapters you'll find online are third-party and DIY-market products, and most of the cheap ones simply remap the contacts without converting the voltage — which, per the math above, is exactly what you don't want feeding an 18V-class device. There's a warranty angle too: Ryobi's warranty terms exclude damage resulting from alteration or modification, and the same logic applies to the horn's own warranty when it's run on a power source it wasn't built for. We break down the whole adapter question — including the rare cases where adapters make sense — in our guide to cross-brand battery adapters on a train horn gun.
If you're thinking "but the DeWalt® FLEXVOLT® 60V battery works on a horn gun — why not Ryobi 40V?", the difference is in the pack, not the horn. FlexVolt packs have dual-voltage switching built into the battery itself: they physically fit the standard DeWalt 20V rail and automatically drop their output to 20V when mounted on a 20V-class device. We covered that in detail in our FlexVolt 60V train horn guide. Ryobi 40V packs have no equivalent mechanism — they're single-voltage batteries with a mount that exists nowhere in the tool line.
The Battery That Does Work: Any ONE+ 18V Pack
Every horn gun we build for the Ryobi platform mounts a standard ONE+ 18V battery — any capacity, standard or HIGH PERFORMANCE, the same pack that runs a Ryobi drill. A typical 5Ah-class pack is good for roughly 1,500+ blasts on a single charge, so even a modest pack from a drill kit covers months of use. Full pack-by-pack details, including which capacities balance weight against runtime, are in our Ryobi ONE+ 18V compatibility guide.
On the horn side, the ONE+-compatible lineup runs from a dual-trumpet model for Ryobi® batteries to a quad-trumpet version, up to a premium build with a wireless remote — all hitting up to 150dB, all running off the pack you already charge for your tools.
And if your garage runs on more than one battery brand, the same horn exists for every major 18V/20V-class platform. The loudest configuration we make is the 5-Trumpet Horn Gun for Milwaukee® 18V Battery, which runs on any Milwaukee® M18™ pack — worth a look if there's red on your charger shelf instead of green.
FAQ
Will a Ryobi 40V battery physically fit on a train horn gun?
No. The 40V pack is a surface-contact slide-on design and the horn's port is a stem-style ONE+ 18V receptacle. Nothing aligns, so there's no way to force it — the incompatibility announces itself before any electricity is involved.
Are 40V and 40V HP batteries different platforms?
No — they're the same platform. Ryobi states that any 40V lithium-ion battery powers any of the 85+ products in the 40V system, HP or not. HP simply marks the newer high-output packs. But HP or standard, no 40V battery works on ONE+ 18V equipment, and none works on a train horn gun.
I only own 40V batteries. What's the cheapest way to run a horn gun?
Pick up any ONE+ 18V pack — even a compact one from a starter kit will fire the horn reliably, since the horn draws from the pack in short bursts. Alternatively, if you own a brushless drill on another platform, the universal DIY 4-trumpet horn kit pairs with the drill you already have instead of requiring a dedicated battery mount.
Does any high-voltage battery line work on a train horn gun?
Only one we know of: DeWalt FLEXVOLT, and only because the pack itself steps down to 20V automatically on a 20V-class mount. High-voltage lines without built-in dual-voltage switching — Ryobi 40V included — stay incompatible, full stop.