FLEX® 24V showed up at Lowe's in spring 2021 and has been growing fast ever since — pouch-cell Stacked Lithium packs, charges as quick as 18 minutes, and a catalog that adds tools every season. What most owners don't know: the same 24V pack that runs your drill can fire a 140 dB train horn. Here's exactly which FLEX packs fit, what the 24V label really means, and how many blasts each battery size delivers.
The short answer: every FLEX® 24V pack runs it
A battery-powered train horn is built like a cordless tool. Inside the housing there's a compressor, four tuned trumpets, and a battery dock molded for one specific platform — no air tank, no 12V wiring, no compressor kit under the bed. BossHorn builds a dedicated FLEX dock: the Quad Train Horn for Flex® 24v Battery. Slide your pack onto the rails until it clicks and the horn is live.
The spec sheet in plain numbers: 140 dB maximum output with three volume settings (full, medium, soft), four powder-coated aluminum trumpets in staggered lengths — 14, 12, 8, and 5 inches — for that layered freight-train chord, audible up to a mile out. It accepts FLEX 24V packs from 2.0 Ah to 12.0 Ah, aftermarket FLEX-compatible packs included, and comes with a 160 ft wireless remote (a 2,000 ft long-range remote is an optional upgrade). The unit weighs about 4 lb before you add the battery.
The FLEX® 24V platform in 60 seconds
If you're newer to the platform, here's the context that matters for a horn buyer. FLEX cordless tools launched at Lowe's in April 2021, with nationwide store availability that spring. The brand itself has German roots in high-end grinders and automotive polishers, and today it's owned by Chervon — the same manufacturer behind EGO outdoor equipment and Skil tools. FLEX backed the launch with a limited lifetime warranty on tools, batteries, and chargers for buyers who register.
The practical takeaway: this is a current, actively supported battery platform sold at one of the two biggest home centers in the country. Batteries are easy to find, and every 24V pack FLEX has shipped — first-generation cylindrical-cell packs and the newer Stacked Lithium line alike — is stated by FLEX to be 100% cross-compatible with all FLEX 24V tools and chargers. The horn's dock rides on that same compatibility.
What the 24V label actually means
Here's the fine print FLEX doesn't hide but most people never read: 24V is the pack's maximum initial voltage, measured fresh off the charger with no load. Under load, the pack settles to its nominal 21.6 volts. The reason is cell count — a FLEX 24V pack runs six lithium-ion cells in series instead of the five you'll find in 18V and 20V MAX™ class packs. Six cells at roughly 3.6 volts nominal each is 21.6V working voltage; six at a fresh-charge peak of about 4 volts is where the 24 on the label comes from.
That sixth cell is a real difference, not marketing. An 18V-class 6.0 Ah pack stores about 108 watt-hours; a FLEX 6.0 Ah pack stores roughly 130 Wh at nominal voltage — FLEX's own label rates it at 144 Wh using peak voltage. Either way you count it, that's around 20% more energy per amp-hour riding on the same size battery. Kobalt® packs are built the same way — six cells, 21.6V nominal — which is why the two 24V platforms sold at Lowe's answer this question identically; we broke that one down in the Kobalt 24V compatibility guide. The horn's compressor is spec'd for the FLEX pack's full voltage range, so you get the benefit as runtime, not as risk.
Which FLEX® batteries fit the horn
All of them. FLEX has shipped two generations of 24V packs, and both mount on the horn's rails the same way:
- Standard lithium-ion packs — 2.5, 3.5, 5.0, 6.0, and 8.0 Ah. The original cylindrical-cell line that launched with the platform. The 2.5 Ah is the light kit battery; the 8.0 Ah is the big cylindrical pack for high-draw tools. Any of these from 2.0 Ah up runs the horn.
- Stacked Lithium packs — 2.0, 3.5, 6.0, and 10.0 Ah. FLEX's newer pouch-cell design. The 6.0 Ah version weighs 2.2 lb, carries THERMA-TECH™+ heat management, and FLEX rates it to fully charge in as little as 18 minutes on its fast charger. For a horn, the honest note is that stacked cells buy you charging speed and cooler discharge under heavy tools — they don't change how loud the horn gets.
- Aftermarket FLEX-compatible packs. Third-party 24V packs that mount on FLEX rails work too, up to the horn's 12.0 Ah ceiling. If it clicks onto your drill, it clicks onto the horn — though factory packs are the safer bet, for reasons we covered in our aftermarket battery guide.
Runtime: blasts per charge, size by size
A horn is about the friendliest load you can hand a battery. A saw or mower pulls sustained current for minutes; a horn fires in one- and two-second bursts with rest in between. That's why even a compact 2.5 Ah pack is genuinely useful here. The manufacturer rating for the FLEX-compatible quad horn is 500+ short blasts or about 200 sustained blasts on a 6.0 Ah pack — the table below scales that rating linearly across the lineup, so treat the other rows as close estimates:
| FLEX® 24V pack | Stored energy (nominal) | Short blasts (est.) | Sustained blasts (est.) |
|---|---|---|---|
| 2.0 Ah Stacked | ~43 Wh | ~165 | ~65 |
| 2.5 Ah | ~54 Wh | ~210 | ~85 |
| 3.5 Ah | ~76 Wh | ~290 | ~115 |
| 5.0 Ah | ~108 Wh | ~415 | ~165 |
| 6.0 Ah | ~130 Wh | 500+ (rated) | ~200 (rated) |
| 8.0 Ah | ~173 Wh | ~665 | ~265 |
| 10.0 Ah Stacked | ~216 Wh | ~830 | ~330 |
Two practical notes. First, nobody fires 500 blasts in a day — a 2.5 Ah pack covers a full tailgate, a trail ride, or a season of scaring geese off the dock before it asks for a charger. Second, the fast-charge story matters more than the capacity story: if you own the Stacked Lithium 6.0 Ah and FLEX's fast charger, an 18-minute top-up means the horn effectively never sits dead.
FAQ
Does the horn come with a battery?
No — that's the point of the platform-specific dock. Your existing FLEX packs are the power system. If you'd rather keep tool batteries on the tools, a compatible 24V battery and charger are sold as optional add-ons at checkout, but neither is required.
Do I need a Stacked Lithium pack, or will my older FLEX battery work?
Any FLEX 24V pack from 2.0 Ah up works — first-generation cylindrical packs included. FLEX states its 24V batteries are fully cross-compatible across the platform, and the horn's dock follows the same rule. Stacked Lithium buys faster charging, not more decibels.
Is there anything louder than 140 dB for the FLEX platform?
140 dB is the current ceiling for the FLEX-compatible quad model. BossHorn's louder Extreme Series — larger trumpets, 150 dB-class output — currently ships on other docks, like the Extreme Series Train Horn for Milwaukee® 18v Battery. If maximum output matters more to you than staying on one battery wall, that's the route.
Is 140 dB safe to fire?
Respect it like a power tool. OSHA's technical guidance treats 140 dB peak as the ceiling for impulse noise exposure. Fire the horn pointed away from people, keep it at arm's length, use short bursts, and don't set it off in an enclosed space. The three-level volume switch exists exactly for close-quarters use.