Train Horn Battery Drain Calculator: Runtime in Minutes
How long before your truck battery dies running a train horn compressor? Enter Ah capacity, amp draw, duty cycle, alternator output — runtime with Peukert correction.
Battery type
Engine state
Amp-hours at the 20-hour rate
Actual output now: 52 A
Peak current when compressor runs
% of time the compressor is actively running
Stereo, lights, fans already running
Alternator keeps up
∞
Alternator delivers 52 A, which covers the 22 A average load. You can run this setup indefinitely at the current engine state.
Load vs. alternator
Peukert check
- Rated capacity (20-hr)
- 70 Ah
- Peukert-adjusted capacity
- 48.5 Ah
- Usable before damage
- 38.8 Ah (80% DoD)
- Average total load
- 22 A
Lead-acid batteries deliver less than their rated capacity at high discharge rates — a 100 Ah battery rated at the 20-hour rate only gives ~65 Ah when pulling 30 A continuously. We apply the Peukert equation (t = H × (C / I·H)^k) with k = 1.2 for AGM (XS / Optima / Odyssey). Lithium's flat discharge curve means Peukert barely matters (k ≈ 1.05) — one reason LiFePO4 drop-ins punch above their rated Ah.
Embed & share
All inputs are preserved in the shared URL and iframe code.
How to calculate battery drain from a train horn compressor
The naive battery drain math looks simple: runtime in hours equals battery Ah divided by load amps. Reality has three complications, and this calculator handles all of them:
- Depth of discharge. You cannot pull 100% of a battery's rated Ah without damaging it. Flooded lead-acid tops out at 50% DoD; AGM at 80%; LiFePO4 at 95%.
- Peukert's law. Rated capacity assumes a slow 20-hour discharge. At 30 A continuous, a 100 Ah flooded battery delivers only ~65 Ah.
- Alternator contribution. With the engine running, the alternator offsets part or all of the load — often making the setup sustainable indefinitely.
The runtime formula
t_hours = (C_rated × DoD × Peukert_factor) / (I_load − I_alternator)If I_alternator ≥ I_load, runtime is effectively infinite — the battery stays topped off. That's the reality for most daily-driver train horn setups with the engine running. The calculator flags this case and tells you your coverage margin in amps.
Alternator output at idle vs. cruise
A healthy alternator makes 30–50% of its rated amps at idle (600–900 RPM), and 85–100% above 2000 RPM. That's why your headlights dim when you hit the horn at a stoplight — the alternator isn't spinning fast enough to carry the extra load. Typical numbers:
- 70 A alternator (compact car) — 25 A at idle, 60 A at cruise
- 130 A alternator (mid-truck) — 50 A at idle, 110 A at cruise
- 180 A alternator (HD truck) — 70 A at idle, 155 A at cruise
- 250 A HO upgrade — 100 A at idle, 210 A at cruise
Battery type comparison
- Flooded (standard starting battery) — cheap, 50% DoD, Peukert k ≈ 1.3. Fine for engine-running use; marginal for troll horns.
- AGM (XS Power, Odyssey, Optima YellowTop) — 80% DoD, Peukert k ≈ 1.2, tolerates deep cycles, high CCA. The default for serious builds.
- Deep-cycle flooded (marine/RV) — 80% DoD but weak CCA; only useful as a second battery.
- LiFePO4 lithium drop-in — 95% DoD, Peukert k ≈ 1.05, flat voltage curve. Pricey but a 100 Ah lithium beats a 150 Ah AGM in usable output.
When do you need a second battery?
If the calculator shows under 15 minutes of runtime with engine off, and your use case involves engine-off troll blasts or parade stops, add a second battery. Mount it in the bed or trunk with a manual cutoff and wire through a marine-grade battery isolator (Blue Sea, Trombetta) sized for 1.25× alternator output. The wire gauge calculator gives you the right cable for the battery-to-battery run.
Frequently asked
- How long can a car battery run a train horn compressor?
- With the engine off, a standard 60 Ah flooded battery feeding a 30 A compressor at 40% duty cycle runs about 40 minutes before hitting the 50% discharge damage point. An AGM upgrade doubles that to 80 minutes (80% safe DoD). With the engine at idle, the alternator contributes 30–40 A and the compressor typically keeps up indefinitely.
- Will a train horn kill my car battery?
- Not from short blasts. A 2-second blast draws only ~0.03 Ah from the battery — invisible. The risk is troll-running the horn at a stoplight with the engine off, which can drain a weak battery below 12.0 V in 5–10 minutes. If you plan to troll, upgrade to an AGM battery and verify the alternator output at idle covers compressor current.
- What is Peukert's law and why does it matter?
- Peukert's law says battery capacity falls as discharge current rises. A 100 Ah flooded battery rated at the 20-hour rate (5 A draw) only delivers ~65 Ah when pulling 30 A continuously. Train horn compressors pull 23–46 A, so Peukert derates your usable capacity 20–35%. LiFePO4 lithium has a Peukert coefficient near 1.0, which is one reason a 100 Ah lithium outperforms a 100 Ah AGM for high-current loads.
- Do I need a second battery for a train horn?
- Only if you troll or run without the engine for extended periods. Serious show builds add a dedicated AGM or lithium battery in the trunk/bed, wired through a battery isolator or DC-DC charger. For daily-driver use with the engine running, your existing battery is fine as long as it's healthy and the alternator output at idle exceeds the compressor draw.
- How much alternator output do I have at idle?
- About 30–50% of the rated maximum. A 130 A alternator on a truck makes ~50 A at idle; a 70 A small-car alternator makes ~25 A. That's often less than a single Viair 444C (46 A), which is why revving the engine during sustained horn use keeps the battery neutral instead of drawing it down. The calculator above shows whether your engine state covers the load.
- What size battery do I need for a 5 gallon train horn build?
- A 70–100 Ah AGM (Group 31 or Group 34R) covers the vast majority of serious 5 gallon builds. For dedicated show trucks running dual compressors off the key, step up to 150 Ah AGM or a 100 Ah LiFePO4 drop-in. The 20-hour Ah rating matters more than CCA for train horn use — CCA is about cranking the starter, not sustaining a compressor.
- Should I use a deep cycle or starting battery?
- Dual-purpose AGM (both high CCA and deep-cycle tolerant) is the sweet spot. Pure deep cycles don't make enough cranking amps for reliable engine starts; pure starting batteries hate being deeply discharged. Brands to look at: XS Power D-series, Odyssey, Optima YellowTop, Northstar. All are AGM and rated for 80% DoD.
Related