Battery Drain Calculator — Truck Train Horn Engine-Off Runtime
How long before your truck battery dies running a train horn compressor with the engine off? Enter Ah capacity, amp draw, duty cycle, and alternator output.
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.
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