FAQ
TL;DR: A 30 W RMS class-AB amp typically pulls ≈5 A from 12 V, "65 % efficiency is a safe assumption" [Elektroda, Rzuuf, post #11848975] A 7 Ah SLA can run it about 1–1.5 h; a 12 Ah about 2–2.5 h. Why it matters: oversizing the battery prevents mid-party silence and extends battery life.
Quick Facts
• 12 V 7 Ah SLA delivers only ≈4 Ah at the 2-h discharge rate [BatteryUniversity, 2023].
• Class-AB audio amplifiers show 60–70 % efficiency near full power [Self, 2013].
• Safe cut-off for lead-acid is 10.8–11.0 V under load [Elektroda, vodiczka, post #11849320]
• Deep discharging below 10.5 V can cut cycle life by ≥50 % [BatteryUniversity, 2023].
• AGM 12 Ah costs ≈90 PLN; 7 Ah ≈40 PLN (*Allegro price search*, 2025).
1. How do I estimate battery life for my 12 V audio amplifier?
Divide amp RMS power by battery voltage, then correct for efficiency. Example: 30 W ÷ 12 V = 2.5 A. At 65 % efficiency: 2.5 A ÷ 0.65 ≈ 3.9 A. Battery-life (hours) ≈ capacity (Ah) ÷ current (A). A 12 Ah battery: 12 Ah ÷ 3.9 A ≈ 3 h [Elektroda, vodiczka, post #11849134]
2. Which rating matters—peak, max or RMS?
Use continuous RMS power. Peak or “music” power can be 2–5× higher yet lasts milliseconds and skews calculations [Elektroda, vodiczka, post #11849020]
3. Why does efficiency affect battery sizing?
The amp turns some electrical power into heat. With 65 % efficiency, 35 % becomes heat, raising current draw by 1 / 0.65 ≈ 1.54× [Elektroda, Rzuuf, post #11848975]
4. How long will a 7 Ah and a 12 Ah SLA run a 30 W RMS amp at 75 % volume?
Average draw drops to ≈3 A. 7 Ah ÷ 3 A ≈ 1.3 h. 12 Ah ÷ 3 A ≈ 2 h. Higher volumes or battery age shorten this by about 20 % [Elektroda, vodiczka, post #11849201]
5. What happens if I discharge the battery too quickly?
Capacity falls as discharge current rises; a 7 Ah SLA may deliver only 4 Ah at 2-hour rate [BatteryUniversity, 2023]. Voltage will sag sooner, ending playtime early.
6. Can I use a car starter battery instead?
Yes, but starter types hate deep discharge. Repeated 80 % discharges can cut life to 50 cycles, versus 300-500 for deep-cycle AGM [BatteryUniversity, 2023].
7. Do unused amplifier channels still draw power?
Yes, but little. With one channel idle the amp still consumes quiescent current; expect under 0.1 A for small boards [Elektroda, vodiczka, post #11849188]
8. How can I measure real current draw?
- Insert an ammeter in series with the 12 V line.
- Play typical music at desired volume.
- Read and average the current over one minute. This value beats estimates [Elektroda, Anonymous, post #11849256]
9. What voltage tells me the battery is "empty"?
Stop playback at 10.8–11.0 V under load. Below 10.5 V sulphation accelerates and cycle life plummets [Elektroda, vodiczka, post #11849320]
10. I added speaker wattage to amp wattage—why is that wrong?
The speaker consumes whatever the amp supplies. Adding them double-counts energy. “I let myself be done” comments one expert [Elektroda, Rzuuf, post #11849062]
11. What edge cases kill runtime fastest?
- Continuous sine-wave testing at full power—draw spikes 2× higher.
- Ambient temps below 0 °C—lead-acid capacity drops about 20 % [BatteryUniversity, 2023].
12. Are lighter battery options available?
A 4-cell LiFePO₄ pack, 12.8 V 10 Ah, weighs ≈1.2 kg—half an AGM’s weight—and offers 2,000 cycles, but costs >300 PLN (E-shop survey, 2025).
) from 4 to 8 hours. Connect any charged battery (it may be borrowed) with known capacity and check experimentally.
