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Calculating Battery Life for 370W Speaker & Amplifier: Ampere-Hour for 3-4 Hours Playtime

tuniolx 42207 35
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How large a 12 V battery do I need to power a 370 W max speaker/amplifier setup for 3–4 hours?

Do not size the battery from the 370 W MAX figure; use the amplifier’s RMS output and its efficiency, because the speaker power does not get added to the amplifier power [#11849134][#11849073] For the bridged 70 W RMS output mentioned later, the amplifier would draw about 110 W from the battery at roughly 65% efficiency [#11849134] The actual playtime depends on volume, music, and amplifier class, so the thread’s rough estimates range from about 1–2 hours on a 7 Ah battery and about 70% longer on 12 Ah, with a 100 Ah battery only giving about 1–1.5 hours if you were really near full output [#11848975][#11849201][#11848934] The most reliable way to size it is to run the system on a battery, measure the average current with an ammeter at your normal listening level, and use that for the calculation [#11849256] Ordinary car batteries also do not like deep discharge at high current; traction batteries are better suited for this use [#11848975]
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  • #32 11849286
    Anonymous
    Anonymous  
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  • #33 11849304
    tuniolx
    Level 14  
    Posts: 152
    Rate: 12
    and who learned it :D
    there have always been and will be ties
    ps it's junior high
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  • #34 11849320
    vodiczka
    Level 43  
    Posts: 30170
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    tuniolx wrote:
    to know how much energy is left in it

    Unfortunately, you won't know. Buy a voltmeter when the voltage drops to 10.8 V, it will mean that you have discharged the battery to a minimum and you must charge it immediately. You can discharge to 11.0-11.2V safely.

    Added after 2 [minutes]:

    tuniolx wrote:
    and who learned it
    there have always been and will be ties

    Unfortunately, this shows in your attempts to add power to the amplifier and speaker. You will not spend your whole life on cheat sheets.
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  • #35 11849338
    tuniolx
    Level 14  
    Posts: 152
    Rate: 12
    sorry but I'm not that stupid
    anything but physics, this is black magic.
    math chemistry english it's light but it's sorry but never
  • #36 11849372
    jankolo
    Rest in Peace  
    Posts: 32197
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    tuniolx wrote:
    and who learned it :D
    there have always been and will be ties

    So please look for a tie. I close the topic.
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Topic summary

✨ The discussion revolves around calculating the appropriate ampere-hour (Ah) battery capacity needed to power a 370W speaker and amplifier setup for 3-4 hours. Initial calculations suggested a 12V 100Ah battery would suffice, but further analysis revealed that the actual power consumption is higher due to amplifier efficiency (approximately 65%). This leads to a requirement of around 570W from the battery, translating to a current draw of 47.5A, which would deplete a 100Ah battery in about 1-1.5 hours under heavy use. Participants debated the importance of RMS versus peak power ratings, concluding that only the amplifier's RMS power should be considered for battery calculations. Recommendations included testing with a borrowed battery to measure actual performance and using an ammeter to monitor current draw.
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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?

  1. Insert an ammeter in series with the 12 V line.
  2. Play typical music at desired volume.
  3. 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?

  1. Continuous sine-wave testing at full power—draw spikes 2× higher.
  2. 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).
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