What size of 12V battery to run a car stereo?

[rupertlssmith]

What size lead acid sealed leisure battery would I need to run a car stereo, with the volume up for an hour say? Thanks for the help

 

[c_mitra]

I run a car stereo (actually I use the pen drive only) from a 12V 1A (SMPS) power supply that was from a old printer adapter. I guess a 12V6AH would be sufficient to run for 1H with pleasure.

 

[KlausST]

Hi,

Measure the current.
Often this is less than 0.5A.

Then if you have a new fully charged battery with lets say 6Ah,
then divide the 6Ah by 0.5A --> this gives 12h.

Like Ah says it is Amperes x hours.

For sure if the same battery is old it won't have full 6Ah of capacity.
And if it is partely charged it will have "partely" capacitance.

Klaus

 

[thannara123]

To calculate that correctly we need to understand the power of stereo .

 

[Audioguru]

Modern car "stereos" have 4 or 5 channels. I had a car that came with 4 speakers that were 2 ohms for about 25 real Watts per channel plus a subwoofer that had a 128W amplifier. I never played it loud enough for the amplifiers to clip so it had plenty of headroom.

 

[C1500]

If it is only a 4 channel head unit you should be able to use a pretty small automotive battery to get an hour of play time. My job-site radio is basically a head unit with two speakers and I can get about 6 - 8 hours of play time from a 4 amp hour 12v power tool battery.

 

[rupertlssmith]

Thanks for the replies. I wanted to get a rough idea only, but it sounds like 4 or 6Ah will easily be enough. I'm only going to run 2 speakers on it.

 

[Audioguru]

I'm only going to run 2 speakers on it.

But you forgot to say what is the impedance of your speakers. 8 ohm speaker will play all day. 4 ohm speakers will play for a few hours. A little battery might not produce enough current for 2 ohm speakers.

 

[rupertlssmith]

But you forgot to say what is the impedance of your speakers. 8 ohm speaker will play all day. 4 ohm speakers will play for a few hours. A little battery might not produce enough current for 2 ohm speakers.

I haven't got the speakers yet, but I'll bear that in mind when I do. Thanks.

 

[Warpspeed]

Just run it the way you normally run it, and measure the current.

Amps x hours = amp hours.

If you expect the battery to have a reasonably long life, its rated capacity should be at least double what you expect it to provide, and preferably more.

 

[c_mitra]

But you forgot to say what is the impedance of your speakers. 8 ohm speaker will play all day. 4 ohm speakers will play for a few hours. A little battery might not produce enough current for 2 ohm speakers.

I thought it all depends on the output power. When you regulate the output, you control the output power and the speaker will adjust the voltage and current to deliver the power. In fact I even do not remember the input impedances of my speakers. Why it is critical?

 

[Warpspeed]

What size lead acid sealed leisure battery would I need to run a car stereo, with the volume up for an hour say? Thanks for the help

That must mean at least 500+ watts of thumping bass that rattles the windows next door.

 

[Audioguru]

I thought it all depends on the output power. When you regulate the output, you control the output power and the speaker will adjust the voltage and current to deliver the power.
I even do not remember the input impedances of my speakers. Why it is critical?

An amplifier does not regulate the output power. The amplitude of the input signal, the volume control setting, the supply voltage to the amplifier (high power car audio systems boost the DC voltage) and THE IMPEDANCE OF THE SPEAKER determines the output power because its current is higher. Use ohm's law:

13V battery, amplifier output is 10Vp-p which is 3.5V RMS.
1) 3.5V squared/8 ohms= 1.5W.
2) 3.5V squared/4 ohms= 3W. It might be two 8 ohm speakers in parallel and each one gets 1.5W.
3) 3.5V squared/2 ohms= 6W. It might be two 4 ohm speakers in parallel and each one gets 3W.
Now use a bridged amplifier so that the voltage swing is almost doubled and the power is 3.5 times more.
Then the amplifier produces 10.5W into 4 ohms but the salesman says its battery is overcharging at 14.4V and the amplifier output is clipping badly almost producing a squarewave so they advertise 14 Whats per channel.

 

[KlausST]

Hi,

I don´t think the OP runs the amplifier continously at it´s theoretical maximum.

On the other side I agree that the 4Ohm speaker (with B or AB linear amplifiers) may cause the battery to drain a little earlier - even with same output power as with an 8 Ohms speaker.

****

If the OP wants to play music for a long time, then he should look for a low power standby car stereo with Class D amplifiers inside.

Klaus

 

[Audioguru]

I think acid rock music (noise?) is full continuous power. Ordinary music averages about 1/5th of the maximum output power.
I have never seen an ordinary car radio that uses class-D amplifiers because the car battery is massive and can power class-AB amplifiers for a very long time. How many people listen to the car radio for hours with the engine charging system turned off?

 

[c_mitra]

An amplifier does not regulate the output power. The amplitude of the input signal, the volume control setting, the supply voltage to the amplifier (high power car audio systems boost the DC voltage) and THE IMPEDANCE OF THE SPEAKER determines the output power because its current is higher.

Absolutely! What I was trying to say (perhaps I did not communicate well) that we regulate the output power as per our preference. We have only one control, the volume control, that we adjust to listen to the "sound" at a comfort level. That is what I meant by "output power". It is possible to have the same "sound level" with a 8 Ohm or a 2 Ohm speaker.

Most modern audio amplifiers are high tech and have low output impedance: they can drive 8 Ohm or 2 Ohm speaker without loss of efficiency. In other words, the efficiency of the amplifier is not *significantly* affected if you use a 8 Ohm or a 2 Ohm speaker.

I believe that if the output power (electrical energy fed to the speaker) remains *approximately* the same, the power consumption from the battery will also remain *approximately* the same.

13V battery, amplifier output is 10Vp-p which is 3.5V RMS.
1) 3.5V squared/8 ohms= 1.5W.
2) 3.5V squared/4 ohms= 3W. It might be two 8 ohm speakers in parallel and each one gets 1.5W.
3) 3.5V squared/2 ohms= 6W. It might be two 4 ohm speakers in parallel and each one gets 3W.
Now use a bridged amplifier so that the voltage swing is almost doubled and the power is 3.5 times more.

Perfectly correct but it is not DC and the impedance of the speaker varies with the music. If we have a battery at a 12V connected to a 2 Ohm speaker how do we get different output at all?

Most of the current will be coming from the buffer capacitors and we can have variable power output even with a const volt power supply and a low Ohm load.

Only thing that will be affected is the max power output- 2 Ohm speakers will naturally handle greater power - because the voice coils are thicker and stronger and can take higher currents...

But how many of us listen to the music at the full volume? Even in a car?

Then the amplifier produces 10.5W into 4 ohms but the salesman says its battery is overcharging at 14.4V and the amplifier output is clipping badly almost producing a square wave so they advertise 14 Whats per channel.

A car audio amplifier output is clipping badly at 21W/ channel? Assuming 4 channels, it will be about 80W total output- and assuming 50% amplifier efficiency, the total input power will be 160W- more than keeping all the lights on (including the head lamps).

 

[BradtheRad]

We dropped a car off to get a new windshield installed, and picked it up at the end of the day. The mechanics had run down the battery listening to the car stereo for hours. It had 4 speakers. Apparently they played it loud while they worked around the outside of the car. We could not start the engine, so they had to attach jumper cables and give us a boost.

 

[ads-ee]

We dropped a car off to get a new windshield installed, and picked it up at the end of the day. The mechanics had run down the battery listening to the car stereo for hours. It had 4 speakers. Apparently they played it loud while they worked around the outside of the car. We could not start the engine, so they had to attach jumper cables and give us a boost.

Dang! Did they give you a discount for reducing the life of your car battery doing that? :x

If you have an Optima battery with the deep discharge capability I suppose they could claim you don't deserve any discount ;-)

 

[BradtheRad]

Dang! Did they give you a discount for reducing the life of your car battery doing that? :x

I thought they ought to offer us some break, but running down the battery can't easily be quantified. I had trouble believing it could happen by playing the radio a few hours... Unless they had the key on engine-running position (energizing the alternator at 3 or 4 Amps) all that time. However I thought for sure they would know better than to do that.

So I wondered if our electrical system might have something wrong with it. The alternator quickly recharged the battery. There were no bad aftereffects, and the windshield kept out water, so I forgot about it.

I have also seen a dome (or trunk) light drain a battery overnight, which seems unbelievable as well. I guess the battery was a few years old on those occasions.

 

[c_mitra]

If you have an Optima battery with the deep discharge capability...

I have always been very suspicious about the "deep discharge" claims- lead acid batteries are notoriously unable to recover fully from even a partial discharge.

I believe it is related with the formation of lead sulphate on the plates and it will not go away easily.