annoying sound when battery not fully charged

ghanchi · 02-08-2009, 04:57 PM

Hey,

I have my laptop in the trunk and I recently put in a battery.
The battery is basically there so i can turn the car off n the laptop can go into hibernate w/o using the car's battery.

But now whenever the battery is not fully charged, I get an annoying buzzing sound. The sound stays there until the power meter is at 100%. So usually, it takes like 10-15 mins of driving for the sound to completely stop.
The screen also gets lines, and the lines are synchronized with the buzzing(stops when sound stops, comes when sound comes).

I already have a ground loop installed, and that didnt help get rid of the sound.

Anyone have any idea how this could possibly b fixed? Or what exactly is causing the interference?

WuNgUn · 02-08-2009, 05:07 PM

Are you powering the laptop from a brick, or...?
Also, is it the laptop battery your refering to (at 100%) or the additional battery?

Bugbyte · 02-08-2009, 05:10 PM

Why don't you use the laptop battery to allow it to hibernate?

ghanchi · 02-08-2009, 05:34 PM

laptop is connected from a brick. i bought the DC version of the PSU and hooked that up to the the car's 12V source.

the sound stops when the laptop battery is charged 100%, the sound comes if its anything less. The more de-charged the battery is, the worse the sound is.

I put the battery in so that it can go into hibernate while on battery power. that time, plus the interval between the next time i drive discharges it to about 93%. And then the sound comes until i drive for 10 mins and the battery is back to 100% power. Then the sound stops and everything is fine.

Bugbyte · 02-08-2009, 05:44 PM

Only thing I can think of is to ground the audio output wire to the same ground as the laptop adaptor brick.

Also, do any of your audio wires run near the power wires? Are the good quality, shielded wires?

ghanchi · 02-08-2009, 06:00 PM

Quote: Originally Posted by Bugbyte

Only thing I can think of is to ground the audio output wire to the same ground as the laptop adaptor brick.

Also, do any of your audio wires run near the power wires? Are the good quality, shielded wires?

my audio wires are all in the trunk, running straight from the laptop to the amp. From the amp, it runs to the OEM audio wires which lead to the speaker. I took out the original BMW amp and replaced with aftermarket.

the power wire runs from the trunk to the center console, where I have a main power switch to turn it on/off. The brick is in the console too, behind the A/C unit(if you're familiar wit the BMW center console). so technically, they are completely isolated from each other, running in seperate directions.

I was thinking it might be caused by lack of either voltage or amps since the sound only comes while the laptop battery is being charged. perhaps the power from the car battery isnt enough to run everything and thus causing voltage drops or something? im not too experienced with current, but just have a basic understanding, so its just a theory.

anik321 · 02-22-2009, 06:45 PM

First an explanation:

The brick power supply that you are using to charge the laptop (when you are driving) is a switch mode power supply (SMPS).

This is what all laptop bricks and cell phone chargers are made of nowadays and gets rid of the need to use a huge expensive 60hz transformer.

SMPS works by chopping a higher voltage and adjusting the dutycycle to create a smaller voltage (buck converter) or a viceversa (boost converter)

unfortunately the chopping of voltage/current creates a lot of Electromagnetic interference that can either be radiated or conducted or both. This is EXACTLY what is causing the buzzing in your stereo. The brick is radiating or conducting noise that your amp+stereo is picking up. It will be worst at AM stations, then FM.

Solution:

1. First with your laptop battery charging (drain it down to say 50% first) ... move the brick out of the car (with everything else, including laptop still inside) and see if you can close the trunk lid. Get inside your car and listen to the radio and stereo. It should be remarkably less noisy. If this is the case then you basically need to get that brick as far away from everything else as possible. Though this means running wires back and forth (which is also carrying the noise!!

)

2. If the noise is being conducted (through the wire from your laptop to your amp) .. then you have a bigger problem because I dont know how you are going to remove the noise. An optocoupler wouldnt really do much here I dont think.

Sorry to have made this such a lengthy post. Just wanted to give you a better picture to make it easier.

IT THE BRICK! inverters are the same way - thats why you always here a buzzing. When the battery is done charging, the control loops turns off the chopping and thats why your noise goes away.

ghanchi · 02-22-2009, 08:58 PM

Quote: Originally Posted by anik321

First an explanation:

The brick power supply that you are using to charge the laptop (when you are driving) is a switch mode power supply (SMPS).

This is what all laptop bricks and cell phone chargers are made of nowadays and gets rid of the need to use a huge expensive 60hz transformer.

SMPS works by chopping a higher voltage and adjusting the dutycycle to create a smaller voltage (buck converter) or a viceversa (boost converter)

unfortunately the chopping of voltage/current creates a lot of Electromagnetic interference that can either be radiated or conducted or both. This is EXACTLY what is causing the buzzing in your stereo. The brick is radiating or conducting noise that your amp+stereo is picking up. It will be worst at AM stations, then FM.

Solution:

1. First with your laptop battery charging (drain it down to say 50% first) ... move the brick out of the car (with everything else, including laptop still inside) and see if you can close the trunk lid. Get inside your car and listen to the radio and stereo. It should be remarkably less noisy. If this is the case then you basically need to get that brick as far away from everything else as possible. Though this means running wires back and forth (which is also carrying the noise!!

)

2. If the noise is being conducted (through the wire from your laptop to your amp) .. then you have a bigger problem because I dont know how you are going to remove the noise. An optocoupler wouldnt really do much here I dont think.

Sorry to have made this such a lengthy post. Just wanted to give you a better picture to make it easier.

IT THE BRICK! inverters are the same way - thats why you always here a buzzing. When the battery is done charging, the control loops turns off the chopping and thats why your noise goes away.

makes alot of sense. thanks alot. ill try it out.
I currently have the brick in the center console, running the power from the cig lighter to the trunk. So its right near the screen, but not any sound wires. i can try and move it to somewhere else, like under the seat.
if not, i was thinking abt upgrading to a computer from a laptop anyways to get better performance, so that should solve all my problems.

Thefdog · 02-22-2009, 11:28 PM

Quote: Originally Posted by anik321

IT THE BRICK! inverters are the same way - thats why you always here a buzzing. When the battery is done charging, the control loops turns off the chopping and thats why your noise goes away.

I have to respectively disagree. When the battery is fully charged and the computer is on, the "BRICK" as you call it, is still running to keep a 100% charge in the battery.

It sounds like you have a ground loop problem. Especially, if the buzz or hum increases in frequency or volume (as you perceive it) when you rev your cars engine. Try to ensure that all of the electrical equipment for your carputer and stereo are connected to the same ground. An ensure that the ground is to bare metal and fastened tightly. I recommend reading about this issue in more detail in other posts...

A ground loop isolator or anything that you place inline between your computer and the amplifier to get rid of the high pitched hum, will also get rid of all your high pitched vocals, snares, guitar, etc. So don't do it.

This is the single best way to get a clean, noise free signal through your car stereo. I spent as much or more time grounding everything well, than wiring the power in the first place.

anik321 · 02-23-2009, 12:46 PM

Fdog,

I definitely agree with you if he had mentioned anywhere that the pitch of the noise increases with throttle but I dont think he did.

He did mention that the noise goes away after 10-15 minutes when he believes the battery is fully charged. This led me to believe that it was the brick. It seems after a full charge it is providing minimal current to trickle charge the lithium cells and not its full power rating.

I wonder if laptops always run off their battery or run of the powersupply when docked? Time to go hook up a current probe.

Gangchi,

1. I would take both the solutions under consideration and attempt both. Do you hear noise pitch go up as you push down on your gas pedal/throttle?

2. You said the nosie goes away after 10-15 minutes of driving. Does it ever come back at anypoint while driving? or only when you leave the car and have the battery discharge..?

3. Remove the battery from the laptop but have the brick connected to it. Is the noise always there now?

EDIT: I was also wrong about the 60hz transformer in my original post. This is only in a AC-DC conversion. Not DC-DC. SMPS DC-DC is simply far more efficient than just burning that power off somewhere. Not that anybody here really cares.

ghanchi · 03-21-2009, 01:06 AM

hey, sorry for the late reply, was really busy wit school.

1. Yea, there is a pitch that increases as I press the gas. I already have a ground loop installed, so that noise is very minimal

2.the noise is only there when the battery is being charged. It goes away after 10 mins and doesnt return again until the next time i drive.

3. noise isnt there when the battery isnt in the laptop.

Josho · 03-21-2009, 07:46 PM

This happens on the laptop when plugged in at home. Removing the earth cable stops it.

ghanchi · 03-22-2009, 12:26 AM

Quote: Originally Posted by Josho

This happens on the laptop when plugged in at home. Removing the earth cable stops it.

ill give this a try in the morning

NSFW · 03-25-2009, 04:16 AM

Sounds like ground loop to me.

My laptop is powered from a 12v-120v inverter and I'm not getting any buzzing noises or artifacts on the display. If you can't find/fix the ground loop you might be able to filter the power supply with a capacitor and/or coil.

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