Powering a laptop in a 300cd... Help! - PeachParts Mercedes-Benz Forum

derherr65 · #1 02-08-2007, 02:10 PM

My new(to me) laptop draws a maximum of about 10 amps @ 12 volts. The cigarette lighter fuse in my 300cd is 8 amps. I don't want to risk overloading the stock wires, so I'm looking at installing a power socket. Has anyone been down this road? Pictures, hints, and advice welcome.

pawoSD · #2 02-08-2007, 02:17 PM

I am pretty sure thats not the right fuse for that...I am thinking its supposed to be 12 or 20A....also, I highly doubt in normal operation the laptop is going to use 10A, thats over 100 watts, very unlikely. In normal operation a laptop uses about 40-70 watts. 10A is what the power transformer is rated for, not what the laptop will actually use. I have powered some heavy duty stuff (200watts of stuff) with an inverter off my cig outlet wires, and have never blown a fuse or had a problem....a laptop is minimal.

vstech · #3 02-08-2007, 02:31 PM

yup, any standard 150 watt 110V inverter would be overkill for a laptop a/c adapter. the laptop may pull 10 amps, at the laptop dc side, but the 110V power supply that powers the laptop will barely pull 2 amps of your 12V... I promise.
if you are concerned about it, do what I did to my TD, run #6 wire directly to the battery, and direct wire a 1500Watt inverter in. NO WAY is your laptop gonna pull more than that!
most modern laptops have 90 watt power supplies. on the OUT PUT side.
John

Jeez, what was I thinking when I wrote that? watts is watts, the inverters are gonna loose some efficency to heat, so you're gonna need about 20% more watts than the inverter puts out.
divide by 13.5 and that is how big a circuit you're gonna need.

Jeremy5848 · #4 02-08-2007, 02:40 PM

I'm not sure what the correct fuse is for your lighter socket but it is unlikely that your laptop would ever draw 10 Amps.

In any case, when I took the ashtray out of my 1985 300D Turbo (to install a 3-gauge panel), I wired in a new electrical line to power laptops and things. I ran a wire from the main electrical terminal strip near the battery, through a fuse (30 Amps), through the firewall on the passenger side, behind the glove box, picked up chassis ground from a heavy bolt next to the glove box, and ran the wire out from under the passenger's under-dash panel next to the transmission tunnel. I connected the wire to an accessory holding three sockets for lighter plugs and such. I haven't connected the accessory to the car yet because I haven't figure out how to do it without either damaging the car (drilling holes) or getting in the way. So it just hangs from the wire for now.

Jeremy

derherr65 · #5 02-08-2007, 02:47 PM

I don't have the seat heater option, so it uses a smaller fuse. It matches the descriptio on the fuse box lid.
The laptop itself doesn't use that much, but when running the laptop and charging the battery, and bearing in mind 2 converters(12vdc-120vac & 120vac-18.5vdc) at 80% efficiency, it will heat a 14-16ga wire quite quickly.
The other issue is the large, clunky converter in the way of the shifter and control panel.

Quote:

Originally Posted by Jeremy5848

I ran a wire from the main electrical terminal strip near the battery, through a fuse (30 Amps), through the firewall on the passenger side, behind the glove box, picked up chassis ground from a heavy bolt next to the glove box, and ran the wire out from under the passenger's under-dash panel next to the transmission tunnel. I connected the wire to an accessory holding three sockets for lighter plugs and such.

Sounds similar to what I was thinking.

2.5Turbo · #6 02-08-2007, 09:31 PM

Just throwing it out there...

I blew the fuse in a rental car with my laptop once on a 400 watt inverter. I was playing a resource-hungry 3D game at the time, which makes the processor and other components suck more juice. Add that to the fact that the inverter is not nearly 100% efficient, you'll be drawing more amps than you'd think in no time. If you really don't want to worry about it, run some lines directly from the battery (with fuses that correctly match the wire gauge) to a pair of crimp on disconnects. Be sure to make the positive wire from the battery a female connector, and the negative wire a male connector so you don't ever accidentally get them reversed. Or you can get a male/female set of these. They are good for 20 amps.

vstech · #7 02-08-2007, 10:25 PM

Quote:

Originally Posted by 2.5Turbo

Just throwing it out there...

I blew the fuse in a rental car with my laptop once on a 400 watt inverter. I was playing a resource-hungry 3D game at the time, which makes the processor and other components suck more juice. Add that to the fact that the inverter is not nearly 100% efficient, you'll be drawing more amps than you'd think in no time. If you really don't want to worry about it, run some lines directly from the battery (with fuses that correctly match the wire gauge) to a pair of crimp on disconnects. Be sure to make the positive wire from the battery a female connector, and the negative wire a male connector so you don't ever accidentally get them reversed. Or you can get a male/female set of these. They are good for 20 amps.

I think it was the 400Watt inverter that blew the fuse, not the laptop's need. if you had a smaller inverter plugged into the cig liter it would not have blown. the laptop's max draw is only 100Watts, so it's unlikely that the laptop would draw the max there, don't forget the laptop has a battery for large draws of power.
I agree if you are going to have an inverter that pulls more than 300 watts, you need to have it directly connected to the car's battery with a dedicated circuit. like the instructions that come with any inverter will tell you.
John

deerefanatic · #8 02-08-2007, 10:41 PM

Quote:

Originally Posted by vstech

I think it was the 400Watt inverter that blew the fuse, not the laptop's need. if you had a smaller inverter plugged into the cig liter it would not have blown. the laptop's max draw is only 100Watts, so it's unlikely that the laptop would draw the max there, don't forget the laptop has a battery for large draws of power.
I agree if you are going to have an inverter that pulls more than 300 watts, you need to have it directly connected to the car's battery with a dedicated circuit. like the instructions that come with any inverter will tell you.
John

As an A+ Certified PC Tech who owns his own business, I must hereby respectfully disagree with you.

The average new laptop today has a 130 watt power supply. NO, the battery in the laptop wont cover for large drains....... It either powers the laptop 100% or is not powering it at all and is in a charging/holding state. Secondly, if the power brick for the laptop has a 130 watt output, it will have a 160 watt peak draw assuming 80% efficiency (about right for the norm...). Now add an inverter (Usually about 75% efficient) and you're now drawing 215 watts from your car, or about 16 amps at 13.5 volts.

So, no, a 10 amp fuse wont power the average laptop today that's also trying to charge it's battery or run heavy applications..... You gotta remember, these babies generate some serious heat, and being a space heater isn't their primary purpose!

Therefore, they really draw some juice.......

Now, if you're talking a thin & lite unit with small hard drive, screen, and low CPU speed, then that's another story, though I bet you'd be surprised at the current consumption even then.

Hope this settles the confusion for everyone.

I also recommend the direct wiring idea, as I mentioned on another forum............. That's what I've done with my 800 watt inverter.......

derherr65 · #9 02-08-2007, 11:45 PM

Ditto on the battery comment. I think the confusion comes from general car knowledge, where the battery can act as a backup to the alternator... though you want to avoid that situation naturally.

My laptop today has a 120 watt power supply. Watts = Amps x Volts, remember?
So 6.5amps * 18.5volts = 120.25watts
The "brick" being 80% efficient could draw up to about 150watts.
My cigarette converter, being 80% efficient could draw up to about 190watts.
190watts / 13volts = 14.5 amps peak draw possible from car.
Does it awlays draw this much? No, right now it's almost cold as the battery is charged and it's idle. And there should be a safety margin built in such that the original 6.5 amps is never reached. But I anticipate a lot of gps mapping immediately after leaving a house where it ran on battery. That would be a significant draw. And mine's neither thin nor light nor brand new with a 80gb hard drive, 800mhz processor, and 15.4" widescreen.

Does sound like I need to build for 15 amps and then I can be completely worry free. I'm hearing a lot of talk, but I don't see any pictures of this 800watt converter install!

deerefanatic · #10 02-09-2007, 08:53 AM

OK, OK, I'll pony up some pictures when I come back this evening!!

Do you need to see the relays and such too? I'll have to pull the carpet for that.

Oh well, I'll take a bunch of pics anyway...... Should have in the first place....

derherr65 · #11 02-09-2007, 06:47 PM

I actually miked a couple of wires. The wire that got hot was a 20 gauge. The wire that got warm was a 17 gauge. I aslo found out that a 7 amp fuse does not blow with the laptop on, mapping and GPS running, and battery charging.

derherr65 · #12 02-20-2007, 12:57 PM

Ok, final results, I think.
Will not blow a 7 amp fuse with fully chaged battery and running DVDs.
Will blow a 7.5 amp fuse with partially depleted battery.
Will not blow a 10 amp fuse with partially depleted battery, running computer, running gps, ect.

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