Truck battery powering up windows can?

Discussion in 'Mac Pro' started by Josias, Jun 17, 2006.

  1. Josias macrumors 68000


    Mar 10, 2006
    Hi y'all
    My dad is dreaming of a little place in the woods, where he can sit and write with his laptop (my old wincrap thingy, 2.4 GHz Celeron D, 256 MB DDR PC-2300 RAM, 30 GB 4.200 rpm. HDD, 16 MB Intel GMA/852 and a 14" screen). If he got a 12 V truck/car battery, how long could he sit in the woods and write, running nothing but Word. Don't worry about rain or stuff, just tell me how far he could go on it. I know the computer has a maximum voltage consumption of 19.1 V. That's all I know.:confused: :D
  2. Josias thread starter macrumors 68000


    Mar 10, 2006
    Now I found out the AC adapters output ia 19 V/3.42 A. I really need to know this. Please help me!:D ;)
  3. Timepass macrumors 65816

    Jan 4, 2005
    if I had to guess I would say a few days or so of power could be taken from the bat. Laptops are not going to draw that much power off of it so he could more than likely go more than 24 hours off of it running it. So he could just go recharge it after a while.

    The rating on car batteries is based on how many mins it can run a car drawing I want to say 100 amps. Dont really rememeber the exact amount of power they base it on. But seeing as most batteries say 70-100 mins on about an average car. it a safe bat that it could power a laptop for a few days.
  4. Josias thread starter macrumors 68000


    Mar 10, 2006
    Ok, spankyou very much. We have a charger for the battery, but we just need to know how long it can run, before we buy a battery.:D
  5. TheMonarch macrumors 65816


    May 6, 2005
    Bay Area
    100 amps?! No way!

    Try 1/4th that, and now we're talking.

    BTW, Mac+Lead Acid battery, sounds like a bad idea...
  6. j26 macrumors 65832


    Mar 30, 2005
    Slap a solar panel with it an it could last forever (or at least until you have to reinstall Windows;) )

    Heres a sample - I'm sure you could find cheaper if you hunt about.
  7. Fisheke macrumors member

    May 16, 2006
    Antwerp, Belgium
    19V * 3,24A = 65W. So you'll consume (at max) 65Watts.

    An average lead-acid car bttery has a capacity of about 70Ah's, at 12V. So a car battery has a capacity of 840WattHours. It'll be able to power your 65Watt laptop for about 13hours.

    This being ofcourse with max power consumption from the laptop (it won't even get near that when you're using word), and you'll have to account for losses in the transformer (12VDC->220VAC). Say you'll get about, a rough guess, 16hours of fun with the battery.
  8. Timepass macrumors 65816

    Jan 4, 2005

    Just have to make sure he runs it though an DC/AC power converter. those will be just fine after running it. Power my laptop that way for a long time.
  9. Moe macrumors regular

    Apr 27, 2003
    First, you want a deep-cycle battery, not a car starting battery. The latter is designed to deliver high-current for short periods of time and not be discharged very deeply at all. If you do, it'll have a very short life.

    A Group 27 size deep-cycle battery typically has a capacity of 100 amp-hours at a 5A (20 hour) discharge rate. However, if you want reasonable life out of it, you won't discharge it below 50%.

    An 85% efficient inverter (converts 12VDC to 120VAC) typically draws 10% of the appliance's 120VAC wattage as amps from the battery. So a 120VAC appliance that really uses 50 watts will cause the inverter to draw about 5 amps from the battery. In theory, that appliance will discharge a fully charged 100 amp-hour battery from 100% charge down to 50% charge in 50amp-hour/5amps=10hours.

    On a notebook power supply, you have to consider whether the power rating is on the input or the output. If it's output, you need to multiply that by about 1.15 to find the wattage it inputs because the power supply is also less than 100% efficient. If you only know the output voltage and maximum output current, you can multiply those together and then by 1.15 to estimate the input wattage. In your case, that's 19 V x 3.42 A = 65 output watts x 1.15 = an estimated 75 input watts.

    Keep in mind that the notebook power supply's rating is its maximum capacity, i.e. what it could deliver when operating the notebook while charging its battery if heavily discharged. If you're constantly operating the notebook off the deep-cycle battery, it's like being plugged into residential power all the time, so the notebook battery won't discharge, and the notebook power supply will never be drawing near it's maximum.

    If we estimate the power supply to use 80 watts maximum, we might guess it uses 40 watts or less when the notebook battery is fully charged, causing the inveter to draw 4 amps from the deep cycle battery. In that case, 50 amp-hours divided by 4 amps will give just over 12 hours of use on 50% of the Group 27 battery's amp-hour capacity.
  10. reh macrumors 6502a

    Oct 24, 2003
    There's no sense going from DC to AC back to DC. Get a cigarette lighter power cable for the laptop and your battery will last a bit longer since there's no power loss from an inverter.
  11. imacintel macrumors 68000

    Mar 12, 2006
    Yeah, that should work better. No power losss from inverters.:)
  12. Josias thread starter macrumors 68000


    Mar 10, 2006
    Me daddy got his little chair to write in now, and he spoke to my mum's brother, who knows a lot of mechanics. He just dropped it and went for solarpanels, though it's not very stable in the dansih weather.:D :p

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