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Thread: Running Without Engine

  1. #1
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    Running Without Engine

    Hey guys, I'm looking at running an inverter to power my laptop while I'm on site for work. I'll be there for a couple hours, tethered through my Crackberry, and already have a car charger for that. But as much as I'd like to carry extra batteries for my laptop and call it done, the applications I run run battery time down to 40 minutes at best.

    That said, I was thinking I'd install a voltmeter in the car to be able to tell when the battery is close to being too weak to start the car. Is this the right way to go? What level am I looking at being unable to start the car?

    I may also invest in an ammeter, since that seems to be the more realistic measurement of how much juice is left in a battery.

    My current ride is a 2006 WRX. The Miata I could pushstart (minimal interior weigh-in was something like 2200 lbs, VERY easy to push start... Subby not so much at 3300+!)

    Oh, and I also have a Kinetik HC1400 power cell on the way I may order another with isolator and all the extra wiring and box to get it to the point of I can sit in the backseat with the laptop overnight music playing and crunching numbers and still go an start the car at sunrise.

  2. #2
    FLAC
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    Ive always wondered that myself too. For a normal car battery, I am paranoid about draining it too much and causing permanent damage from sulfation so I make it a habit to try not to drain it past 80% SoC, which is 12.42v (with no load). But I have no idea how you would measure state of charge when the battery is being loaded. When loaded, I dont think the measured voltage will correlate to a specific state of charge because the voltage will vary depending on load.

    If you are going to regularly be draining the battery past 80%, maybe you should get a deep cycle instead.

  3. #3
    Maximum Bitrate FusionFanatic's Avatar
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    Quote Originally Posted by a13n0rma1 View Post
    I'm looking at running an inverter to power my laptop while I'm on site for work.
    don't use an inverter unless there is no other option. your laptop runs on low voltage DC (just like your car). and inverter changes the car's 12vdc to 115vac, then your laptop's power brick will change is back down to low voltage DC again. this double inversion/conversion process is very inefficient. it wastes alot of power, so if you're looking to get the most life out of your car battery, then this is NOT the way to go. get a DC-DC power supply (even a cheap auto/air adapter from eBay is better than using an inverter).

    Quote Originally Posted by a13n0rma1 View Post
    I was thinking I'd install a voltmeter in the car to be able to tell when the battery is close to being too weak to start the car. Is this the right way to go?
    it may help, but it's not going to guarantee the ability to reliably start your car each time.

    Quote Originally Posted by a13n0rma1 View Post
    What level am I looking at being unable to start the car?
    that depends on the battery rating/age/condition, engine temperature, etc... in general (with a good/new bettery) you can start your car down to around ~10v, but again, it depends on other factors.

    Quote Originally Posted by a13n0rma1 View Post
    I may also invest in an ammeter, since that seems to be the more realistic measurement of how much juice is left in a battery.
    useless... amperage (current) is how fast electricity is flowing through a given circuit. when you measure amperage, you're only measuring how much juice is being drawn from a specific circuit. unlike voltage, there will be no reading unless something is drawing power (in other words, it cannot be used to gauge the remaining power of your battery)

    Quote Originally Posted by a13n0rma1 View Post
    Oh, and I also have a Kinetik HC1400 power cell on the way I may order another with isolator and all the extra wiring and box to get it to the point of I can sit in the backseat with the laptop overnight music playing and crunching numbers and still go an start the car at sunrise.
    that will help, but if you run your radio and laptop all night long you'll still have a dead battery in the morning. that HC power cell (like most car batteries) is designed for high-current 'instant' loads (like amplifiers and engine-starting), it is not designed for low-current sustained loads (like the radio or a laptop).

    regular car batteries can provide a lot of power in short bursts, but they also discharge very quickly. crank your engine for a couple minutes and your battery is dead, leave you car sitting for several days and you might have a dead battery just from the clock slowly discharging it. a deep-cycle battery is designed for this type of purpose (slow, sustained load), but is not very good for starting a car.

    the bottom line is; a high power battery replacement (and/or a second battery) will help, but will not give you the desired effect you want. if you want to run your laptop all night long then your best best is to simply carry a couple extra batteries with you. it's simply not worth beefing up your car's electrical system just to power a non-installed laptop.


  4. #4
    The Last Good Gremlin GizmoQ's Avatar
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    I actually have a dual battery charger in my trunk for my son's car that we drop batteries in whenever we're gonna be out somewhere away from the car for extended periods. The charger is only powered when the car is running, but the trip to the site is usually long enough to charge them so they're ready when we arrive.
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