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Thread: number of days of standby (S3) time.

  1. #1
    Maximum Bitrate kbyrd's Avatar
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    number of days of standby (S3) time.

    I've run some numbers. I'm hoping you all can confirm or blow holes in my conclusion.

    These are probably specific to my setup.
    - My VW GTI has a 60 amp hour battery. (I assume that's the 20 hour discharge rate).
    - I configured WinXP to do suspend on power button, set my via m10000 bios to go to S3, and turned off everything I could find, so I think it's just refreshing RAM.
    - I'm using an M1-ATX, which I've set to 5sec off delay, NEVER power off. When I turn the ignition off, the m1-atx hits the mobo's power button. I intend for the carPC to be in the S3 state most of the time when I'm not in the car.


    What I really want to know is how long I could leave the car with the PC in S3 without doing killing my battery. To do that, I need to know how much the whole car (including the carPC in S3) is drawing from the battery when I've walked away and turned on the alarm.

    Here was my test:
    - I hooked up the multimeter in series between the battery negative terminal and the cable that normally attaches to the negative terminal. Is this correct?
    - I turned on the key (not the engine!!) and let the carPC power up. The fuel pump and such kick on, drawing about 9A max here when everything stablizes.

    - I turn off the key, let the carPC go to S3, get out and lock the doors. There's still some pump/fan noise on in the engine bay, drawing 1.1A here.

    - A bit later, the pump/fan noise turns off. This should be my "walk away and leave it overnight" draw, right? 470mA.

    - About 15 minutes, I check again (just to be sure), and the draw is down to 180mA! Maybe some other component in the car powers down after the long delay? I did the test twice with the same results. So I'm pretty sure this is the "walk away and leave it overnight draw".


    Here's my math to guesstimate how long my actual draw will last on my 60AH battery.
    Code:
    60AH / 20H = 3A
    3A / 0.18A = 16.67
    16.67 * 20H = ~333 hours = ~13 days
    So, I could leave my car for 13 days!?! To be safe, let's half that and round down. That gives me 6 days. That's not bad! The reality is that I want to install a kill switch that cuts the constant 12V to the PSU when I plan to leave the car for a while (more than 1 day). Also, the M1-ATX says it'll cut all power if the battery voltage drops below 11V. I don't know how long that would be for a new, healthy batter. Does this all sound reasonable or have I missed some important point?
    My worklog.
    Status: VM GTI sold, got out of the CarPC tinkering hobby, but I still think about getting back in.

  2. #2
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    completely reasonable.

    do keep in mind that using all 60 amp hours leave nothing in the battery to crank the engine though

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    Variable Bitrate StrataG's Avatar
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    If you halve your figures, that'd be 6 days to get your battery half drained. If you can start your car off of even 7V, I'd ask what scientist you had setup your electrical system. Your car will not start if it goes below ~10V. I'd say you'd be lucky if you could go 2 days and be able to start the car. Keep in mind the stress you'll be putting on your battery though with the constant discharge. You most certainly will want to get a deep cycle battery if that is your plan.

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    Quote Originally Posted by StrataG
    If you halve your figures, that'd be 6 days to get your battery half drained. If you can start your car off of even 7V, I'd ask what scientist you had setup your electrical system. Your car will not start if it goes below ~10V. I'd say you'd be lucky if you could go 2 days and be able to start the car. Keep in mind the stress you'll be putting on your battery though with the constant discharge. You most certainly will want to get a deep cycle battery if that is your plan.

    wrong!


    battery capacity to voltage relationship is not linear

    drain a battery halfway and it drops about a volt

    drain it the other half and it drops a lot more

  5. #5
    Variable Bitrate StrataG's Avatar
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    Quote Originally Posted by greenman100
    wrong!


    battery capacity to voltage relationship is not linear

    drain a battery halfway and it drops about a volt

    drain it the other half and it drops a lot more
    So please explain to me how long his system will last. If he is even close to correct, why aren't more people doing it? Why do so many people end up with dead batteries who try this? I'd love to be able to use standby, but from what I understood, it was not viable for people who do not drive their car everyday.

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    I have left my system for a week several times on standby without any problems. Although, once i left it for over 2 weeks and my car started normally but the computer did not resume form standby, it seems the opus cut the 5vsb line when it sensed the battery level was too low, i have a pentium m micro atx setup. For the people that end up with dead batteries, i don't think their motherboards cut power to their usb ports which puts a lot more drain on their battery.

  7. #7
    Variable Bitrate StrataG's Avatar
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    Quote Originally Posted by Kalt
    I have left my system for a week several times on standby without any problems. Although, once i left it for over 2 weeks and my car started normally but the computer did not resume form standby, it seems the opus cut the 5vsb line when it sensed the battery level was too low, i have a pentium m micro atx setup. I think the people that end up with dead batteries motherboards don't cut power to their usb ports which puts a lot more drain on their battery
    Have you ever tested your amperage? I'd love to be able to do that, but my question is, if that is the case, why aren't more people doing it? Everyone complains about startup/resume times and coming out of standby is typically a lot quicker. If you have any further info, lmk. thanks in advance.

  8. #8
    Tainted Love Cris's Avatar
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    Well, I have an M10k, with a lot of USB devices, that are still power after the pc goes into standby or powers off. I keep mine in standby, and when the battery was new, and not so cold outside, it would hang on for about 5-6 days. Now, I'm really really lucky if the car starts the third day...

  9. #9
    Variable Bitrate StrataG's Avatar
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    Quote Originally Posted by Cris
    Well, I have an M10k, with a lot of USB devices, that are still power after the pc goes into standby or powers off. I keep mine in standby, and when the battery was new, and not so cold outside, it would hang on for about 5-6 days. Now, I'm really really lucky if the car starts the third day...
    How old is your setup? I felt what happened to you is what typically happens. Any battery, even deep cycle will lose it's ability to hold a charge a lot faster if it's constantly drained. Obviously you are not completely draining it, but I know the lifecycle of a battery is longer if the discharges are very small and vice versa. If I needed to replace my battery every few years, it wouldn't be a big deal. If I'd have to replace it every year though, it would have me avoid this type of setup.

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    I tested my amperage a while back, it was low enough that i was comfortable enough to try it but i don't remember the exact number. I know they are a few other members doing this too, but from what i have seen majority of mobos usb ports stay active durring and some mobos dont turn off the cpu fan durring s3. I'm sure this stops a lot of members going this route, cause all their usb devises would be active. From what i understand only memory requires power in standby.

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