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Thread: No permanent 12V - advice on secondary battery solution

  1. #1
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    Question No permanent 12V - advice on secondary battery solution

    Hi, all,

    I'm in a rather special situation : the car I'll be driving belongs to my company, and I'm not authorized to make any modification to it, so my carputer must be removable in 2 minutes without any permanent modification to the car.

    Consequently, I indend to use a cigar-lighter socket to power up the PC (one of the cigar-lighters is on a 20A fuse). The problem is that this socket is not permanent : it is only powered when the engine is on. Without a permanent +12V, no clean shutdown can happen unless I have a secondary battery powering the PC while it is shutting down, right?

    I am thinking of using the following circuit to power the PC, and wanted to submit the idea to you all...

    Any advice/suggestion/warning is highly appreciated.

    Vicne
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  2. #2
    Variable Bitrate
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    two suggestions:

    one, just manually hit a soft off switch before you shut down the car and wait until it's shut down before you turn off the car. Easy, and it works.

    two, put some wires into the fuse box to get a permanent 12V... chances are, they'll never see it (I don't know how careful they are with leases, never done one myself), and you'll be able to do a normal shutdown controller like everyone else.

  3. #3
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    Let me thank you first for your suggestions.
    Quote Originally Posted by wizawuza
    one, just manually hit a soft off switch before you shut down the car and wait until it's shut down before you turn off the car. Easy, and it works.
    True. However, I am so absentminded that more often than not, I'll remember to hit the soft-off button *after* I turn off the car
    Quote Originally Posted by wizawuza
    two, put some wires into the fuse box to get a permanent 12V... chances are, they'll never see it (I don't know how careful they are with leases, never done one myself), and you'll be able to do a normal shutdown controller like everyone else.
    That could be an option indeed, but are you sure the fuse box is *before* the ACC relay switching off power to the cigar-lighters. I admit I have no idea if this is standard or not...
    The circuit would also guarantee 100% that whatever happens, the carputer would be unable drain the main battery, as it is physically disconnected from that battery once the engine is turned off...

  4. #4
    See me in my wet t-shirt. stevieg's Avatar
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    As the power will, whatever go off when the car goes off I would either live with it killing power and take your chances on the HDD, or alternatively look in the Operating System Optimisation section in Software Development sub-forum into compact-flash XP booting.

    If you wanted to go for a secondary battery, obviosuly it would do the trick without the above faffing around, but you have to be mindful of the amount of current it will draw to re-charge itself, you may need to limit the current flow to ensure it doesn't either heat up the wiring or burn out the 20A fuse, and as you have already added a diode to prevent it feeding the rest of the car!
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  5. #5
    Variable Bitrate kmcniece's Avatar
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    Laptop?
    with a cigarette adpater/inverter?

    just a thought....
    EPIA M10000, 512 Ram, Opus 90, GPS, XM, 7" Xenarc, Pioneer HU
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    Banned xdjxklusivex's Avatar
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    jack it from ur fuse box works fine just get a tester light turn ur car off and c what fuses have perminent 12 volt usualy the power door locks hazard lights sometimes the headlights... shudnt effect n e warrenty or n e thing ur just basicly tapping into the fuse perminently for ur use but temporarly also because u can just pull it all out and pop the fuse back in thats how all my lights that i added in my car run and its fine :-P

  7. #7
    Variable Bitrate
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    kmcniece,
    Quote Originally Posted by kmcniece
    Laptop?
    with a cigarette adpater/inverter?
    That would obvioulsly do the trick... Only thing is the budget (laptop with decent HD capacity + adapter) and it's much less fun .
    I might however try to find a second hand one in last resort...

  8. #8
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    xdjxklusivex,
    Quote Originally Posted by xdjxklusivex
    jack it from ur fuse box works fine just get a tester light...
    True, I can just get power from a permanently powered circuit (provided it can supply enough Amps of course).
    Quote Originally Posted by xdjxklusivex
    shudnt effect n e warrenty or n e thing ur just basicly tapping into the fuse perminently for ur use but temporarly also because u can just pull it all out and pop the fuse back in
    Oh, I see, you mean just plugging a homemade plug in the fuse "socket" to get the 12v from there ? That's an elegant solution. Just have to make sure I put back a fuse in the derivated circuit and feed back the original one. Did you hack an old fuse or find such an existing "fuse plug" ?

  9. #9
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    stevieg,

    thanks for your suggestions. Replies below :

    Quote Originally Posted by stevieg
    I would either live with it killing power and take your chances on the HDD, or alternatively look in the Operating System Optimisation section in Software Development sub-forum into compact-flash XP booting.
    Hmm. If I choose to let the power go down, I would definitely stay with an OS without a swap file to avoid corruption, so most probably DOS. For mp3 playing, mpxplay could do what I want and more, but limiting a carputer to playing mp3s is a bit silly, isn't it ?

    Quote Originally Posted by stevieg
    If you wanted to go for a secondary battery, obviosuly it would do the trick without the above faffing around, but you have to be mindful of the amount of current it will draw to re-charge itself
    Yeah, that's one of my concerns.
    In permanent use, the current drawn from the socket would obviously be exactly the one used by the PC on average, not a big deal. Moreover, in everyday use, as the 2nd battery would only be used during shutting down (or hibernate) time, let's say 30s or so per trip, it wouldn't be enough to drain it empty.
    However, when first connecting the secondary battery or if the shutdown fails and the PC remains powered until the M1 ATX switches the power off, the secondary battery would probably be far from full and the transient currents on next start could (hopefully ) blow the fuse...
    Quote Originally Posted by stevieg
    you may need to limit the current flow
    What kind of device would you advise to limit that current ? I don't think putting a resistor in series would be suitable, because of the inefficiency and voltage drop in permanent use (*), except if it is automatically shunted by a relay after 10secs or so...
    Best would be a component with self-limiting current (variable resistor ? IC ?) ...

    Any suggestion regarding this is highly appreciated.

    Vicne

    (*) Correct me if I'm wrong, but in the worst-case scenario, the secondary batt is drained dead (0V on its terminals). I'd then get a 12V difference between batteries, and limiting the current to 20A would require a 0.6 Ohm resistor in series with the diode. That means that when the PC pumps e.g. 60W, it would draw 5A (60/12) through that resistor, leading to 15 Watt (0.6*5) spill dissipation + 3V (0.6*5) voltage drop

  10. #10
    Variable Bitrate
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    Replying to myself

    Quote Originally Posted by Vicne
    I don't think putting a resistor in series would be suitable, because of the inefficiency and voltage drop in permanent use (*)
    Ahem, after a bit of thought, I think I found the solution (maybe it's what you meant, stevieg, but it wasn't obvious for me until now) : I don't have to put the resistor in series with the diode on the main path to the PSU. I can put it on the secondary battery branch, so that it only dissipates power when the secondary battery is charging or discharging, and not in permanent use (see schema below). Well, of course, it will spill energy, but not permanently, and it prevents the voltage drop too.
    except if it is automatically shunted by a relay after 10secs or so...
    Hehe, why do I always think complex first and simple afterwards : to limit charging current, but not discharging one, I can simply shunt the resistor with a diode, right ?

    I'm thinking of using this as the secondary battery (1.3 Ah is more than enough : 5 minutes at 60W is 0.4Ah or so...). According to the specs of a similar model, I should limit the charging current to 0.48A (can anyone confirm my understanding is correct ?) That would mean a 25 Ohm resistor to be safe in the worst-case scenario ... And that would also mean a rather long charge time...

    Anyway, does the following schema make sense ?
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