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Voltage drop causing Mac Mini to turn off

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  • Voltage drop causing Mac Mini to turn off

    Hello everyone,

    I am currently 98% finished with my mac mini car computer project but there is one small issue that is impeding my progress.
    I have the Mac Mini powered by a 115 watt inverter that is directly wired to my battery. The Mac Mini works perfectly when the car is off and when it is on, the issue is in the start up period.
    When the car starts the voltage drops from 12v to roughly 10.5 volts. This causes the Mac Mini to turn off. I have exhausted all of my options. I already have a capacitor in line with the Mac Mini and a Yellow Top D35 Deep Cycle Battery.

    If anyone has any other ideas I would much appreciate it. As soon as it is done I would love to take alot of pictures and post it on this site because I know alot of you wanted to put a Mac Mini in your car.

    I have a 2005 Subaru Legacy 2.5 Gt


  • #2
    Get a carnetix automotive power supply and all your problems will go away. They have built-in circuitry for that start up voltage drop. They even have a cable kit for the mac-mini hope this helps SNO


    • #3
      Well I have researched that cable and it is only for the old 2008ish generation of the mac mini. So the plug is different from the new ones. Although would there be a way to cut the old plug and cut the new power cable and splice the new cable on to the regulator?? let me know if that is possible or not.

      Thank you!


      • #4
        i don't believe that is possible-- if i remember right, the newer mini's require bypassing the internal psu(there was a howto in the mac area, and on a computer site somewhere..), where the older ones that the cable sets were for were plug-n-play because the psu was external to the mini..
        My OLD 2001 Mitsubishi Eclipse GT:
        "The Project That Never Ended, until it did"

        next project? subaru brz
        carpc undecided


        • #5
          Alright here is my plan on how I can make the Mac Mini work in my car, here me out.

          I will have the Mac Mini in my glove box wired to a 115 Watt inverter. That inverter will be attached to another smaller battery that I will buy and place in the engine next to the other battery. Then I would buy a battery isolator kit to wire to the alternator so it will charge separately along with the regular battery. The computer will be on at all times.

          My biggest concerns and questions are:

          1. What kind of battery would be ideal for powering the Mac Mini but a convenient size to fit in the engine compartment

          2. What battery isolator kit would be ideal for the job that needs to be done?

          3. The Mac Mini runs at 85 Watts at full power and 15 Watts in idle. Will that amount of energy drain the power of that battery while the car is off in 1 night? 2 nights? or a week?

          The Mac will start up by a setting from the touchscreen so it can be on at all times.


          • #6
            If you have a charge light, that can switch a relay to act as an isolator (eg, search for "UIBI").
            You may also want a battery protector (low voltage disconnect) to protect the Mac's battery (eg, an MW728 but with a raised cut-off voltage).

            An engine bay battery can be a typical wet cell (whether a cranking battery or deep discharge) but it will live longer if placed elsewhere (boot/trunk or in the cabin) due to the lower temperature, but then, being "internal", it must be a sealed type (eg, AGM) unless it's in a sealed enclosure vented to the outside, and wet cells tend to be more robust than AGMs (depending on abuse).

            Can you fit another battery the same as your main (cranker)? The advantage is then having 2 of the same so you may be able to use the Mac's as a spare cranker/main battery.
            It is also a good starting point to see how long that battery will last.

            A battery's AH rating is the number of Amps it can supply TIMES the number of hours it supplies it.
            Vehicle AH ratings are typically based on a 20-hour discharge, aka C20.
            Hence if your main battery is 60AH (@C20), it will supply 60AH/20hrs = 3A for 20 hours to reach its specified discharge limit (typically recommended as 20% for cranking batteries and 50% for deep discharge batteries).

            Assume 80% inverter efficiency, then the 85W Mac means 85/.8 = 106W input.
            At ~12V input, that's 106/12 = 8.8A or about 9A, or 3 times the capacity of a 60AH cranker for 20 hours. Though that suggests 9A for 20/3 = ~7 hours, in practice it will be more like 5 hours. (Battery AH capacity decreases with increasing discharge current).

            At 15W idle, assume 50% inverter efficiency - ie, 30W input = 30W/12V = 2.5A - close to your 60AH "rated" battery C20 or 20-hour discharge current. The 60AH handles 3A for 20 Hours (3A x 20hrs = 60AH), so it should handle 2.5AH for something over 60AH/2.5A = 24 hours.

            If you do want several days battery reserve, then look at "solar" batteries. They are C100 rated (100 hours or ~4 days) and better suited to long & low discharge rates (but check their recharge rates and current limits).
            For your 85W = 106W for 100 hours, you need a 9A x 100hr = 900AH battery capacity. Of 4 days at idle (3A) - 3A x 100hr = a 300AH C100 battery.

            Substitute your own battery's AH capacity. But with the Mac idling, a 60AH cranker will last one day, and will probably live for about 2 years or longer (ie, 2 x 365 = 730 cycles) assuming it is recharged reasonably soon after hitting its 20% discharge limit. (Most batteries tolerate ~500 to 1500 cycles based on their nominal rating. Go higher than their rated discharge rate, or deeper than their rated discharge limit (say 20% for crankers; 50% for deep discharge), and their life is reduced - as does elevating their rated temperature (eg, 25C/75F). (The general rule is that battery life halves for every 10C increase in temperature.)

            The above are merely examples that condense the art of battery science. [ See eg (revised 2Jan12) for more detail. ]
            Other than adding that most battery specs are conservative (ie, they should be the minimum battery performance/life that covers 95% or 97% of that model battery), and that most designs assume 80% of battery performance (because that's when they are normally replaced), I'll leave my reply simple since I suspect you will find a suitable battery or full-time operation impracticable.
            Last edited by OldSpark; 01-24-2012, 06:47 AM. Reason: extra info