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Brief power loss between ACC and start

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  • Brief power loss between ACC and start

    I am searching for a solution for the brief power loss my system encounters while actually starting the engine. I have things worked out so that the system wakes from sleep automatically when the car power is turned on, as well as an external hard drive. The computer is a PowerBook, so the brief power loss is no problem since it uses its batteries during the engine start. The external HD is another story. I need the power to the HD constant once the car power is enabled. Any suggestions?
    I'm thinking about a backup power supply (a small UPS maybe). But i'm not sure how practical it would be. Where can I find a SMALL one? I only need a few seconds to a minute of power - so I dont want a "solution overkill".

  • #2
    There will be a line somewhere in the car that is powered while the car is cranking and when it is finally running. The ECU and other computers need such power. Get a service manual, and you will probably find several places where you can connect your HD. If necessary, use a relay to switch a direct connection from the battery.

    Aaron Cake
    London, Ontario, Canada

    Player: Cyrix 200, 32MB RAM, 10.2Gig Quantum HD, Onboard EtherNet/Sound/Video, Custom Lexan Case, Arise DC-DC, Win95 Kernal w/Custom Player
    Car: '86 Mazda RX-7 w/Basic Performance Upgrades
    Player: Pentium 166MMX, Amptron 598LMR MB w/onboard Sound, Video, LAN, 10.2 Gig Fujitsu Laptop HD, Arise 865 DC-DC Converter, Lexan Case, Custom Software w/Voice Interface, MS Access Based Playlists
    Car: 1986 Mazda RX-7 Turbo (highly modded), 1978 RX-7 Beater (Dead, parting out), 2001 Honda Insight
    "If one more body-kitted, cut-spring-lowered, farty-exhausted Civic revs on me at an intersection, I swear I'm going to get out of my car and cram their ridiculous double-decker aluminium wing firmly up their rump."


    • #3
      Put a relay in series with your battery line and your external hard drive. Control the coil of the relay by diode OR'ing the START and ACC lines. This will do it.


      • #4
        Ok, we tried the relay thing and it did not work out as expected. It did keep the power on during start, but the relay caused the engine to continue running untill ACC power was cut off, which is not acceptable. I did not do this myself (I had a pro do it) so I'm not sure exactly how he hooked things up. I do know he used two relays, one for ACC and one for "start". The relays contacted power from a second battery to my MP3 equipment. How do most of you out there deal with this power loss? When you listen to music with only ACC power, and want to start your car, do you shut down first? ... Still looking for the solution...


        • #5
          I have the same problem.It seems like battery voltage drops to 10-10.5v during the engine start,and my HD "goes to sleep" at these 3 seconds,although he's connected to the battery by the relay.I think I'll maybe use a small 12v battery,about 5-10 Ah as backup for engine start time.These are actually backup batteries for car alarm systems.Also using powerful (2A at least) flyback regulator will solve this problem.You know where I can find one? I tried only gives 1.2A-not enough for HD spinning.
          Enjoint life!


          • #6
            The problem is not really where you connect to the car power, ( even if it is the ACC line) it wont mater if you run direct to the batery because the batery just cant supply a steady curent while cranking. the best solution I can think of ( im working on a schematic , and will post it as soon as I can) but hwere is the theory :

            1. you need a seprate batery to power the computer while the car starts
            2. you need a way to recharge the batery ( remember if just plug it into the car electrical system with no limiting circuit, the car will just suck the cranking power from both baterys.

            use a relay with the coil conected to the IG line so when the ignition line (car starting) is hot, the relay will close a connection for the computer to run from the AUX batery, at the same time the relay will break contact from the computer to the regular car batery.

            Then I would also recomend you get a voltage regulator ( around 1 amp ) or some resistors to limit current to use to charge the AUX batery. you should also hook this up so it only tries to charge the AUX batery when the car is running.Hook the charge circuit to one of the ACC lines ( hot when car is running ) If you dont do this you will notice it's hard to charge your AUX batery when your car batery ( without the car running is only about 11 volts) but with the car runing its up around 14v. the curent limiting is importiant because your car batt and/or alternator will put out 35 or more amps of power, that will fry the 1 or 2 amp AUX bat if hooked up directly.

            It sounds complex but hte parts are like 10 bucks or so, and the wiring is not to hard if you know what to look for. Ill post a schematic as soon as I can.

            Techonlogy on Wheels


            • #7
              why not get a time delay unit, and just have the system boot up, AFTER the engine is started.


              • #8
                Ryan212, Thanks for your suggestion, but that would defeat what I aiming for here. I want to have music in ACC as well as while the engine is running. My system is currently set up so that when power is on (ACC) the external drive wakes my PowerBook from sleep. This works well, and music automatically resumes play in about 10 seconds (No boot time). It will also wake from sleep properly if I start the car quickly. The problem I have is when I let it wake from sleep in ACC mode, listen to music, and then attempt to start the car. That brief power loss screws things up by putting the PowerBook to sleep again, even while the external HDD is powered. I can wake the system from sleep by pressing the power button (on the dash), but this defeats my goal of a totally automatic system - no buttons necessary to turn things on or off. By the way - My web site is in the works now, and I will have it up (I hope) in about a week. I have installed a PowerBook in my 2001 Nissan Pathfinder, and It came out pretty clean. --gizmomkr, I'm very interested in seeing your schematic!


                • #9
                  I have come across a simple build kit from Altronics (aussie company as far as i know). This diy kit controls two batterys, your main one, and a backup. This backup charges when your car is running, but when you stop it, the second battery is isolated.

                  Have a look at the description at: and see what you think.

         Plans and progress on my install - (Updated 28 June 02)
                  Australian mp3car Portal
                  mp3car related links Check them out / Add some
                  mp3car FAQ - RTFF

                  [########==]80% Assembling
                  -=Current Progress: Working Sproggy supply, building box.=-


                  • #10
                    Destin -

                    What powers your external HD right now ( 12 V or plugin to 110 AC Inverter ?

                    In any case, The weather got warm this weekend and I was able to get into my dash to rewire my circuit.
                    It worked just like I planed, and the parts were about 8.00$ from radio shack.
                    Im about to install Visio so I can make a schematic.

                    Hornet - the device looks great for people with dual car bats, One thing I noticed was - based on the description, it outputs a pretty high amperage. A lot of people dont use a full second car bat, they use like a 10 amp alarm back up batery. If the charger was designed for high amp baterys, it would melt down a 10 amp batery.
                    Techonlogy on Wheels


                    • #11
                      gizmomkr - my current setup contains a two batteries. I have already got things working as hornet mentioned, where the secondary battery only charges while the engine is running. The external hard drive is plugged into an inverter which is connected to the second battery. I did not install the electrical part of this system myself, so I'm not sure exactly how things are connected, but I can find out (and I will).


                      • #12
                        Whoever wired up those relays didn't know what he/she was doing. Did he diode 'OR' the ACC and START lines? THey can't backfeed each outher if he used diodes.

                        That said,
                        I do make an Auto off unit that really is a DC power controller. It will not turn off the power to your computer unless the ACC line goes dead for over 8 seconds. This will bridge the time while you are cranking.

                        My unit ties into your ACC and BATT line and provides a smart +12V output.

                        Check it out at

                        Though if you do diode 'OR' the lines properly that will work too.


                        • #13
                          gizmomkr - You mentioned that you did this successfuly, if you could, please post a note on how you connected things. I have just given my car back to the shop and they are going to give it another try. The guy told me he did everything he could think of (with diod 'ORing the relays) and no matter what he did the ACC line backfeeds into the IG line keeping the engine running. He claims to have over ten years experience with diodes and realys and such (I know zilch myself). I suggested using two relays, one for the ACC and one for IG, so the two never touch. Both would be in line with the second battery to the external hard drive. Do you think switching like that would be bad, or cause a spike or drop in power? You help is appreciated...


                          • #14
                            Ok, keep in mind I suck at schematics in visio, but hey.... I use this circuit in my car to prevent temporary power loss due to excessive drain during ignition.

                            I used a small 7 Amp AUX bat, and a 1 Amp voltage reg.

                            Email me if you have questions.

                            Here is <a href=""> the schematic </a>

                            [This message has been edited by gizmomkr (edited 01-19-2001).]
                            Techonlogy on Wheels


                            • #15
                              Here's a hand drawn schematic on how to accomplish what you want.

                              This in no way can backfeed the ACC into the START or vice versa.

                              [This message has been edited by Probedude_2000 (edited 01-20-2001).]

                              [This message has been edited by Probedude_2000 (edited 01-20-2001).]