Apologise to all for this long reply, but even this just skims over one of the problems with a PC in a vehicle – powering it.
The initial installation of a CAR-PC seems so simple at first glance, but in reality involves some serious thinking to overcome problems in the majority of PSU’s that behave like this and other system silliness in complex interfaced installations. If it were not for my initial use of STR (Suspend to RAM) I would not have noticed anything wrong with that part of the PSU initialisation (there was also a major noise problem with the M4 that I had to overcome).
Most installations end up with multiple ground points for various parts of the installation along with lengthy cable runs in a hostile electrical environment. It’s difficult in most installations to isolate the LCD ground casing, USB HUBS and amplifier chassis etc from the car frame (there are also reasons not to) and/or there are long earth connections joining two points. If a long cable connects the screen or other device to the PC then all kind of noise signals can develop between the two earth points, even with chassis grounding (and some case because of it.) RF pulse noise from the PC, HUBS, data lines and switch-mode PSU crawl over everything to some degree and the PC/PSU effects increase rapidly with a more powerful (power draw) system.
My installation has a small 14AH AUX battery that is “supposed” to maintain the PC and stop cranking voltage pulse/variations from upsetting the various microcontrollers when starting the car. Of course this depends on maintaining a correct state of charge in the AUX battery and comes back to how the system is used, wired and controlled. So every installation is different, and that’s without even getting into different Car design, charging systems, ground systems, chassis construction etc used by each vehicle maker which effects noise, starting voltage variations, pulse currents/voltage and chassis induced currents.
A lot of CAR-PC PSU units are supposed to run down to 6v or so (for brief periods like engine cranking) and they do, but that doesn’t mean the PSU wont spit the dummy for a few hundred MS just as it’s starting, in other words, it starts, stops (if in STR mode it drops +5v standby) and restarts, the end result is a failure to resume from STR.
I can only talk about my installation as without a block diagram, lead length and grounding etc I have no idea of your system connections, isolator or power switch timing. I think this is now at a point where my current basic power layout circuit is needed.
I ran 35A cable, both positive and negative, directly from the battery to an Alpine AMP mounted in the trunk. About 3 feet from the AMP I paralleled a Pos and Neg cable to the CAR-PC, also in the trunk but forward of the AMP.
The negative wire from the main battery and a short heavy chassis wire go to the same point on the CAR-PC chassis along with the negative cable from the AUX battery. The AMP is also grounded straight to the car chassis.
I spent a few days trying every type of chassis isolation and grounding scenario while measuring radiated noise, interference and audio noise, and this was the best layout. I would think this a pretty standard cabling connection for a lot of CAR-PC users.
Although my PC system only draws around 4A, in retrospect I could have gone with even heavier cable but it’s all I had at the time.
Now to the circuit, I designed my own Isolators as I had the bits in my junk bin and they cost me nothing. In reality you should only need one in this system, but I do NOT trust the M4 or any other PSU that is microprocessor controlled (or not) to drop the power in the event of a system failure to shutdown or other condition causing the main vehicle battery to bleed down to a critically low voltage.
I still like Diode isolation for a “SMALL” AUX battery as it allows the Car Battery to feed voltage during cranking without pulling current from the AUX. I do this in an attempt to maintain a voltage above 7 volts if the AUX battery is weak in charge and the PC is starting or has started. Of course you open the PC supply to positive voltage spikes from the cranking process; however that’s the reason you take a two wire feed directly from the battery terminals. The battery is such a huge low impedance load at that connection point so starting spikes should (I said should) be greatly reduced.
The forward voltage drop of around 450mV (for this device) across diode (D1) in that isolator is shorted by a Power FET when the voltage input rises above 13v, so it’s only a direct connection when current is flowing “into” the PC-AUX battery from the alternator and have a few mV drop across the FET/Diode when shorted.
The low voltage Isolator has a hold time so no matter how low the cranking voltage goes it won’t disconnect for 10 seconds. In this situation there is no sudden switching of input voltage to the PSU at the point of cranking.
However there is a rise in voltage as the Alternator voltage comes up and it depends on the state of the AUX battery, and therefore the AUX voltage, so there can be a sudden change. I believe that in my system this is the point causing a problem in the M4 and other PSU’s I have tried. When the AUX is just at the right voltage and the car is cranked just at the right time then the PSU does its double start silliness.
The second problem I found and posted long ago was the IGN line to the M4. If it swings at just the right time at start-up it causes the M4 to drop +5 standby if in STR. A cap and diode helped this problem but you are limited to the size of the cap unless you want the Power-off sense time greatly extended.
I also had a problem with my Monitor under the right starting conditions. The Monitor would be blank, would not respond to the remote, would not show the reverse camera and appeared dead. I supply the monitor with ATX +12v and +12v (nom) from the IGN switch. I have a reverse camera that is auto detected by the monitor when powered in reverse and so the monitor runs without the PC powered. When the ignition is off I can watch the status of the PC by keeping the screen powered by the PSU.
Removing and reconnecting power to the monitor made no difference. I found in the end that if I disconnected the main video cable and reconnected it to the PC the Monitor immediately displayed a picture. Seems that the PC does not see the monitor as it starts and only detects it when physically reconnected as above, however that does not explain why the Reverse camera and all controls on the Monitor appear dead, so maybe the video input circuitry locks on the monitor and only resets when it’s unplugged from the VGA port although you would think powering it on and off would unlock it.
The fix was that 10,000uf cap across the monitor supply socket on the PC case. It smooths out the sudden voltage fluctuation under some start-up conditions.
Now, at last, to the question you asked about what I rewired in the system to stop the majority of false start-ups. I initially has that low voltage isolator “sense input” switched via the ACC switch so that with the IGN was completely off, the Main battery was isolated. During cranking the ACC voltage is interrupted and that causes the low voltage isolator to switch and cause a sudden change in supply to the AUX battery. My design mistake was not having a 10 second delay built into the isolator.
When I modified the isolator I also decided to run the sense straight to the input supply and leave it powered at all times and have faith in the isolator, hey if it fails there is still the M4 low voltage drop out, is that built in redundancy? Or just redundant!
Those final two changes have made this so solid with STR, although I now hibernate the system instead. Another change was all USB devices now power up every time, where as I would occasionally get one (usually GPS) not initialising correctly. But it all relies on the AUX battery keeping a good state of charge, which is why I went to hibernate instead of STR. I have a few ideas to try down the track with powering only a couple devices in STR, but I’ve spent too much time on this project for now, besides it works perfectly at last so I just want to enjoy it, also the front and rear Hi Def web-cam drive-recorder is now working perfectly and completely embedded in the front end.
When you consider that I have 10 USB ports at the PC with 1 spare and one 4m lead to the front of the car with 3 x 4 Port Hubs on that and only 3 spare on those , that’s not a bad effort.