"Surviving Crank"

A Guide to Understanding How to Survive Engine Cranking


Introduction

The term "Surviving engine cranking" means that your CarPC will continue to run without crashing while you crank (start) your car's engine. This sounds trivial, but it is a very difficult thing to accomplish. It is highly dependent upon having the right kind of Power Supply Unit (PSU), as well as several other important factors which will be highlighted below.

Some people are not concerned with surviving crank since they routinely manually start and stop their CarPC. There is nothing wrong with this technique, but for some CarPC users it is inconvenient. Most people want a completely automatic system that can be used not only by them, but also by people not accustomed to using the system (ie wives, friends, parents, siblings, etc). Most don't want to have to "think" about starting, stopping, or hibernating their CarPC when they 1) loan their car to a friend or relative, 2) make a short stop for gas, 3) turn off the engine briefly while still listening to MP3's. This is when "surviving crank" is important.

Important Factors

Below are several important factors to consider when designing a CarPC system which will survive crank.

PSU

By far the most important aspect of surviving crank is having a PSU that is DESIGNED for automotive applications and can provide a properly regulated output with an input (from the car battery & alternator) that varies from about 7.5volts (minimum) to over 18volts (maximum). PSU crank survival is totally dependent upon the lowest input voltage it sees during cranking. The CNX-P1260 is designed to provide an output of +12V(+/-5%) while supplying 5Amps with a minimum input voltage of 7.5Volts. If the load is lower than 5Amps, the input voltage can go even lower while still providing a constant, well regulated +12Volt output. For example, with a 3.3Amp load, the CNX-P1260 can tolerate an input voltage of approximately 6Volts. At 1.67Amps, it can go down to 5.5Volts. This is very difficult to do and MOST PSU's on the market cannot accomodate this type of input voltage drop while providing full load and tight output voltage regulation.

Wiring

Another very important aspect of crank survival is choosing the right wire and connectors to be used in the installation. You must take into account length, size (gauge), current draw, and connections when planning your install.

All wire has inherent resistance (acts like a resistor), and the resistance varies with it's length and diameter. Resistance causes a voltage drop as current flows throught the wire. The longer and thinner the wire, the greater the voltage drop across the wire (resistor). For example, 18 gauge wire has approximatley 6.4 ohms per 1000 ft. (or .0064 ohms per foot). If you decide to run 18 gauge wire from your battery to your CarpPC in your trunk (boot for you Brits) which is 12 feet away (NOT a good idea, but a good example of a bad mistake) you would end up with a .0756 ohm resistor in series with your battery. If your PSU needed 10 Amps during engine cranking (not unusual) you would end up with a voltage drop of (10 x .0756) .756 volts, just due to the wire. If you then add a small amount of resistance for every connection (splices, connecting block, etc) in the link, as well as the fuse, you could easily end up with well over 1 volt of loss between the battery and the PSU. If your battery dropped to 7.5 volts during craning due to the load from your starter, plus the load of your CarPC, you would end up with (7.5 - 1) 6.5 Volts to feed your PSU! You can see why you don't really want to use 18 gauge wire for this application.

This is an extreme example, but it illustrates the point that, you must use heavy gauge wire and good connections for connecting the PSU to the battery. Choose as large a wire as practical to connect your battery to your PSU. Take into account the fuse (very important to include a fuse), any connections, as well as wire size and length when installing your system.

Load

Another important consideration for surviving engine cranking is the load (the number of Amps) that the systme places on the PSU. Each system is different and many small ITX form factor systems place a small load on the PSU. For example, a Casetronics C134 case with an internal Morex 60 watt PSU, laptop HDD, and slim CD/DVD drive require only about 3.5 Amps, even during startup (when the load is the heaviest). If you add a Lilliput LDC screen (approx .5 amps), and one or two USB devices (GPS, 802.11), you could easily be well under 5 amps total. The lighter the load during cranking, the easier it is for the PSU to continue to provide a well regulated output. Make sure your system load (amps) don't exceed the rated output capacity of your PSU. The CNX-P1260 is designed to provide 5 amps continuous, even during engine cranking, but is capable of providing up to 6 amps for short periods of time. If you sustain a load greater than 5 amps for more than 10 seconds, the CNX-P1260 will go into "forced shutdown" and gracefully shut down your CarPC.

Battery

Many newer, stronger batteries will not dip below about 9 or 10 volts during cranking. But as batteries age, they tend to develop internal resistance. This shows up as a deeper voltage drop when they are supplying current to the starter (during cranking). It is not unusual for 3 - 4 year old batteries to dip as low as 7.5 volts during a normal engine crank. The problem is much worse in the winter when your battery is cold. A well-designed PSU can survive engine cranking with a "decent" battery, but most will not survive if a battery has developed significan internal resistance with either age or cold weather.





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