The magazine of the Melbourne PC User Group Travelling Tom Coleman

Those who know me, know all too well that I can be terribly grumpy when the weather turns cold. I like to think of it as a genetic urge to migrate North with the pigeons. There are those who disagree.

Anyway, I knew that I would either have to suffer another winter of misery or get out. Those dreading another winter of my discontent were hoping that I would go to pastures warm.

I seem to be able to meet most of my computing needs with the set up I have at home. However I have taken to much more computing related photographic activities since acquiring a digital camera. The problem arose, how to download from the camera, edit pictures and generally do with a travelling computer, what I had been doing before on the desktop at home.

Looking at the minimum requirements it seemed like I should either purchase a laptop for about $2000 or, install a desktop in the car. Well! Am I going to spend $2000 on a laptop that would be of use for only a few months at the most, or cobble together something from all the bits and pieces in the workshop?

I would not be writing this article if I had opted to buy a laptop.

Fundamental Needs

Trying to create a computer power supply that would run off a 12-volt car battery is not an option. Far better to get an inverter that generates 240v AC from the car battery. This generates several other problems too. I knew from a previous experience, installing a computer on a floating platform on a pearl farm, that it was feasible. However that was many years ago and computers have become more sophisticated since then. They certainly use more power. After doing my sums I figured that the least I needed was a 300 watt inverter. That's 300 watt continuous, not maximum.

Glory be, Cheap Autos sold me a 330 watt continuous inverter for less than a hundred dollars. Now that solved the first problem. The next was how long could I run the computer before the battery went flat and the car would not start. That was solved by adding a second battery and isolating it (more or less) from the rest of the car's electrics What I did was divert the cable that carries the charge from the alternator through an either/or switch. One side of the switch returned the charge to the car battery and the other side to the extra battery that I installed on the floor on the passengers side.
 

The second battery I purchased for $35, factory reconditioned, complete with 3-month warranty, and it has about twice the ampere hour capacity of the car battery.

If I were to seriously run down the second battery I could be looking at a short term initial charge rate of about 25 amps which is pretty gutsy on a 12 volt system. The alternator is capable of 40 or more but rarely ever gets anywhere near that. So I need some fatter than the average 12-volt wire. I am not talking about starter motor cable, which are the finger- thick cables attached to most car batteries. This is charging amps not starting amps; starting amps are much, much higher.

My second battery was not coupled to the starting electrics. The cable had to handle only charging amps. These are usually quite modest unless a battery has been run flat.

I set out to find a switch that would handle a 25-amp current. They are expensive, so I cheated. You can buy 20 amp 12-volt switches at any electronics store. I bought mine from Dick Smiths for about $4. It was a double pole/double throw switch, designed to trip both the positive and the negative wire.

This is how I cheated. If it is a 20 amp switch then either pole of the switch must be rated at 20 amps. So I joined the incoming charge wire to both poles in the middle of the switch. Then I joined the wire to the car battery to both poles at one end of the switch and the wire to the second battery to both poles at the other end of the switch. This effectively made the switch a single pole switch but as the current was being carried by both sets of contacts it theoretically doubled the carrying capacity. That may not be strictly true as there are other considerations but it comfortably raised the capacity of the switch to greater than 25 amps.

There is a trap for young players in this switch business. Some either/or switches are just that. They switch directly to one or the other. There is another type of either/or switch, which has an OFF position in the middle. Most modern computer controlled cars become very cranky if you run them without a charging connection to a battery. If you accidentally put the switch in the OFF position you could do some damage to your car. Be sure to have a yarn with your friendly auto electrician or mechanic before you attempt anything like what I am discussing here.

I purchased an oversized battery case and mounted all of the appropriate contacts and fuse (20 amp) at one end of the case and the battery at the other. I could not find a suitable double contact plug to connect to the inverter and finished up using a car cigarette lighter, which works fine.
I did consider adding a few instruments to monitor the state of the batteries but I settled for a multimeter and find I don't need built-in gauges.
I used a power board with switch on each outlet to take off from the inverter. If I am anywhere that there is regular mains power I can simply plug in an extension cord and I'm in business.

So there you have it. The charge from the alternator is diverted through an either/or switch that selects either the regular car battery of the second battery. The second battery powers an inverter that turns out 240 volts AC that will run a regular desktop computer.

Will it Work? Read On

As all good electronic gurus know that an inverter does not turn out 240 volts AC in quite the same way as the stuff that comes out of the plug in the wall. The 240 AC coming out of the wall is cycling from positive to negative 50 times a second. So is the stuff produced by an inverter. However, out of the wall power builds up to 240 volts. The inverter just sends a virtually instantaneous blast of 240 positive then a blast of 240 negative. Out of the wall power follows a sine curve. Out of an inverter the line is almost dead flat.

Many devices do not like flat AC — they only like sine waves. The switch mode power supply on most computers can cope with a flat wave. They may not like it but they manage. Some can be a bit temperamental. However I was surprised that the monitors are much more sensitive. I had to swap around several monitors before I found one that coped.

I have deliberately chosen a CRT monitor as they have a better performance in the colour and resolution area. I could have got a good digital monitor that would have used only half the space (very tempting), but this was going to be a photography machine so I opted for the better monitor and saved many beautiful dollars to boot.

I also discovered that the switch mode power supply in the box gives less trouble starting if you do not shut down Windows as per the correct procedure. When you are done, save and shut down all programs then once you are at the Desktop, just turn the power off. OK, so next time you start Windows will check the drives and mutter something about not being shut down properly but that is the lesser of two evils. The alternative is to sometimes have 5 or 10 attempts at starting before it powers up. In other words go directly to a hardware shut down and avoid the software shutdown.
 

I have been using this set up for a couple of months on the road now. It has performed very well. Once in a while I need to have two or three goes at starting. When i turn it on the lights flash on and the fans give a spin and then the power fails and I have to wait a couple of seconds before trying again. I haven't worked out why but sometimes holding the power button on while switching on the inverter seems to do the trick. However it is no big deal and it always starts soon enough. I have also found that having the power board switched on and powering up from the inverter seems to give fewer problems.

There is a possibility that the whole start up business would go away if I had used a more powerful inverter. I don't know. If I ever get the chance it would be interesting to find out.

The second battery sits on the passenger's side floor so there is a gap between the battery and the seat. This just fits a milk crate, which is used to hold floppy disks, CDs and various spare parts The box sits on the passenger seat and the monitor on top of the box. Both face the drivers seat. An early problem was that the monitor became mobile going around corners. So I found a wire office tray that just fitted between the monitor and the dash, pushing it gently enough into the back of the seat and stopping it from rolling around. As it also rested on the top of the box there was a gap between the wire tray and the milk crate on the floor. I found a plastic cutlery draw that just fills the gap. It only sits on the milk crate and will readily slide out like a draw. So that's how I use it, It currently holds spare camera batteries, a pen, a pencil, some small change, a spoon, the mouse when not in use, my mobile phone and a pamphlet on the Queensland gem fields. Just the type if thing everyone should have in their car, I have a wooden board that sits on top of the wire tray, which serves as a mouse platform, coffee table; it's the only bit of free space in the front of the car. Very useful. See Figure 4.

With the box as close to the drivers side as convenient, and the monitor up against the passengers door there is just enough room on the box for the keyboard.

It is cramped but it works.

After using the whole setup for a couple of months I can recommend to anyone else doing this that you install the largest capacity battery that you can. The one I have goes for two or three hours before the inverter starts to beep at me, warning that the battery is running low. I have never carried on until that inverter dies for lack of battery power, so I have no idea of how much slack there is in the warning system.

Reprinted from the November 2005 issue of PC Update, the magazine of Melbourne PC User Group, Australia

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