An cheers for sorting all this out Dino

Trap.

Am I too late to add my name to the list for a heated screen?

Roy

Umm, well yes ... but of course it's not that simple!!

The first batch - which was originally supposed to be the only batch!! turned up short which was a bit so I took the decision to over-order this time.
I've also actually bought them myself rather than relying on the importer, hopefully this way we'll actually get the number we want!
Of course if they all turn up I'll have more than needed, in which case obviously you can have one.

I've sent you a PM suggesting what we do.

No problem mate, glad I could help

The final screens from the first batch are being sent out this week, so if anyone hasn't heard anything by Friday give me a bell and I'll chase it up for you.
D

They are delivering mine this afternoon. Has anyone measure the current that the screen pulls yet? I've been told that it is in the region of 40amps.

If no one knows, then I might have to run an experiment this week.

It was suggested that'd be more like 20-25A but of course we won't know until someone (you ) checks!

Any more than that and it would fry the altenator/regulator....

Er, not really. Why do you say that?
If your alternator/dynamo can't keep up it'll just flatten the battery.

I wouldn't run the heater through the regulator, the regulator is for things that need an accurate supply voltage, like the gauges. I'm planning on a 55amp alternator, which might be a bit low, but it should only struggle if the screen is a very high current, with the wipers, heater, headlights all turned on. I'm hoping it will be more like 20 amps, but I'll let you know when I measure it.

well this tallys with what the min boys were saying when they use a screen from ricky, and im sure rickky told me they were 25amps.

Mine has arrived today apperantly, will have a look at it when im back up the parents over christmas

WEST

Following on from the earlier posts and some initial testing, here are some thoughts and suggestions regarding fitting:

It should be noted that it is not possible to accurately determine the current draw of a heating element by simply measuring it's resistance at room temperature, as the effective resistance reduces as the temperature rises.
Nevertheless some initial measurements suggest the following:
The room temperature resistance is ~13ohms
The room temp resistance of a 21W bulb is ~ 62ohms
As both the bulb and the screen filaments are heating elements a guesstimate would suggest that the screen may draw something in the region of 80w-100w (resistance is somewhat less than 1/4 therefore current draw is ~4-5 times)
This is quivalent to ~8A at 12v.

I connected a 12v 7Ah sealed lead acid battery across the screen and measured a current draw of 6A I'm not suggesting the installed screen will only pull 6Amps, but it's a ball park indication.

The importer suggests current draw could be similar to a Mini screen ~20A-25A as it is of 'similar size and construction.'

I think based on all the above we can safely assume the screens will not draw more than 25A, and in all probability quite a lot less.

So my initial suggestions for fitting:

As with all high current items, the screen should be wired through a relay, a standard 30A type is fine.
The positive feed should be fused, I would suggest at 20A initially and ideally fed straight from the battery - under no circumstances should the screen be run from the existing wiring without thorough checks.
The cable size for both positive feed and earth return should be 44/0.30mm, (3mm2, 27.5amp), in the light of experience it may be possible to reduce this to 28/0.30mm, (2mm2, 17.5amp) but I wouldn't recommend it at this stage.
It's probably a good idea to have an illuminated switch.
The screen is safe to be left running continuously, but there is little point as it will defog/defrost very quickly.
There is no possibility of the screen damaging the alternator/dynamo even if the charging output is insufficient to power the screen and other items. All that will happen is that the battery will lose charge whilst the screen is running.

At the risk of sounding like the HSE if you're not confident with wiring it in you should seek assistance from an auto electrician.

If what I was told about the screens, and what seems right from what I've seen of mine, the screen is heated by two separate elements, one for each side of the car. Did you wire up both sides, or is this for just one?

Your figures sound promising at 6amps, obviously you would need to add a bit as the alternator would be running at 14ish volts, so I'm guessing that 20 amps won't be far off the mark (for both sides).

I think I am going to wire mine up with the ground for each side connected at the top (maybe earthed to the pillars) and the positive feed connected at the bottom. It might be worth wiring up the circuit so that it has to be turned on each time (like many rear screens on modern cars). It's called a soft latch and I would think it is possible with the right relay and switch

A more accurate figure might come from connecting it to a car with it's engine running. That is my plan when I out into the garage. My poor old pug 106 won't know what hit it!

Right, just a little update. I have measured the current drawn by the screen ran on a battery (with no alternator) and it has come out at 6.6 amps, so even at 14 volts (with the alternator) should only be drawing 7.7 amps, which I've got to admit is significantly less than I originally thought it would be.

So, at least it means that I'm not going to need a massive alternator.

My screen should be wired up and fully operational by Friday thanks to a trusty auto electrician.

Trap.