[paulgato]

Aha! Thanks jasantal!

This is what we need.

My goal is to do away with the accessory belt entirely on my Golf TDi.

The belt drives the air conditioning, the alternator and the power steering pump.

The air conditioning system can be binned in its entirety, and that's the first thing I'll do as I never use it, and removing the system might free up some space for fitting an electric power steering pump later on. It will also save some significant weight. I'll leave the actual compressor in place temporarily as it's not in the way, but all other parts of the a/c system can be binned now.

An electric power steering pump can then be fitted (thanks again jasantal for the info on wiring!) and plumbed up to the existing power steering hoses. When that is all working satisfactorily, then both the original power steering pump and the aircon compressor can be removed, and a shorter accessory belt fitted to drive the alternator.

The alternator can remain in place for the time being as a back-up, but the belt can be removed for daily local driving, as I'll be charging the battery up at night from the mains, and for short drives I should never need the alternator. I can carry the belt and a spanner in the car in case I need the alternator, and maybe re-fit it before long journies away from mains power. (It is very easy to fit the belt - just pull the spring tensioner away from the belt with the spanner and slip the belt onto the pulleys. Release the spanner and all is done.)

As for the power steering using more electrical power than the engine-driven pump, well it will, and that is kind of the point as it it precisely there that the fuel savings can be made if electrical power is generated off the vehicle. So the electric power steering will eat into the useful battery life, but for short local journies that won't matter so much, as I'll have enough power in the battery for short journies, and for longer journies which involve a lot of motorway cruising, I won't need power steering, and can switch it off to save amps. A simple ON/OFF switch on the dash should do it.

I've also looked into solar power to top up the battery, and it does seem to be feasible these days. In theory I can fit about 225w of cells on the roof. (I may need to custom build them from bare cells and embed them in resin permanently on the bodywork to accomodate the curvature of the roof, as ready-made 'semi-flexible' panels won't bend in two planes at once.)

As far as I can determine, a nominal 225w of cells should produce something like 80w (or 6A) even on grey winter days, and that happens to be the minimum my car needs to run the engine and nothing else (6.4A). Over the 5 hours of daylight one can expect in winter around here I'll be putting 30Ah back into the battery, which represents approximately the amount of power I'll use in daily local driving over an hour and a half or so. In theory this 6A should be enough to keep the car going indefinitely if I restrict long journies to the hours of daylight. And building solar arrays oneself is actually quite cheap, if time-consuming. Even 400w worth of cells is cheaper than a new alternator for example. Now I MAY have got the calculations wrong here but it does seem that solar power for recharging a car battery is entirely feasible. Of course in summer the electrical load is lighter and solar power production much higher but I want to design around the worst case winter scenario. (This is really a summer project because it will involve lots of bodywork. Summer is finished here now so maybe next summer I'll tackle the solar assist side of things.)

But one thing at a time. Well, two or three at most! I'm going ahead with the electrical alternator disablement plan and nightly battery re-charge off the mains. Also with the engine pre-heater. These two things both involve plugging the car in to the mains at night, so they kind of go together and I'll concentrate on those for now. But all this power steering pump info is getting filed away for the medium future. Really useful.