[paulgato]

I did a little testing today. With the alternator connected I drove over a 30mph test route of a few miles, both with a 'minimum' current load and with a 'maximum' current load. I noticed quite a marked difference in mpg at 30mph between the two current loads.

At 17.1A (engine and headlights only) I got an average of 93.2mpg (as shown on dash)
At 44.4A (as above + fan at max speed and rear demister) I averaged 79.9mpg.

At a constant 30mph that gives approx 0.5mpg improvement for each amp I can reduce my consumption by, so reducing the current draw from 17.1A to 0.0A should result in a further improvemment of 8.3mpg at a steady 30mph on that route, giving a maximum of 101.5mpg (as shown on the dash display, which is about 7% optimistic.)

Assuming an average current draw of 22A, I calculate that I should expect to save 10.7% on fuel (at 30mph!) by 'deleting' the alternator. Obviously that does not apply to long journies, and also the faster the speed the less impact electrical loads will have per mile, ...though most of my journies are in fact short local journies at low speed.

Soon I shall test with the alternator off. I'm a little nervous of doing much alternator-off testing with my current battery, as athough it appears to be fine, I don't know it's age or its history and I don't want to get stuck somewhere with a dead battery. I'll rig up an alternator ON/OFF switch on the dash so I can switch it back on with the engine running if the battery voltage starts to look low on the meter during a test.

I was thinking today that if I were to get the biggest leisure battery I can fit in my standard battery tray and use that as my starter battery (this happens to be that same 110Ah leisure battery I was going to use in the spare wheel well as an auxilliary battery) then I might be able to use 50% of that battery's capacity (55Ah) before it becomes difficult to start the engine with it. I was thinking that for my normal daily driving pattern that might be enough, and I wouldn't need to use a 2nd battery at all, as long as I was a bit frugal with the amps used. (Once I have the engine pre-heater installed my (winter) days won't be starting with idling on the driveway while scraping off frost, followed by 20 minutes of heavy blower, demister and windscreen wiper use, so amps used will be lower.)

With this 'plan B' one-battery solution I would save all the weight of a 2nd battery, I would save a LOT of complication and I would save a lot of expense too. And if it turns out I need more juice then I can go back to 'plan A' with nothing lost, and with nothing purchased that I wouldn't have purchased for plan A anyway. I can't think of a reason for not 'starting off small' like that, so that's what I'll do. While I'm testing things to see how low I can take the new battery and still start the engine on it I shall carry my current battery in the passenger foot well as a spare.

One downside to the one-battery solution is that I'll be running the car at a lower voltage (Down to 12v instead of 14.5v) and my headlights will be dimmer. I'll try and get some better, brighter, more efficient and possibly slightly 'whiter' headlight bulbs and see if that helps at all. I kind of like seeing where I'm going though. But we'll see...

...Ahh! I've just been reading up on what to do about dim headlights. It seems on the Mk4 Golf all the power to the dipped headlight bulbs goes through the headlight switch, round the houses, back down through the dingly dell and finally on to the headlight bulbs when it's all tired out and a few volts down on what it starts out from the alternator at. I'll try upgrading the circuit with relays close to the headlights and see if that brightens them up enough for me to feel comfortable about running on the lower battery voltage with no alternator and no alternative charging voltage from a DC/DC converter present.