Well, I now have some data for how much electrical power my 2002 1.9 TDi PD VW Golf uses.
I parked up today, disconnected the alternator field wire and started measuring. I used my recently-acquired DC clamp meter, which I later checked against my two digital volt/amp meters and found to be reasonably accurate. If my DVM's are accurate (they both agree) then the clamp meter over-reports DC current by 2% at about 2A and by 3% at about 7A. That's within specification and accurate enough for my purposes.
With the alternator disconnected, the battery voltage was about 12.3v. At higher voltages (e.g. 14.4v) the current draw will be correspondingly higher. E.g. If a test shows 12A at 12v then at 14v the current draw would be 12A / 12v x 14v = 14A. A 24A draw at 12v would become 28A at 14v and so on. This is assuming the loads are simple resistive loads like light bulbs kind of are.
(By the way, just two bulbs have already been swapped out for LED's: the two 5w rear number plate bulbs. All other bulbs are standard incandescant filament bulbs. One of the four 5w rear tail light bulbs is blown and needs to be replaced. I am assuming the two LED's use 20% of the power of the 5w filament bulbs, so that's 8w saved, plus the 5w of the blown bulb makes 13w. So I have added 13w (call it 1.0A) to any test results where the lights are switched on. I intend to replace all the bulbs I can with LED's but I want to start with a baseline current requirement assuming the use of standard bulbs. Later I can calculate how much lower the requirement will be using LED's.)
So, with all the doors closed, the bonnet ('hood') up and the meter clamped around the battery's ground wire I got these results. The bonnet has no electrical connections to it as far as I know so the car is not 'aware' it is up and its being up does not affect the car's current draw in any way.
Ignition off...
Key out of ignition, car unlocked ...... 0.025A
Key out of ignition, car locked ........ 0.020A
Key in; ignition off; sidelights on .... 3.7A
Ignition on...
Ignition on; everything else off ....... 1.7A
Ignition on, plus...
Sidelights on .......................... 5.2A (- 1.7 = 3.5A)
Sidelights on; dash illumination high .. 5.3A (- 1.7 = 3.4A)
Sidelights on; dash illumination low ... 4.9A (- 1.7 = 3.2A)
Headlights on (dipped) ................ 13.2A (- 1.7 = 11.5A)
Headlights on (high+dipped) ........... 21.5A (- 1.7 = 19.8A)
Front wipers on low speed (approx) ..... 5.0A (- 1.7 = 3.3A)
Front wipers on high speed (approx) .... 7.0A (- 1.7 = 5.3A)
Rear wiper on - 20% duty x 4.5A ........ 4.5A (- 1.7 x 20% = 0.56A)
Blower on (speed 1) .................... 6.0A (- 1.7 = 4.3A)
Blower on (speed 2) .................... 8.5A (- 1.7 = 6.8A)
Blower on (speed 3) ................... 12.4A (- 1.7 = 10.7A)
Blower on (speed 4) ................... 17.5A (- 1.7 = 15.8A)
Radio on (volume = 0) .................. 2.7A (- 1.7 = 1.0A)
Radio on (volume = low) ................ 2.7A (- 1.7 = 1.0A)
Radio on (volume = medium) ............. 2.8A (- 1.7 = 1.1A)
Radio on (volume = high) ............... 3.5A (- 1.7 = 1.8A)
Radio on (volume = max) ................ 4.0A (- 1.7 = 2.3A)
Front interior lights on ............... 3.2A (- 1.7 = 1.5A)
All interior lights on ................. 3.7A (- 1.7 = 2.0A)
Engine OFF; reversing lights on ........ 5.2A (- 1.7 = 3.5A)
Engine on...
Engine on at idle (850RPM) ............. 6.4A (- 1.7 = 4.7A)
Engine on at all other engine speeds ... 6.4A (- 1.7 = 4.7A)
Engine idling; sidelights on ........... 9.6A (- 6.4 = 3.2A)
Engine idling; headlights on dipped ... 17.1A (- 6.4 = 10.7A)
Engine idling; headlights on high ..... 26.0A (- 6.4 = 19.6A)
Engine idling; blower on (speed 2) .... 22.6A (- 6.4 = 16.2A)
Engine idling; rear demister on ....... 24.1A (- 6.4 = 17.7A)
And finally...
Ignition on; field wire reconnected .... 2.1A (- 1.7 = 0.4A)
So there we have it. A few minor anomalies where results were slightly different on different occasions, but it's basically sound data I think. Sound enough for the purpose at hand anyway. Some predictable results - such as lights, where nominal wattages are already known - and some surprises.
I was pleasantly surprised that it only takes 4.7A to keep the engine going, irrespective of RPM. 1.7A is needed just to have the ignition switch on, so the minimum requirement for daytime driving is just 6.4A. Much lower than I was expecting. (I'm SURE other diesel cars I've had didn't run very long at all when I've driven them on a faulty alternator.)
The heater blower motor, even at its lowest speed, uses almost as much electrical power as the engine! (4.3A) And at its highest speed it uses a massive 17.5A! I normally drive with the blower at speed 2, which uses 6.8A, but that more than doubles the minimum day-time driving requirement. Wow! I might need to re-think that habit!
Then the wipers use about 4A (50w?) which must contribute to the higher fuel use in the rain. The rear wiper doesn't use a lot (0.56A). The rear demister uses a massive 17.7A, but I guess it's normally only on for a very short period, and when you need it, you need it.
The lights use a huge amount of power. The sidelight bulbs can all be replaced with LED's which could bring the use of sidelights down from about 3.5A (measured once at 3.2A and once at 3.7A - not sure why) to about 20% of that, which would be a mere 0.7A. That would save a useful 2.8A for all night-time driving, and at 0.7A total that would make it feasible to use daylight running lights too. (Not currently a legal requirement in the UK.)
I don't use high beam all that often but it is a shame that VW decided to keep the dipped lights on when the high beam lights are switched on. There might be a software adaptation that switches off the dipped lights when high beam lights are being used. That would save 7.5A every time the high beam lights were being used and (this might be useful for me when I'm deciding how large a DC/DC converter to use) it would reduce the maximum current draw of the car by 7.5A. At some point I'll look into what software adaptations are possible in that regard, but as I say, I don't use high beam much so I think I can probably ignore that extra current draw for now.
So, the maximum likely
sustained current draw of this car is likely to be when the engine is running, radio is on, blower is on speed 2, wipers are all on low speed and lights are on dipped. Lets look at that...
Engine on ............... 6.4A
Radio on high ........... 1.8A
Blower on speed 2/4...... 6.8A
Headlights dipped ...... 10.7A
Wipers on low speed ..... 3.9A
Total max sustained .... 29.6A
I can reduce that by 1.8A by not using the radio (but I like the radio!) and I can reduce it by 2.8A by using LED bulbs in the side lights, the number plate lights and the tail lights. So with LED's installed that's 26.8A at 12.3 volts. At 14.4 volts it would be 31.4A.
The suitable DC/DC converters I have come across are either ready-made and built-for-the-purpose modular units of 12A; 24A; 36A, etc. (from a really helpful and knowledgeable local manufacturer, at a cost of about £70/module) or else impressively sophisticated and great-value-for-money bare-circuitboard-and-heatsink units of 10A each (on e-Bay, sent from Hong Kong at £12 each delivered to my door.) It looks like a 24A unit from the UK manufacturer would do the job (£140 approx) or else I could use three of the Honk Kong units (£36 plus all the other bits and pieces required to make them into useable units.)
There will always be a useful amount of buffering, or peak current supplementation provided by the starter battery, and since the alternator-free range of the car will be limited anyway, a certain amount of discharge of the starter battery would be quite acceptable at times of high current demand. By the time the auxilliary (donor) battery has got down to its 50% cut-off level, it is very unlikely that the starter battery would have lost more than 20% of its charge, and at that point, the alternator would be fired up anyway. So a 24A DC/DC converter would be adequate, and a 30A converter would be more than adequate. This simplifies the wiring somewhat as I can safely feed the current from the DC/DC converters to the main starter battery via the existing cable that feeds the 12v power outlet socket in the load area, which is fused at 35A. Indeed, as a temporary measure I can simply plug the converter output into that socket.
OK, so a 24A or 30A converter will be fine. (12A will not be enough unless I also plan to use a deep discharge battery as a starter battery and plan to partially discharge both batteries regularly.)
Now for battery capacity. For this I need to know the
typical current draw on a typical day. (Over-specifying will lead to unnecessary battery weight being carried in the vehicle, although it may possibly lead to increased battery life.) Lets assume a typical (working) day is not dark and is not rainy.
Engine on ............... 6.4A
Radio on high ........... 1.8A
Total minimum current ... 8.2A
I don't have a regular commute; I drive out to various local customer addresses during the day, and then to suppliers to pick up parts. I should really have monitored my typical daily driving patterns, but for now I will assume that I spend less than 3 hours driving per day. (Electrical power is consumed purely on a per-hour basis. Mileage is irrelevant in this context.) 3 hours at 8.2A is 24.6Ah. Assuming I will be limiting the donor battery discharge to 50% of its total rated capacity, I would only need a 50Ah battery.
My inuition tells me that 50Ah is not enough, and sure enough I have not taken into account losses through the converter, and then there's all the other little power usages, such as indicators, brake lights, charging mobile phone, use of interior lights, and so on. (All interior lights should be changed for LED's if at all possible!) So what then? 70Ah? 80Ah...?
Arrgh...! But somehow I struggle to accept the use of anything less than 100Ah, despite what I said about adding unnecessary weight. I actually found a source of 110Ah leisure batteries which are low-profile and will not only fit in my spare wheel well, but will also - at a pinch, and with a little battery-tray modification - fit in the engine bay as a starter battery when my current one gives up. They are big enough that, although they are deep discharge batteries with thicker plates, thay actually have a greater CCA rating than my current starter battery. They cost £75 each delivered, and are guaranteed for four years. There is no way one of those will last for four years being 50% discharged every day but that's not my problem and a guarantee is a guarantee.
(I guess they reckon that most leisure batteries are only used a few times a year.)
OK, so that's interesting. I was wondering if the 110Ah battery (26kg) would be enough, but it seems it would be more than enough. If I could find a pair of slightly smaller batteries that are of the deep-discharge type and yet are still big enough to start the engine, then that would extend the alternator-free driving time with minimal weight penalty, as I can then safely discharge both batteries to a useful extent. I only need enough power left in the starter battery to start a WARM engine at the end of a day, and on a longer run, when the lights start to dim I just switch on the alternator while driving and all will be well.
I know I said mileage is irrelevent, but just as an exercise, if I drive at my normal 50mph cruising speed, a 110Ah battery should give me 55Ah of power at 50% discharge, which - if I have done the maths right, should give me a dry-road daytime range of six hours or 300 miles before the alternator needs to be switched on. And that's without discharging the starter battery at all. (That doesn't sound quite plausible. I might have to go over those figures again when I'm less tired.)
On a dark, rainy, winter day I might have less than two hours driving time out of this system before the converter cuts out and the main battery starts to discharge.
So, a really long post, and a long testing/calculating session, but I feel the basic specification is done, and it's all looking very feasible.
The next step is to measure actual fuel consumption at various speeds with the alternator switched off. I'll also plug a laptop in with VagCom and see how much fuel is being used at idle with the alternator off.
Then I'll double-check all those my figures, and if they all still add up I'll cost the various options and make some decisions...
[Edit...] Ah, I've just seen a mistake. I think it is probably necessary to have the blower on at least at low speed to prevent the windscreens misting up. I'll experiment with other methods of ventilation but assuming I do need the fan on at speed 1 or speed 2 then that is going to double the current requirement, so a 110Ah battery is NOT an overspecification; it's about right, ...and the maximum alternator-off range is actually looking more like 150 miles, not 300 miles! 150 miles sounds more plausible, and sounds more like the kind of range that others have reported. Still, for daily driving - and most of my driving is daily local driving - I shouldn't need to use the alternator at all with this set-up.