Alternator bypass & engine preheater mod

[paulgato]

I have a 2002 VW Golf 1.9 TDi estate, and I'm planning to get some mpg gains by not using the alternator to generate electrical power but using an onboard leisure battery to do the same job.

This would involve plugging the car into the mains every night to charge up the leisure battery, so I thought it would make sense to also pre-heat the engine every night/morning using the same electric hook-up. This will reduce the strain and wear on the engine at start-up and improve the mpg I get in the first few miles. Cold engines also pollute more. The engine pre-heat is of more benefit in the winter of course but will save fuel and reduce wear all year round. An added benefit is that I get to start the day in a WARM car with frost-free windows!

I'm hoping for about 10% improvement in mpg following this combined mod. But whatever improvement I get (and I will certainly get some) the added benefits of reduced engine wear and increased comfort convince me it is worthwhile in any case.


The possible downsides of the plan are...

1. Increased weight. Maybe 30kg. (Battery, etc. fits below rear load floor.)
2. Cost. This mod may not pay for itself, ...and batteries are consumables.
3. Added complexity.


Upsides summarised are...

1. Improved mpg.
2. Lower pollution at start-up
3. Greatly improved comfort, safety and time-saving on winter mornings.
4. Reduced engine wear.
5. Insurance against main starter battery failure or alternator failure (battery backup!)
6. Main starter battery is also charged fully every day.


The basic plan is to leave the normal starter battery as it is and to use a second, auxilliary battery of the deep-discharge type (i.e. a 'leisure battery') to replace the function of the alternator. The 12 volt leisure battery will charge the main battery via a dc/dc converter set to somewhere between 13.8 and 14.4 volts output (to be decided.) That way, the voltage the car's components see will be the normal 14.x volts and not the 12 volts or lower that it might see using just one partially-discharged battery. Also, the starter battery will remain fully charged at all times. To keep things simple to start with I'm planning to have a simple on/off switch for the alternator and a simple on/off switch for the dc/dc converter. Later on I might automate the system so at certain voltage levels in the auxilliary (donor) battery the dc/dc converter switches off and the alternator simultaneously switches on.

(At some point in the future I may consider permanently bonding one of those flexible solar panels on the roof of the car to increase the alternator-free driving range, but that's expensive and won't help for for daily short journies. It can easily be added to any planned system that incorporates a storage battery so I'll ignore solar power for now.)

So anyway I decided to document the process, and post some of that process up here, in the hope that someone might be able to point out my errors before I make them!

OK, so apart from research, all I've done so far is to identify the field wire to the alternator, cut into it and put a pair of bullet connectors in line so I can disconnect for testing and then connect in a switch to turn the alternator on and off from inside the car. Identifying the wires wasn't so straightforward (for me!) and involved a good bit of reading up on how alternators work. In my car, there are three wires (disregarding the -ve ground connection) connected to the alternator:

1. A big fat wire that routes to the battery via the starter motor. This carries the charging current.

2a. There are two smaller wires that terminate in a two-pin plug to a socket on the alternator body. One of these will be the +ve battery voltage 'reference' wire, which the alternator uses to monitor battery voltage.

2b. The other small wire in the two-pin connector will be the field wire, which is live when the ignition switch is on, and is used to 'kick-start' the alternator by energising the rotor's magnetic field.

The field wire is also in series with the dash warning 'no charge' light (battery symbol) and that's how I identified it. I disconnected the two-pin plug and grounded each of the two pins in turn, both with the ignition on and with the ignition off. I didn't want to risk grounding it 100% so I put a multimeter in series on the amps scale as that has a fuse (of which I have plenty of spares!) I think you could just use a test light bulb, but I didn't have one. The field wire is the one which lights up the 'no charge' warning light when it is grounded.

After cutting the field wire and putting the bullet connectors on, I reconnected the two-pin plug and checked with a volt meter plugged into the cigarette lighter socket. Sure enough, the alternator was not charging when that field wire was disconnected, as volts remained down at about 12.5v even when the engine was running. Success! That's the only alteration of the car's electrical system I need to make.

So the next step is to MEASURE the electrical current my car actually uses in various states: how much it uses just to keep the engine going, how much the lights use, the heater blower, the radio, everything. I need that data in order to determine what size auxilliary battery to fit, what size mains charger, what size dc/dc converter, etc.

I could have used my multimeter to measure the current used but it would have been tricky, involving disconnecting and reconnecting cables while the engine is running, and of course the risk of blowing fuses in the meter. (http://ecomodder.com/forum/showthrea...olts-8908.html) So I went out and bought a small DC clamp meter that will measure down to 1mA and up to 80A. That cost £82 but I will certainly use it for other things. Measuring current is always a pain if you have to disconnect things every time.

In the next post I will list the current draw for all the equipment on my car including the engine...

[Frank Lee]

I can't speak for motorists in other locales, but here in the Rust Belt the engines and transmissions are typically the last things to fail. Everything on the undercarriage succumbs to rust and falls apart and the drivetrain is still going strong. I've had cylinder heads off engines with nearly 200,000 miles on them that had virtually no ridge and still had clearly visible cross-hatching- easily good for another 200,000. Just sayin', engine longevity doesn't seem to be much of an issue anymore.

P.S. Unless it's a VW.

[paulgato]

Good point Frank. It's pretty damp and salty around here too. I'm currently sorting out some rust on the underbody because I am planning on keeping this car for as long as possible. Encouragingly, the only rust I've found so far has been where accidental damage has taken place, such as previous owners dropping the sills on kerbs or jacking up on the wrong places. Elsewhere, it all looks 100% rust free, which is not bad after 11 years on Britain's salty roads. I also took off one of the front wheel arch plastic liners and found half a kilo dry weight (more than a pound) of compacted mud filling the lower rear section of the wheel arch. It was full to a depth of about six inches. That mud will have been almost constantly damp and soaked with road salt for years. No rust whatsoever underneath.

And I'm not having to start at -30 degrees C, so engine wear is probably not a huge issue, as you say. Still, eliminating any possible wear from cold starts is a kind of 'free benefit' of doing this mod and it cheers me up to count the benefits!

The mpg benefit of starting the day with an already-warm engine is my main motivation for fitting the pre-heater. In the summer, the car's mpg meter will show about 70mpg on my five mile run into town, and on the way back it will show about 90mpg (imperial) even if the car has been sitting an hour or two in between. (In the winter I would expect this difference to be even greater.) With an already warm engine it should be showing close to 90mpg both ways. (The car's mpg meter is about 7% optimistic, but consistently so.)

[Frank Lee]

Yes, absolutely, I should have noted before that all the other aspects are well worthwhile... and the engine wear thing is nice too, even if it doesn't really matter.

[cbaber]

I'm doing something similar, but with a different strategy:
http://ecomodder.com/forum/showthrea...tml#post384677

I haven't looked, but I am sure a 12V DC-DC converter capable of car level amperage is going to be rather expensive. You really have to know what demands your car has, and make sure your DC-DC converter can supply more. Also, DC-DC step up conversions increase voltage but you loose some current in the process. Be sure to know exactly how much current is reduced at the output. It will factor in your calculations to figure out how long the deep cycle can charge and run your system.

The only reason (that I can think of) to stay at 14 volts is headlight dimming. The cars electronics will run fine on 12V. Deep cycle batteries often have sufficient CCA ratings to start our little engines, so that isn't a concern unless your under extreme demands on starting. You'll have to decide if the dimmer headlights are still acceptable safety wise. For my car 14V vs 12V is a noticeable difference, but just because you notice the dimming doesn't mean its unsafe. Faded lenses and old bulbs are more of a safety risk than running headlights on 12V in my opinion.

There is also problems with running multiple batteries in the car. For your system it's a bit different so it may not apply. If I were to simply connect a deep cycle in parallel with my starter battery I run the risk of damaging the starter battery. Those batteries are not designed to discharge much at all. If I run them in parallel they share a voltage and combine capacity, but I am still discharging it more than I would want to everyday. This could cause it to go bad earlier, further making my system inefficient and costing me more in batteries. Having a single deep cycle, or multiple deep cycles in parallel provides a more flexible setup with discharging.

Overall I think cutting out the alternator could yield good MPG results, but it probably won't be economical overall. When you factor in the cost of batteries, the cost of the extra components, and then the cost to charge the battery from your house, there is very little room for any actual savings. But for me it's not all about actual savings. Some of it is just seeing how high my MPG number can go, regardless of what it costs. If that is you too, then go for it.

[paulgato]

Thanks cbaber,

I haven't looked, but I am sure a 12V DC-DC converter capable of car level amperage is going to be rather expensive. You really have to know what demands your car has, and make sure your DC-DC converter can supply more.

Absolutely. So I'll be measuring my amps usage in the next couple of days, and then I can decide on size of battery/size of dc/dc converter, etc. So far I have found one UK company who makes suitable dc/dc converters designed for charging one battery from another. They make them modularly at 12amps; 24amps; 36amps; 48amps, and so on. From memory about £70/module. They will put out a predetermined voltage and shut off with an audible alarm at a predetermined donor battery voltage. Because they make them locally they can set those voltage levels to whatever I want, within limits.

Another possibility is these dc/dc boost units available from Hong Kong on e-Bay. 10 amp units, sold as a bare circuit board on a heatsink with no case. They have adjustment screws for regulated output voltage and I believe they have a contact for remote switch on/off. Up to 95% efficient, so they claim, and any number of these 10A units can be run in parallel. They look good to me and cost £12 each delivered to the UK. Three of those units would cost £36 and put out 30A. I'm sure that would be more than adequate. (I am, after all, only having to provide the average power needs through the converter, as any peaks will be taken from the starter battery.)

I'll probably go with the UK firm though because a guy in the tech support department there is an mpg nut himself and would be keen to help me get this working well. He was giving me advice about LED lighting. I also like the idea of the automatic shut-off alarm at low donor battery voltage. I'm told it can extend battery life significantly if you don't go down below 50% charge. Also I can use that shut-off signal to poke the alternator back into life automatically.

But this all depends on the data. I was playing with the clamp meter tonight. It hooks nicely over the battery's earth cable and seems to work well. Maybe not quite as accurate as a multimeter in series, but they say it's accurate within 3% or so, and that's good enough for my purposes.

...PS - I just read your thread. So you removed the alternator/power steering belt completely, eh? Power steering delete (or fitting an electric p/s pump) is also on my list to do, but one thing at a time... I like the battery relocation you've done but I'm wanting to go down the two-battery route. I was nervous about having a long cable run to the starter in a diesel. At 300A cranking current you'd need a monster cable for such a long run. As it is, I think I can safely use the existing cable to the 12v power outlet socket in the load area as that is fused at 35A and I doubt my dc/dc converters will be rated higher than that. I'll just plug the converter in there to start with.

My 2nd battery is going to be located down in the spare wheel well and the spare wheel is going to stay at home. I thought about the 'no spare wheel' issue a long time and it does make me slightly nervous as I did lose a wheel in an accident a year or so ago in the snow. Tyreweld would not have helped in that situation. But it's rare these days. That was the first time I'd had to change a wheel by the roadside in 20 years. In that time I've had quite a few breakdowns due to faulty batteries and alternators, as well as other breakdowns for other reasons. Tyres are tough. If they're in good condition and you drive carefully I reckon they're no more likely to let you down unexpectedly than any other component. With two batteries (and all the voltage and current monitoring guages I'm going to have on the dash!) I'm far less likely to find myself with a dead battery or be caught out by a failing alternator.

[cbaber]

On my engine the accessories are driven by their own belt. So I can remove just the power steering belt and not worry about effecting or modifying the other systems. The cable I used was 2 gauge welding cable, which can handle my Honda's starting amps. For 300A I would have used 0 gauge welding cable. Yes it's a pretty big wire, but the safety component is the inline fuse mounted just next to the battery. If anything were to short out on the long cable run that fuse would protect the car from a fire.

I'm willing to bet that during normal operation your car uses more than 35 amps. Especially at night with the headlights on. I could be wrong though, as I have not measured my own current draw. But it does pose a problem if you plan on using the existing 35 amp outlet. Your DC-DC converter has to provide the current to run the car systems, but also a little more to make sure the starting battery stays topped off.

It is true that how much you discharge the battery is related to how many cycles it can perform and how long it lasts. Even deep cycle batteries, which are designed to be deeply discharged, will last longer if you discharge down to 50% capacity rather than down to 20% capacity. That is one of my concerns as well. My deep cycle battery may not even last 2 years if I go through 300+ charging cycles a year. So I may end up spending 2-3 times more on batteries than I would without deleting the alternator.

[paulgato]

Well one option is to buy a leisure battery with a 4 or 5 year warranty. Most leisure batteries get used just a few times in a year so manufacturers can afford to offer long warranties like that, even though they only expect them to last for 500 full charging cycles.

[paulgato]

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.

[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.