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...