Alternator disable switch -- how much MPG improvement?

[mechman600]

I'm stuck with a Mazda Tribute (AKA Ford Escape) V6 AWD because it's my wife's. My vehicle is a motorcycle. Anyway, I'm thinking of doing a very simple alternator mod. I'd like to try running the output wire through a high amp relay/solenoid. A simple on/off, with a small 12V battery or capacitor on the alternator side of the relay to avoid large voltage spikes when the relay opens. The relay will be controlled by the brake light switch. When the brakes lights are on during decel, the alternator charges. The the brakes are off during accel, the alternator is idle. The control switch would be a three position switch, easy to figure out by my less than mechanically inclined wife:
1. Highway: alternator functions as normal.
2. City: alternator functions as described above, controlled by the brake light switch.
3. Alternator off: the alternator does not charge at all.
Option 3 will be most beneficial with a deep cycle battery and a small charger under the hood for charging over night - sort of a very cheap man's PHEV.
This setup will be very easy to achieve in my Tribute, as there is an alternator output master fuse where I can make my splices/modifications.

In the wise words of Jeremy Clarkson: "How hard can it be?"

[Doni_99]

Hi Folks!

As I see this is not the most frequently visited/commented forum.

I want to instal a switch on/off for alternator in my car (1982 Volvo 244, 2383ccm, diesel).
BUT I have a serious question about it.
In my country drivers must use light day as well as at night.
The battery in my diesel car is a big one (90AH)
My question is that how can I count the time within the battery goes dead if I use only the lights and sometimes the brake lamp?

If the answer for my question above would be discouraging:
Is it an ulta silly idea?: I would solve this problem with a mini windmill under the engine hood? Of course not for cruising in the city but outside it at higher speeds?

Best mpg!
D99

[chrisoverson]

A mini windmill would still slow the car down as the air hitting the front of the car is only doing so due to the engine pushing the car through it.

It would mean no load during town driving etc though.

One thing that concerns me about this, although I'm thinking about trying it, is that the ECU, fuel pump and injectors are designed to function on 13.5-14.00 volts. On a lower voltage, would everything still function as normal (i.e. fuel pump pressurising the same, sensor voltages ok etc)?

I understand a lot of sensors run on 5v supply from the ECU. If the ECU uses a voltage regulator to provide this then no problem, as there will be a tollerance. If the ECU uses a resistor setup for example then the voltages would no longer be correct.

I suppose this is less of a problem on cars designed to adjust the alternator output such as the Honda Civic, but more of a problem on cars reliant on a steady supply voltage.

I'm tempted to try this regardless on my 1998 rover, to see how much difference the alternator does make. I think it would be very important to ensure that in between trips the battery can be recharged to full to ensure that it is never putting extra load on the engine to recharge a depleted battery when the alternator is connected.

I currently have two 1.5w solar panels on my parcel shelf. Now these are very low power, but they can help to keep my battery topped up in between uses as long as it is in direct sunlight, and I have not been sapping the power too much.

For the future, if I ever develop a plug-in hybrid type car which I really want to... then I'll be adding a 60w solar panel to the roof. It will look a bit odd... but it would be able to recharge my car battery in around 10 hours. Baring in mind this is a 12v set up, and I don't yet know what I'll need for the electric motors.

It needs further testing really, but I don't think an unloaded alternator would sap 5% mpg by just spinning... so the first thing I am going to attempt is just a on/off switch for the alternator which would eventually be set to automatically reconnect under overrun and braking.