How to Bypass Alternator with a Switch DIY

[Mr. Previa]

Hi,

I recently performed this on my 92 Previa and thought it would be a helpful write up.

Part one: How to Bypass Alternator with a Switch
Part two: Add a Solar Array for Trickle Charging

The first part of this project was to replace the existing starting battery with a deep cycle battery. I decided to go ahead with this project now because my starting battery was dying anyways, and because of that, it is a fairly cheap mod. Some people use 2 batteries though. I will try one for now, due to my short commutes and other tricks up my sleeve.

The next step is to turn the alternator on and off. Listen up, this is very important! You can not simply disconnect the alternator from the battery. The alternator will continue to output power and will blow up.

“Automotive alternators require a voltage regulator which operates by modulating the small field current in order to produce a constant voltage at the battery terminals.” – Wikipedia

So, by disrupting the Field Circuit, we stop power output even thought the alternator is still spinning. And, because the field in no longer complete, there is no resistance that the alternator has to over come via the engine.



I confirmed that I had the correct wire (B/O) by running the van and checking the voltage at the battery. It was only 12.5 , so I knew the alternator was not producing and charge. Had it been charging the multimeter would have read ~14.5 volts. The engine ran smoother also.

Next, I installed:

• a 40 amp relay

• a 20 amp fuse

• ran the wire for the dash switch

• the dash switch

• spliced into the field circuit wire and connected it to the relay

This finishes the alternator control circuit. I can keep tabs on the battery with a volt meter. I also posted an image of a relay diagram that I found.

I will post the solar installation when the parts come. I hope this helps some one.

[California98Civic]

Nice! And I like the concise discussion of the field wire and the reason for disrupting it. When you ran the wire for the switch, how did you go through the fire wall?

[Mr. Previa]

Originally Posted by California98Civic

Nice! And I like the concise discussion of the field wire and the reason for disrupting it. When you ran the wire for the switch, how did you go through the fire wall?

Thank you. I ran it through the same rubber grummet as the radio antenne.

[California98Civic]

Originally Posted by Mr. Previa

Thank you. I ran it through the same rubber grummet as the radio antenne.

You used a hanger maybe to thread the wire through? Taped the wire to the hanger? Or something else?

[Mr. Previa]

Originally Posted by California98Civic

You used a hanger maybe to thread the wire through? Taped the wire to the hanger? Or something else?

Yep, I used a hanger. I taped it to the hanger with some black tape. I first "massaged" the opening with a screw driver.

[Daox]

Nice write up.

My 2 cents. The 40A relay and 20A fuse are major overkill. When I did mine I just added a switch (no relay). IIRC the field wire carried less than 1A through it.

Here is my setup:
Daox's 1997 Paseo Build Thread

[serialk11r]

^^ 1A? I read the current can get up to 4A :O Maybe that's wrong though.

So if a switch is attached to the brake and gas so that whenever the engine is under very light load or when the car is decelerating, that would be a very weak sort of regenerative braking. Basically collect electrical energy when it's cheap/free from the engine. I think BMW is doing this in new cars. Since these switches would be modulated a lot a relay might be a good idea.

[Mr. Previa]

Originally Posted by serialk11r

^^ 1A? I read the current can get up to 4A :O Maybe that's wrong though.

So if a switch is attached to the brake and gas so that whenever the engine is under very light load or when the car is decelerating, that would be a very weak sort of regenerative braking. Basically collect electrical energy when it's cheap/free from the engine. I think BMW is doing this in new cars. Since these switches would be modulated a lot a relay might be a good idea.

Mine is 7.5 amps. Using a relay is simply a cheap safety measure.

[Daox]

I measured something low around 1A when I measured mine. In any case you can get away with just a switch. Mine has been installed for over a year now and works great. However a relay definitely does allow you to do some nifty things like activate it while braking. Also, if you use a normally closed relay it will fail in the closed position so if it does fail your alternator is connected.

[deathtrain]

Originally Posted by Daox

I measured something low around 1A when I measured mine. In any case you can get away with just a switch. Mine has been installed for over a year now and works great. However a relay definitely does allow you to do some nifty things like activate it while braking. Also, if you use a normally closed relay it will fail in the closed position so if it does fail your alternator is connected.

yes that would be the best thing to do for this mod.