Alternator delete with regen braking modification for better MPG

[Daox]

Alright, lets think about V3.0. This time lets read the service manual... There are some real nuggets in these things if you actually READ them. I'm talking to myself of course.



Let me quote the important bit for you:

When the battery voltage (generator "S" terminal voltage) reaches a regulated voltage of approximately 14.4V, the field current is cut off.

Hey, that sounds a lot like what we want to do. Thanks helpful service manual.

So, we just need to apply 14.4+V to the S terminal, and it'll stop charging. Now, where can we get 14.4V? Well, unfortunately, there isn't anything on the car that we can steal power from. However, they do make pretty small and cheap devices called dc-dc converters. These take one voltage on their input side, and spit out another voltage on their output side. So, I set out and found one that takes in 12V and spits out 15V. Sounds like it'll work, right?

Here is a picture of said dc-dc converter, and a link to its specs.





So, I came up with this pretty simple wiring diagram. This circuit should allow me to switch from the actual 12V signal from the battery to the false signal sent from the dc-dc converter to disable the alternator.

[Loren]

So, I tested it. I pulled the fuse, fired the car up and... 15V! Haha, the opposite of what I wanted. If the alternator can't sense the battery voltage, it just blasts out 15V.

While annoying for what you're trying to do, it's actually brilliant! You lose the sense wire, instead of defaulting to "all of it", or "none of it", it gives you a safe and adequate (but, not super-efficient) 15v. They really did do a great job of designing this car. (I guess that's probably a standard alternator, so whoever designed the alternator... good job!)

[Loren]

I don't see why V3 wouldn't work. And it has the benefit of fooling the system into thinking that everything is working normally. An excellent cheat!

[Daox]

Alright, I actually ordered the dc-dc converters a few days ago, and they arrived on Friday. So, over the long weekend, I had a chance to build the circuit and test it out. Here is the first bits soldered up for testing:






I chose to tap into the sense wire right at the fuse because that makes it quite easy. There are just 3 wires to connect. One to one side of the fuse, one to the other side of the fuse, and a ground for the dc-dc converter. Pretty simple really. I connected them with alligator clips and I used little blades of metal to push into the fuse area.





Here is a quick video I shot of the circuit in action. Spoiler alert: It works perfectly!

[Fummins]

Dumb question, does that converter draw noticeable or measurable power?

[Loren]

The sense wire should be pulling almost zero current. But, the voltage convertor device itself is rated for a max of about a 1/4 watt. So, either way... it's not capable of a lot of current flow.

I was just geeking out on the spec sheet, hoping it would include a theory of operation. It does not. I know there are several ways of raising DC voltage, like a coil/capacitor circuit that stores energy in mutiple banks, and then essentially adds them together for the output, or making the DC into a square wave to drive a transformer, which is tapped for higher voltage output. But, this device is so small, I was wondering if they'd come up with a way to do it with a solid state circuit.

Great work, Daox!

[Daox]

Loren is correct. I tested out how much current the sense wire was pulling when I pulled the fuse the first time. It measured 1mA on my meter. The DC-DC converter claims to be 84% efficient, so its really not pulling hardly any power thankfully.

[MetroMPG]

I did run into a bit of a snag. Once I revved up the engine a little, the voltage jumped up to 14.4V. Boo! So, it is self exciting somehow. This simply won't work. It started exciting (powering up) at probably a little over 2k rpm.

FYI, the alternator in my Metro/Firefly is the same. Apparently it's a "1-wire" set-up: ground, plus the fat positive wire to the battery is all you need for it to generate charging current.

[MetroMPG]

PS: so, did you try the circuit on your way to work today?

Have you figured out how much power the various systems in the Mirage draw?

I once did that for the Metro/Firefly, as a starting point for calculating how much battery capacity you would need in an alternator-off scenario.

The Metro/Firefly uses a surprisingly small amount of power to run its 1.0L 3-cyl engine (no accessories) - just ~60 watts with the alternator off, and ~90 watts with the alternator charging.

FYI:

Watts ... item 

1.7   key @ ACC, ECU awake
17.8   key @ RUN, fuel pump finished priming
54      parking lights, dash lights high
59      brake lights
112     headlights, dash lights high
120     headlights high beam, dash lights high
36      key @ RUN (fuel pump running - priming)
6       radio low vol
18      radio high vol
96      radiator fan high
30      heater fan low speed (1)
49      heater fan 2
66      heater fan 3
81      heater fan high (4)
58      rear defrost
5       interior light
36      wipers low
53      wipers high
66      cigarette lighter
78      hazard lights (est)
42      horn (est.)
39      reverse lights

60      engine idling @ 880 RPM (alternator disconnected)
62      engine running @ 2500 RPM (no alt)
92      engine idling @ 880 RPM, alternator on (field current is high, topping up battery)
89      engine running @ 2500 RPM, alternator on - lower field current as battery voltage came up

Full thread: Tested: power draw of Geo Metro accessories & systems (volts, current/amps, watts)

[Daox]

PS: so, did you try the circuit on your way to work today?

Have you figured out how much power the various systems in the Mirage draw?

I did not try the circuit on the way to work today. I had a mishap AGAIN where I left my windows open and the car got drenched inside from rain. So, I spent the trip to work today with the A/C blasting to help dry it out. I didn't want to take any chances. However, it is installed. I just went outside, and flipped the switch too. So, I will be using it on the way home.

I also haven't done any electrical load testing on the Mirage. I imagine that it will suck down a bit more power than the Metro with electric power steering and more electrical sensors. However, I don't see it being a huge power pig. There is only one way to know for sure though. Perhaps I'll test it a bit later.