Tricking a Sense Line

[Mozencrath]

So in my Toyota Prius, my 12v system is not powered by an alternator. It is powered via a DC/DC converter whose supply is a 200v battery. It steps it down to anywhere between 12.9v and 14.2v. I do have a 12v battery whose only purpose is to provide the initial startup power of the electronics. The battery is connected to everything that the converter is connected to. The converter has a single voltage sense wire that runs to the + battery terminal. The purpose of this wire is to see what the voltage drop is i.e. gauge the battery level/voltage. This reading that is taken determines whether the converter outputs 13v~ or 14v~...If it senses the voltage drop is great, then it keeps the converter pumping out around 14v, but if the sense line doesn't sense a big voltage difference, it will run at around 13v...

What I need help with is designing a simple circuit that will allow me to fool the sense line. I want the sense line to read a few tenths of a volt higher than the actual voltage at the battery. In fact, the voltage it see's doesn't even have to vary with actual bat voltage. It can be a constant voltage. Anyone know of any simple circuits that can be used to do this?

 

[jackorocko]

So the sense wire senses the 200V bat or the 12V bat?

 

[Mozencrath]

The sense wire senses the 12v bat and it runs from the bat terminal to the converter and the conerter compares this reading with it's own output.

 

[Resqueline]

You'll have to spend some amount of power to do that. Define simple. Do you know the current going in the sense line? What is the purpose of this mod.?

 

[Harald Kapp]

You could use a regulated step-up (boost) converter.

Although with respect to safety messing with the car electronics probably isn't a good idea - your life may depend on it!

Harald

 

[MattyMatt]

I agree with Harald... Those battery banks can be nasty, though your risk may be lower, but I would still sit on the safe side.

Perhaps, as Res suggested, if we knew the purpose for this modification, we may be able to suggest an alternative method that would be safer / cheaper or even potentially easier (though easier maybe not).

Matty-

 

[Mozencrath]

I agree with Harald... Those battery banks can be nasty, though your risk may be lower, but I would still sit on the safe side.

Perhaps, as Res suggested, if we knew the purpose for this modification, we may be able to suggest an alternative method that would be safer / cheaper or even potentially easier (though easier maybe not).

Matty-

There is no danger as the 12v system is completely isolated from the high voltage system. The purpose of what I am trying to do is trick the sense line so instead of reading the battery terminal voltage (lets say its 13.9v) I want to disconnect it from the battery and hook it up to some kind of voltage source that is adjustable from about 13.5v to 14.5v, in order to manipulate the voltage that the converter outputs to the rest of the vehicle. The converter output as I said is determined BY the reading it gets from that sense line. It compares the voltage it is reading at the battery to the voltage coming out of the converter itself (remember the battery and converter are all tied into the same system). The current going through the sense line is a few miliamps..

The only idea I have is to power a boost converter off of the battery, and have the sense line read its output instead of the battery, but I don't know how I would ground it.

 

[Harald Kapp]

Ground is usually any point of the car's chassis. Or better yet, the - pole of the battery.

If the current on the sense line (into the sense electronics) is small, you could (instead of using a step up converter) connect a single 1.2 V NiMH battery in series with the 12 V car battery. from that voltage (12 V + 1.2 V) you can generate any volateg in between with a simple adjustable linear low drop regulator. You'd need a ground connection for this setup, too.
Alternatively, depending on your requirements on stability of the voltage, just add a potentiometer across the 1.2 V battery connecting ends of the pot to + and - of the 1.2 V battery, respectively and the sense line to the pot's tap. This works withot separate ground connection.

But what is the purpose of tricking the generator into outputting a lower voltage in the first place? If the generator outputs constantly less power than needed, there is a risk of discharging the main 12 V battery, thus making it unusable or disabling the rest of the car electronics.

I agree that the 12 V per se is not dangerous, unless you consider:
1) a 12 V car battery can deliver enormous currents. The energy unleashed if a car battery is shorted can melt a screwdriver. I'd call that dangerous.
2) AS far as I understand the 12 V battery powers the complete car electronics. If you mess with this supply, it is unpredictable what kind of tricks the electronics can play and to what consequences. One can only hope that the engineers have put some fail safes into place so the car will shut down bevore running amok.

Regards,
Harald

 

[jackorocko]

Yeah don't count on the engineers thinking of everything. I own a vehicle that is an 07 with tons of electronics. When I had a very weak battery not to long ago my vehicle would barely run, traction control going on and off which would cause dips in the power which would almost make the vehicle stall. You would think that a vehicle this day and age with all the electronics would at least have some safe guard against a weak battery like that, but it doesn't.

Just seems like a really bad idea to mess with the electronics on a car. If you where to ever cause an accident and they investigate to only find it was your doing that caused the accident, may god have pity on your soul if anyone except yourself was hurt.

Carry on!!!