Engine On/Off Indication From Lead Acid Battery

This is like the 5th time ypu have posted this.

Pick one of the existing threads, and just use that one.

The answers are NOT going to change!!!

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Yea, we know. We got it after the first 4 copies of the posting.

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No. You can't. It won't work. Bye...

However you could monitor the rotation of the flywheel with an Hall effect sensor and that will give you a lot of information.

Surely if the batteries are being charged it can only be because the voltage applied exceeds their internal EMF?

What voltage do you see when the engine is on and off?

I would also imagine that the voltage would vary when the alternator is turning (possibly a more or less rectified sine wave). You could even (tongue in cheek) extract a spectrum and work out the engine speed.

Take some measurements.

engine

level

component

voltage

I'm very sorry this was posted three times. I submitted the original post last Monday, but they weren't posted until this Saturday. I thought I did something wrong posted it again.

It won't happen again.

Also, thank you all very much for your help.

e s

I still think your best hope would be to detect current DIRECTION. The

2 problems is, A. Some vehicles at idle with headlights, heater fan and other accessories on MAY not be charging the battery - so current direction would give false indication of engine OFF. B. There would still be 'noise' and , even with Germanium diodes, SOME threshold issues to filter or hysterisis to work around.

But, in a general sense, current direction would be a binary indicator.

Any remotely current car with a on-board electronics worth its salt will up the idle revolution frequency sufficiently to avoid that. Very old ones would do it blindly if you switch on the lights, modern ones have considerably more sophisticated battery energy management.

But none of that bears much relevance to the actual question. The answer to that is a solid "Don't do that!" If you want to know if the main engine is running, acquire that signal at its source, not at some barely related point several massive disturbances away from it. Guessing what the voltage at a terminal shared by an alternator, a battery, and probably a several active loads might mean is just a nightmare waiting to haunt you.

Yes and no. Sometimes allowing a simple retro-fit is worth the effort. ie We have done battery-voltage decision based systems, for coarse power saving decisions - but we did not make mission-critical decisions. .

The OP should measure systems (MORE THAN ONE), and look for the DC and AC (Audio) signatures at the battery. eg A properly advanced system could get Tacho info, from the charge ripple frequency spectrum ;)

Including temperature improves what it possible, as Lead Acid batteries have a clear tempco.

-jg

Jumping into this thread: The suggestion might already be made but every alternator I know of has a 'W' terminal, outputting an AC voltage directly from the windings, commonly used to connect to a revolution counter. That would be your best source to check whether the enginge is running or not.

Meindert

We have successfully done this by monitoring the input voltage. If at 11 to

12.6 the engine is off, if the motor is running it will be at 13.2 to 13.8V. We have used this on motorcycles (6 to 6.8), Petrol and diesel cars as well as Diesel trucks (24, 27.6).

Just a few warnings, when the starter motor is kicking in the voltages go all over the place, especially on older vehicles, so use a TVS at least. Also beware that if the battery gets to the end of its life it acts more like a cap, the voltage will jump up quick but drop if the motor is idling. We used 30 samples on one second interval to overcome most of these issues.

To overcome different readings on different hardware mostly due to resistor values we did not go to the extent of calibrating the ADC but we just added an ignition voltage as a soft setting. Example: my car indicates ignition if the input voltage rises over 12.8V, in my wives car its set as

13V.

Have fun

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