Info From A Lead Acid Battery On An Engine?

You should be able to base it on the voltage. During charging the voltage should be in the 14 (28) volt range. When the engine is not running the voltage should be no more than the max charged voltage. I can not remember the full charge voltage off the top of my head, but it should be easy to find. The only way to charge the battery to full is to exceed that voltage.

Scott

--- and expect the indication to be a lot less reliable than a spark signal, oil pressure, crankshaft once-around signal, or other signal that happens as a direct consequence of the crankshaft turning.

I think they usually design for a 13.8V charching voltage once the battery is fully charged. If the battery has been run down a ways, it might be lower.

It's around 12.6-12.8V.

He'd probably be better off measuring the direction of current flow in the cable to the battery terminal, but sensing it without a shunt resistor will require some sort of inductive sensor.

Yes.

There should also be a lot of RF noise coming from the alternator when it is running.

IIRC: The nominal battery voltage SB 12.6. Float charge is something in the mid 13s. Charging voltage is usually 14+, all for a 12V system. IIRC :-)

I'm certainly no expert but it seems to me that it should be possible to monitor the power supply voltage for the presence of some characteristic motor noise that would be absent when the motor isn't running. On the other hand, my proposal might lead you to an overblown and unreliable solution but it might be worth a look.

JJS

I am developing a system that is powered from a 12V or 24V lead acid battery configuration on an industrial engine. These are the same lead acid batteries used in vehicles. The batteries or battery will always be charged by an alternator running off of the engine.

Is it possible to get a run signal just from the battery? The PIC24F microcontroller being used needs to be able to tell if the engine is on or off. If could monitor the engine operation(on or off) from the power source, I wouldn't need an additional device to monitor this(oil pressure switch).

Yes and No. It depends on the particular engine assembly.

The alternator generates the AC ripples which can be seen at the battery terminals. You can even get the engine RPM by that ripples. However this behaviour is stongly dependent on the particular engine and the associated electrics, and this method won't work at all for some engines. So, if your goal is a mass production device, I won't count on it.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

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Hmmm....maybe use the exciter circuit thru a bridge on-board and detect when the voltage is equalized or , as someone said, perhaps detect the change in current flow to/from the battery. have to create a rectifier circuit that indicates the change in current flow, but it could be built into your on-board power supply I guess.

I dunno if I made myself clear! hah!

Carl

Use the signals available on the CAN bus that describe the situation in which you are interested?

This isn't 100% reliable either since the current flow depends on the engine RPM, the amount of electric load, the temperature and the charging state of the battery. In our days it is all controlled by the MCUs, so you can't take anything for granted.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

I am developing a system that is powered from a 12V or 24V lead acid battery configuration on an industrial engine. These are the same lead acid batteries used in vehicles. The batteries or battery will always be charged by an alternator running off of the engine.

Is it possible to get a run signal just from the battery? The PIC24F microcontroller being used needs to be able to tell if the engine is on or off. If could monitor the engine operation(on or off) from the power source, I wouldn't need an additional device to monitor this(oil pressure switch).

Very true, depends on what reliability you need and what connections you have or can make. The battery voltage has the advantage that you are already connected to it.

Scott

Assuming the feed you are getting is from a very close point to the battery or alternator, not via a 6ft or longer feed, via in line fuses.

Some kindly sole may even put a suppression caps or line choke on your feed somewhere. At start or later modification as people have habits of cleaning DC supplies on motors over time in many places.

All of which is just some of the many ways you may not be able to rely on it being what you think always.

I would be concerned about the reliability of that approach. Will the alternator disconnect for an over-voltage condition? Someone else mentioned a low-voltage condition when charging a discharged battery.

I measured the AC riding on my car battery with my Radio Shack meter. It indicated 6 to 11 mV during idle -- I thought there would be more, but there might be enough hash to detect. Would the sampling be shielded enough from other, non-motor, noises to work reliably?