Engine On/Off Indication From Lead Acid Battery

Started by eeboarder August 17, 2009
"Hans-Bernhard Br&#4294967295;ker" <HBBroeker@t-online.de> wrote in message
> 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.
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

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

Have fun
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