[SOLVED] Engine Running Detector

I decided to install a dual battery system in my SUV since I have a spare battery laying around. I bought a PAC-80, 'battery isolator' they called it... Its just really is a solenoid/relay. I had it mounted, all wired up except for the triggering part. I don't want to have it wired to the accessory on or install a manual switch. I want it to trigger on only when the car is running so that the backup battery can be charged, so I designed this circuit:

Base on the Fact that when the engine is running, most alternator puts out more than 13.6V



Output is taken at R4 and will be driving a suitable FET
Output is HIGH when VCC is greater than 13.6V

PAC-80: **broken link removed**
The relay will handle 80 amperes on a continuous basis at 12 VDC and 150 amperes in rush peak or surge.
Switching power must be between 10 and 14 VDC. The relay coil draws less than 1amp when switched on

I think it should work, I'm posting this here for your opinions/improvement before pulling out my soldering gun.

Thanks

If the 2ndary battery is dead and the primary battery is weak, and the car is idling, the relay will oscillate On/OFF

There are many ways to detect an "engine running" condition, without actually tapping into the vehicle's electrical system.

For instance, you could use a low cost microphone attached near the engine, and detect when there is a significant output from it.

An EFI vehicle wouldn't notice if you tapped off one of
the fuel injector drivers with a >10Kohm impedance and
fed that to a one-shot's retrigger input. Figure 600RPM
idle means about 5Hz minimum rep rate, so a 0.1s timer
retriggered would do the job.

To detect if the engine is running, use its oil pressure switch, or install another for the purpose.
Do not use an oil pressure gauge sender, use the one that contacts either to ground or + when there is pressure, like the one for oil pressure warning lamp, and drive the relay with it.

schmitt trigger said:

There are many ways to detect an "engine running" condition, without actually tapping into the vehicle's electrical system.

For instance, you could use a low cost microphone attached near the engine, and detect when there is a significant output from it.

Click to expand...

Yes, this is another way of doing it, thanks for the idea but I don't think it would be simpler/easier than what I have. However, it would be an ideal input for a micro processor

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SunnySkyguy said:

If the 2ndary battery is dead and the primary battery is weak, and the car is idling, the relay will oscillate On/OFF

Click to expand...

This would be a problem, thanks for pointing it out. I would have to add in some battery level indicators.

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dick_freebird said:

An EFI vehicle wouldn't notice if you tapped off one of
the fuel injector drivers with a >10Kohm impedance and
fed that to a one-shot's retrigger input. Figure 600RPM
idle means about 5Hz minimum rep rate, so a 0.1s timer
retriggered would do the job.

Click to expand...

This is another great idea, thank you.

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Externet said:

To detect if the engine is running, use its oil pressure switch, or install another for the purpose.
Do not use an oil pressure gauge sender, use the one that contacts either to ground or + when there is pressure, like the one for oil pressure warning lamp, and drive the relay with it.

Click to expand...

I have researched about this, but the location of this oil pressure switch in my car is so hard to reach, I abandoned that idea

Just wanted to say, the circuit worked as expected. I built it on a perf board and installed it in my car. Added a 'relay on' LED as a charging indicator. My secondary were around 9V and there were no problem, no oscillation; nothing gets hot. I'm quite happy with it.

Radioshack charged me1.99 + tax for 2 12V 1W zener! Arggghhh

NOTE: a pull up resistor of 100k was added to base of Q2 to ensure that it is not floating

hah . I would have just used my surplus White LEDs 4x3V=12V zener.

good stuff. relay hysteresis must help