I am investigating a project where I will interface an Arduino-based invention with the OBD2 port on a car. This OBD2 port provides 12V fused power from the battery (OBD2 pin 16) but does not - as far as I can tell - provide an "ignition" (i.e. motor running) power supply.
It is simple to put something like a voltage divider or a MC7805 on the battery source, cutting down the noisy and variable 12V - 14.7V to 5V, and use that to power the Arduino Nano via VCC (or VIN). But that leaves me with a problem: a constant current draw from my external voltage regulator (and in the case of VIN, the Nano built-in VIN regulator as well), while the engine is switched off. It will eventually drain my car's battery to the point that the engine wouldn't be able to start.
a solution that will have a very low current draw when the motor isn't running
a solution in which the power draw stops completely when the voltage dips beneath a particular level
or ideally, a combination of the two?
(I edited the post, adding the following requirements)
a solution that will power the Arduino whenever the engine is running
a solution that will draw the power from the available "battery" source, which is variable from 12 - 14.7 V and has noise in it
a solution that will step down the power to something the Arduino can use
It's tempting to tap into my Conecta solution, but I would prefer to manufacture something new and independent from that device. I would also prefer to get everything from the OBD2 port since I am using that to access the CAN bus. So, I am not excited about running a separate wire to an "ignition" power source. It would make my invention to difficult to install.
Could I use something as simple as a backwards diode, with a cut-off at 12 volts? There are so many different types of diodes, I want advice from experienced people before I go driving with something that might be unreliable or dangerous.
Sorry if this is a FAQ. I searched and didn't see anything directly relevant.
Thank you, I think I could have been more clear. The Arduino only needs to be powered when the engine is running. Otherwise, I would like it to power off. When engine is running I expect between 12-14V. When the engine is stopped, I expect 12V or less.
A rough design I have been thinking of is something that blocks the flow of battery current when that voltage is <12V. From that source, I could use a step-down converter producing 5V.
I must have forgotten to mention, I don't need the Arduino running when the engine stops. It's just... I don't want the invention to have a power switch - which if forgotten, will drain the car's battery to depletion.
Yes, there definitely is! CAN bus data from the running engine, as well as a tachometer pulse, and various other ports providing between 3V - 10V while the engine is running. But some of these are undocumented connections to various ECUs, and some may be data channels. I don't think it is safe for me to draw power from these - but more importantly is the chicken-and-egg problem: I would need a powered-up Arduino to determine the result, is the car running?
Do cars still have a power bus that only supplies power when the ignition is ON? Older cars with an ignition switch had multiple switch contacts, one for accessories, one for power to everything when the car is running, and another that only powered the ignition system and starter motor (to reduce battery load while cranking).
Do some measuring on the battery voltage, how it changes and update the numbers You gave. Below 12 volt..... That is a battery being rather low.
Measure the battery before start and then continue during motor running and battery charging. Then turn off the engine and see what the voltage drops to.
I would guess that You will find a significant knee, break point there.
Instead of <12 volt I could think of <13 or even <13.5 volt.
Detecting this can be done with a few simple components, as well as shutting off the controller.
Vin needs at least 6.5V. but if you are regulating the voltage to 5V, you can power the Nano through the 5V pin (before you ask, yes, you can do that).
I did this earlier today. Pin 16 ("BAT") against pin 4 ("Chassis Ground") gave 12.15V when the car was off. I'm sure it's less after the dashcam has run for an hour or the car's sat for weeks; I'm sure it's more in various conditions I can't control.
Pin 16 against pin 4 gives 14.19 while the car is idling. Raising the RPM adjusts it a little, but I think from what I know of alternators, this is also subject to other factors that I won't be controlling or predicting.
The short end of this is, I don't mind if my device depletes the battery down to 12.15V . Going higher could work, but raises the risk the device will power off during use when it's needed.
Do you care to elaborate? I believe you might have already ruled out a Zener diode, but that was perhaps before the requirements were clear.
Nano is not the best choice if you are concerned about draining the battery. It has a usb-serial adaptor chip which consumes some current even when not connected to usb, and a power led. A Pro Mini at least does not have the usb-serial chip. You can remove the power led, with care.
Do you need to use a 5V Arduino, or would a 3.3V Pro Mini be ok? 3.3V Arduinos often draw less power.
Hi David, my car is from 2009 so has a rather "classic" power system. You can think of it in four states, OFF, ACC, ON and START.
During ACC and ON, there is power to accessory things like the stereo and cigar lighter. During ON, there is full power to things like the engine ECUs, fuel pump and steering. During START, I know for a fact my radio powers off.
I hope this answers your question. For my project, I want to restrict myself f̶r̶o̶m̶ to the services available to the OBD2 port, and I don't think it includes any conditional power source like ON or ACC.
Consider this: https://www.electronics-tutorials.ws/diode/diode_4.html section, Silicon Diode V-I Characteristic Curve
Can someone help me identify a zener diode that has a well-defined reverse breakdown voltage, and very low leakage current?
I'll see if I can pull in the exact diagram I am referring to: