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### Topic: split charge auto batteries - voltage sense with separate grounds (Read 619 times)previous topic - next topic

#### Luke-H

##### Apr 23, 2016, 10:22 pm
Hi

Sorry if what I am going to ask is obvious to the experienced people. I am fairly new to Arduino's and my electronics knowledge is average.

I am installing a R1 motorbike in a classic mini and I will have a split charge battery system. The bike engine will have it's own battery and wiring loom and the car will have it's own battery and wiring loom.

I have built a prototype gear position indicator for the bike engine using an Arduino which will be powered by the car battery system. The problem is that I need to sense neutral of the bike engine which is indicated by a wire on the bike loom going to near 12v of the bike battery.

I have built a voltage divider to bring the 12v down to a safer ~5v for the Arduino input pin however I would like to know if I can have the voltage divider using a separate ground to the Arduino ground.

Here are a couple of schematics I've put together to better illustrate my point.

Option 1 - separate grounds

Option 2 - using the Arduino ground.

Any help would be appreciated.

Thanks

Luke.

#### jremington

#1
##### Apr 24, 2016, 12:10 am
In order to make a measurement, the grounds all have to be connected. Ground provides the zero voltage reference.

#### bluejets

#2
##### Apr 24, 2016, 12:13 am
Diagram 2 would be the accepted norm.
Just one problem is you have shown the input to the voltage divider as a negative 12V and that is incorrect.

#### jremington

#3
##### Apr 24, 2016, 12:17 am
Quote
Diagram 2 would be the accepted norm.
Just to clarify, the diagram labeled "neutral_sense_schematic_2.png" will not work.

#### Luke-H

#4
##### Apr 24, 2016, 08:49 am
Thanks jremington and bluejets.

I made a typo with the negative 12v input. It will be positive.

Will i risk introducing potential issues if I connect grounds together from both wiring looms or is this safe to do so?

Worse case I could run the whole thing including Arduino on the bike loom although I'm trying to avoid adding accessories to the bike  loom. I want to keep it mainly for engine management and then have all the car accessories running from the auxiliary battery.

Thanks again for helping.

#### jremington

#5
##### Apr 24, 2016, 05:15 pm
Look up the topic of "ground loops", which can often be a serious problem.

#### Luke-H

#6
##### Apr 25, 2016, 04:08 pm
Look up the topic of "ground loops", which can often be a serious problem.

#### MorganS

#7
##### Apr 25, 2016, 04:33 pm
How can you not have ground joined? You use plastic bolts to attach the motor to the chassis?

Motors usually have rubber vibration isolation mounts but it's difficult to avoid some steel part touching somewhere. That's why most engines have a thick copper ground strap. This ensures any currents have a controlled path to ground, instead of running down the throttle cable and heating it up or causing sparks.
"The problem is in the code you didn't post."

#### Luke-H

#8
##### Apr 25, 2016, 04:48 pm
How can you not have ground joined? You use plastic bolts to attach the motor to the chassis?

Motors usually have rubber vibration isolation mounts but it's difficult to avoid some steel part touching somewhere. That's why most engines have a thick copper ground strap. This ensures any currents have a controlled path to ground, instead of running down the throttle cable and heating it up or causing sparks.
You make a very good point! I hadn't considered that.

The engine will have rubber/poly bushes but as you say it will have a thick ground strap plus other metal contact with the car chassis.

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