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Discussion Starter #1
If you install a 12V supply (for car's lights, radio, etc) in a EV through a switched-mode DC-to-DC converter from the main EV big battery pack, do you make the 12V installation negative (ground) and the (for example 144V) installation negative both connected to the car chassis, i.e. share a common ground, or only one of them and completely isolate the other installation?

What if you use a second 12V battery charged though an alternator, do you make them share ground, or have isolated separate negative wire?

Or are both options used?

What are the pros and cons of common and separate ground?
 

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you MUST isolate the high voltage from chassis ground and low voltage! High voltage traction pack should be completely isolated!

low (12v) voltage dc-dc gets grounded to chassis, and if you want to add a little battery or big steroe capacitor to soak up the momentary sags when 12v stuff comes on... that is pretty common. If you have your dc-dc convertor 'always on' it acts as the main power source, and a little aux battery just kicks in a little as needed, then is recharged.... no need for alternator since the dc-dc will bring it back up to full voltage.
 

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If you install a 12V supply (for car's lights, radio, etc) in a EV through a switched-mode DC-to-DC converter from the main EV big battery pack, do you make the 12V installation negative (ground) and the (for example 144V) installation negative both connected to the car chassis, i.e. share a common ground, or only one of them and completely isolate the other installation?

What if you use a second 12V battery charged though an alternator, do you make them share ground, or have isolated separate negative wire?

Or are both options used?

What are the pros and cons of common and separate ground?
You don't have much of a choice for the 12v system because most items in the car will use the chassis as ground. So your 12v dc/dc or alternator system should use the original 12v common ground.

The high voltage pack should be isolated from the chassis.
 

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Discussion Starter #4
you MUST isolate the high voltage from chassis ground and low voltage! High voltage traction pack should be completely isolated!
Just to have a complete understanding on the issue: why?
What are the negative consequences if they would have common ground?

Not that I need or want to make common ground, I would just like to understand why I should not.
 

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Discussion Starter #5
You don't have much of a choice for the 12v system because most items in the car will use the chassis as ground. So your 12v dc/dc or alternator system should use the original 12v common ground.

The high voltage pack should be isolated from the chassis.
Would it be a good idea to also isolate the 12v system from ground, which of course means rewiring/redesigning all the cars 12v components? I.e. separate the car body chassis from any electrical ground, and use only thick copper wire with black insulation to connect ground to everything?

Or would that be a bad idea?

Or just a waste of time, but with no negative consequences if I do this?

Why does a car normally have ground on the body chassis at all, is there a good reason for it besides saving money on wire?
 

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cars use the chassis as ground probably mostly to save on wire rather than running ground wires all the way back to battery. ;)

in the case of an EV you REALLY don't want high voltage to chassis, because if the high voltage positive touches ANYWHERE, then you blow up electrical stuff, start fires, and maybe electrocute people as the vehicle turns into a giant toaster... how's that for a consequence.
 

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If you don't have current running through the chassy, it'll rust far, far faster. Unless you have an aluminum chassy, current slows down its oxidation.
 

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You don't have much of a choice for the 12v system because most items in the car will use the chassis as ground.
That might of been true 20 years ago, but not now. Most gizmo electronics now days equipment chassis is not bonded to the return circuit, and come with a full length negative conductor to go back to the battery return buss.

Using the chassis as a return circuit is really bad news.
 

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Just to have a complete understanding on the issue: why?
What are the negative consequences if they would have common ground?
As for the high voltage there are two very good reasons:


  • Being isolated reduces the risk of electrocution and as stated already prevents accidental short circuit of the high voltage if and when accidental contact occurs with the vehicle frame.
  • One other major factor is battery self discharge. Problem is batteries get dirty with oils, grease, road grime dirt etc.. Once it builds up a very light coat of grime from the battery terminals or other electrical terminals will allow current will flow.
  • To a lessor extent from safety using a floating ground on high voltage makes it easy to control RFI caused from common mode noise issues.
On the low voltage side using a floating ground eliminates a common mode noise from entering via the power wires to all the 12 volt gizmo's which is a real concern for radios, GPS, data, ect...

In addition just like the high voltage short circuits to the frame work do not cause any damage of outages.

In the ole days auto manufactures use to use the chassis for just about everything which had two benefits:


  • Minimizing voltage drop on high current devices.
  • Saved them money from additional wire.
However with today vehicles are chocked full of sensitive electronics and using the frame of the vehicle allows common mode nose to be generated and enter th electronic system via power wiring, signal wiring, and antennas.
 

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cars use the chassis as ground probably mostly to save on wire rather than running ground wires all the way back to battery. ;)

in the case of an EV you REALLY don't want high voltage to chassis, because if the high voltage positive touches ANYWHERE, then you blow up electrical stuff, start fires, and maybe electrocute people as the vehicle turns into a giant toaster... how's that for a consequence.
Hey Dan,

I have an IOTA DLS-55 and a 18AH motorcycle battery. DLS has pack voltage always at input. On output I have a relay on the positive leg that is switched by the key sending 12v+ to the 12V battery. When the negative side of the HV enters the DC-DC, does that essentially tie the HV system to the chassis or does the DC-DC somehow isolate the HV?

Thanks,
Dave
 

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When the negative side of the HV enters the DC-DC, does that essentially tie the HV system to the chassis or does the DC-DC somehow isolate the HV?
In a properly designed device there will be isolation between the high voltage input side and the 12 volt output side. The high voltage will continue to be isolated from the chassis ground. This isolation is accomplished with magnetic coupling of the input to the output using a transformer for the power conversion stage and the usual approach for isolating the control signals is optically using an opto-isolator which is an LED and photo diode pair.
 

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In a properly designed device there will be isolation between the high voltage input side and the 12 volt output side. The high voltage will continue to be isolated from the chassis ground. This isolation is accomplished with magnetic coupling of the input to the output using a transformer for the power conversion stage and the usual approach for isolating the control signals is optically using an opto-isolator which is an LED and photo diode pair.
Thank you, Doug. This makes sense. Now I am wondering whether people moved away from the DLS-55 because of failure or isolation or both. This is the DC-DC that came with my package, but I am wondering if I should swap it out for something weatherproof and isolated.
 

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(bold mine)

I'm pretty sure you didn't intend it, but an astute observer will find at least two good puns in this one sentence! :D
In the current discussion a spark of hope exists with this connection to good use of english.Good to see you leading the charge which will lead to a complete circuit of this site and a lively discussion:D
 
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