# newbie question on pack grounding



## major (Apr 4, 2008)

vwdevotee said:


> I know this is probably an obvious question, but is the battery pack in an EV grounded too? In all teh wiring diagrams I see a heavy cable running from the motor or controller back to the battery pack to handle the bulk current, but should there also be a smaller wire grounding the negative side of the pack to the chassis?


No,

Keep the high voltage battery pack isolated totally from the chassis and also from the low voltage (12v) system.


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## vwdevotee (Mar 8, 2008)

Could you elaborate? It still occurs to me that if it's isolated then there could still be a large potential between the pack and ground, which might lead to a nasty shock if someone goes to do anything on the pack like unhook it.


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## major (Apr 4, 2008)

vwdevotee said:


> Could you elaborate? It still occurs to me that if it's isolated then there could still be a large potential between the pack and ground, which might lead to a nasty shock if someone goes to do anything on the pack like unhook it.


 
Safety is best served if the high voltage (HV) system is completely isolated. Then if you make the connection between any single point in the HV system and chassis (ground) with the flesh on your body, you are not completing a circuit with said flesh and will draw no current.

If your HV system is grounded to the chassis and you touch a single HV point while in contact with chassis you will have a potential across that flesh and draw current.

Which would you prefer?

Everybody isolates the HV system completely from everything.

major


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## vwdevotee (Mar 8, 2008)

Ok, that makes sense. Thanks for clariying. The other post just didn't offer any explanation beyond "don't do it".


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## colinmcc (Oct 19, 2008)

vwdevotee said:


> I know this is probably an obvious question, but is the battery pack in an EV grounded too?


I'll just repeat what the others have said... NO, NOT EVER!

You should also be aware that many chargers (the Manzanita Micro PFC20 I have is typical example) connect one side of the utility company's input to the neg side of the DC output when plugged into the charging voltage. So, if I am charging from 220v, and touch a battery terminal, it is actually 'live' via the charger.

So it is important if you are running a DC to DC converter to get 12v for the car's electrics from your main battery pack that the converter either isolates it's input from output, or is disconnected from the battery pack while charging.


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## vwdevotee (Mar 8, 2008)

This might be a charger design question, but do the chargers that connect the battery directly to mains just use a non-isolated buck-boost or something similar? Would an isolated AC/DC converter keep from having a live connection on the battery?


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## samborambo (Aug 27, 2008)

vwdevotee said:


> This might be a charger design question, but do the chargers that connect the battery directly to mains just use a non-isolated buck-boost or something similar? Would an isolated AC/DC converter keep from having a live connection on the battery?


Yeah, it would but you'd add to the cost of the charger significantly with the HF transformer.

The whole point in having a common earth is for fault clearing. The idea is that there is an expected potential between earth and the rest of the system. When a fault occurs, the protection system (fuse, breaker, etc) should be sized appropriately to clear the fault as soon as it happens.

If the system is not referenced to earth at all, one does not expect there to be a voltage hazard. However, if two faults occur in the system, the fault current may cause a quite violent arc at the point of fault since the protection system is not designed to clear it.

It takes 30mA of current to cause serious harm to a human. Residual current devices for hazardous areas are designed to trip a breaker when it detects an out of balance of current on phase and neutral or a current flow through earth of more than 30mA.

On my current MR2 conversion, I'm grounding the middle of the 640V pack to the chassis to give +320V, 0V and -320V for a few other reasons. I'm designing a current sensing circuit to trip all contactors on more than 30mA flowing through the single chassis ground point.

Sam.


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## vwdevotee (Mar 8, 2008)

It sounds like grounding the battery pack would work, you would just need a safety system in place. Something like a 30mA fast blow fuse in the ground line, and then a safety circuit on the contactors. I'm sure I'll get an earful for say that. Can I ask what your other reasons for your battery arrangement are?


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## samborambo (Aug 27, 2008)

vwdevotee said:


> It sounds like grounding the battery pack would work, you would just need a safety system in place. Something like a 30mA fast blow fuse in the ground line, and then a safety circuit on the contactors. I'm sure I'll get an earful for say that. Can I ask what your other reasons for your battery arrangement are?


The 30mA fuse idea is good as long as you also have some way of detecting the fuse blowing and tripping all contactors.

I want to build a grunty general purpose inverter for the car. I can build the inverter as a non-isolated half bridge (cheap) if I ground the middle of the pack.

I also believe it makes the cars safer, being able to detect wiring faults as they happen. Many EVs have their battery packs spread the length of the car - mine is no exception. If you can detect an exposed, frayed energy cable or a stray BMS wire and shut down the system, all the better. This is even more important in a high voltage design like mine.

NZ electrical standards require that any appliance with exposed conductive metal must have that metal earthed. Cars have a lot of exposed metal. Electric cars are considered an appliance if plugged in. I'm building a non-isolated fast charger for my pack to charge from a 3 phase 415V (p-p) 50A supply. The neutral will be connected to the mid point of the pack. I can't have the pack mid point directly connected to earth while the car is plugged in as current may flow through the earth conductor of the supply (not allowed for normal operation) so a contactor will be needed at the 0VDC earth point to isolate it while the car is plugged in.


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## samborambo (Aug 27, 2008)

colinmcc said:


> I'll just repeat what the others have said... NO, NOT EVER!


Never say never


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## Amberwolf (May 29, 2009)

One thought you might ponder is that if you have the chassis at pack negative, then anything you might accidentally drop across *any* part of the pack that also happens to touch the chassis could destroy the part of the pack electrically between the contact point and pack negative. Not to mention whatever you dropped on it, plus possibly parts of you (depending on what happens to the pack/etc because of it), and the vehicle. 

This can also happen if a cable rubs on the chassis for any reason, and wears thru the insulation. Or if a crash occurs that somehow breaks something loose enough to have part of the pack's contacts/etc touch the chassis. 

If the pack is isolated from chassis, then something can only happen if you contact across any two points in the pack, which you can make a lot harder to happen much easier than if it is not isolated. 

If the pack is isolated, it takes TWO failures of your insulation and protection to cause a short across any part of the pack. And those failures probably would have to be fairly close together physically, which generally means (depending on how your batteries are arranged) the voltage between them will be lower.

If the pack is NOT isolated, it only takes ONE failure of your insulation and protection to cause a short across the pack, and it could happen anywhere the cabling or battery terminals come near enough the chassis to make contact via whatever method. 

Food for thought. 
________
Jerk2MyTits


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## ClintK (Apr 27, 2008)

colinmcc said:


> So it is important if you are running a DC to DC converter to get 12v for the car's electrics from your main battery pack that the converter either isolates it's input from output, or is disconnected from the battery pack while charging.


Which wires of the DC-DC need to be disconnected during charging?

I have my DC-DC's input negative wire connected after a contactor triggered by the key switch. The input positive wire is connected after a circuit breaker, but I was planning on leaving that closed while charging at it is an emergency only safety. Basically the DC-DC only receives pack voltage when the key it turned on because of the negative break.


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## colinmcc (Oct 19, 2008)

ClintK said:


> Which wires of the DC-DC need to be disconnected during charging?
> 
> I have my DC-DC's input negative wire connected after a contactor triggered by the key switch. The input positive wire is connected after a circuit breaker, but I was planning on leaving that closed while charging at it is an emergency only safety. Basically the DC-DC only receives pack voltage when the key it turned on because of the negative break.


I have a DC-DC converter that has isolated input to output, so I didn't have to bother, but it would depend on which side of your charger has it's output connected to the input bridge rectifier. I'd say if your DC-DC is not isolating input from output, the put a 2 pole relay in the input cable and disconnect both from the battery pack while charging to be safe.

My Manzanita micro charger can take 110 or 220v in so as far as I can see I get one side of the battery pack directly connected to neutral if plugged into 110V, and to 110V if charging off 220. That of course presumes that the house outlet I plug into for 110V has it's pins the right way round, else I could get 110V on my pack if the outlet wires are back to front.

When plugged in at home I'd be ok, but other folk's houses, municipal parking spots etc, I'd be worried!


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## colinmcc (Oct 19, 2008)

samborambo said:


> Never say never




I'd thought of earthing my pack mid point, to halve the potential to ground.

But when I found out my charger's design connected it's output straight to one side of the Utility co's supply, I realized that was not a good idea!

It seems that most high power chargers have the same foible, hence *never* seems an appropriate piece of advice, especially since the questioner calls himself a newbie.


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## samborambo (Aug 27, 2008)

Amberwolf said:


> One thought you might ponder is that if you have the chassis at pack negative, then anything you might accidentally drop across *any* part of the pack that also happens to touch the chassis could destroy the part of the pack electrically between the contact point and pack negative. Not to mention whatever you dropped on it, plus possibly parts of you (depending on what happens to the pack/etc because of it), and the vehicle.
> 
> This can also happen if a cable rubs on the chassis for any reason, and wears thru the insulation. Or if a crash occurs that somehow breaks something loose enough to have part of the pack's contacts/etc touch the chassis.
> 
> ...


Food for thought: In a fault, are you trying to save equipment or human life?


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## samborambo (Aug 27, 2008)

colinmcc said:


> I'd thought of earthing my pack mid point, to halve the potential to ground.
> 
> But when I found out my charger's design connected it's output straight to one side of the Utility co's supply, I realized that was not a good idea!
> 
> It seems that most high power chargers have the same foible, hence *never* seems an appropriate piece of advice, especially since the questioner calls himself a newbie.


<rant>

Fair point. I'd go further to say that if one doesn't understand voltage potentials, isolation devices, earthing systems, etc. then they should get an auto electrician or an electrical engineer to design their system. Its not enough to see other people using a particular charger, battery system, controller and thinking that everything is interchangeable. Unless you're exactly copying a working and proven safe design, get it checked out by someone qualified. Also, don't rely on opinions or information you get from forums such as this. You have no idea whether the person giving you this information is qualified or experienced to do so.

There are a LOT of members posting misinformation sounding authoritive. I think we should take a more serious response to those that do so, especially for information regarding safety.

</rant>


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## vwdevotee (Mar 8, 2008)

That's sort of what I was thinking. If there is a potential between even the low side of the pack one could get a nasty zap, or if there is any fault, even on the low side, there could still be a bad zap.

Fortunately, I'm planning a service plug on my pack so that I can completely disconnect it before there are any cables to cause problems. I'm also planning a battery box lid switch to open all contactors if I even open the lid.


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## Woodsmith (Jun 5, 2008)

vwdevotee said:


> That's sort of what I was thinking. If there is a potential between even the low side of the pack one could get a nasty zap, or if there is any fault, even on the low side, there could still be a bad zap.


I think that this is where a lay persons [lack of] knowledge and understanding of electricity can land them in trouble. I often have students at college who don't understand why birds don't get zapped when they land on 400kv overhead power lines because of this lack of knowledge and understanding.

If the pack is floating, ie. no part of it is grounded to the vehicle structure then there is no 'low side' to the pack. The voltage is fully floating and the 144v, 156v, or whatever only has any meaning in reference to itself. If a person was to touch the body of the vehicle and one of the pack leads then that person will bring that part of the pack down to the same voltage as the vehicle, ie. zero volts. The other end of the pack then becomes dangerous to touch.

If the person was to let go of that first lead the pack will be floating again. The person can then touch the other pack lead and bring that end to zero volts.

It means that one contact is safe and two contacts is not.

However, that in itself causes problems.

With a floating pack any one of the leads on the pack, or the brush tails in the motor, can be damaged and make contact with the vehicle. Nothing will happen and the driver will not know anything was amiss until any other point of the pack is touched and a shock received.
Grounding one end of the pack would show any faults by tripping the breakers if fitted or possibly starting a fire if not!

It would then be prudent, when examining the vehicle, to disconnect the pack completely. If the examination requires the pack to remain connected then a quick voltage test of the pack to the vehicle will show if it has been grounded, intended or not, at any point.

Ultimately, if you need to ask whether to ground or not then it is a good indication that you need an expert to examine the way the vehicle has been wired and what sort of circuit protection is in use or intended. Without this information, the 'to ground or not to ground' answer will be meaningless to your specific case.

However, if this is in anyway confusing to you then best leave it to those who know what they are doing to wire your vehicle or provide you with specific instructions to suit your case.


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## vwdevotee (Mar 8, 2008)

I think I get it now. _ I was thinking in terms of potentials only and not the charge involved in bringing one side of the pack down to zero. But it sounds like the current needed to bring part of an isolated battery pack to zero is infintesimally small so it's a non-issue. Is that getting any closer to the right thinking?_


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## ClintK (Apr 27, 2008)

colinmcc said:


> I have a DC-DC converter that has isolated input to output, so I didn't have to bother, but it would depend on which side of your charger has it's output connected to the input bridge rectifier. I'd say if your DC-DC is not isolating input from output, the put a 2 pole relay in the input cable and disconnect both from the battery pack while charging to be safe.


The specs for my converter include: Isolate between input and output 

So it sounds like I'm good!


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## Woodsmith (Jun 5, 2008)

vwdevotee said:


> I think I get it now. _ I was thinking in terms of potentials only and not the charge involved in bringing one side of the pack down to zero. But it sounds like the current needed to bring part of an isolated battery pack to zero is infintesimally small so it's a non-issue. Is that getting any closer to the right thinking?_


Yep.
With a floating pack so long as you are only bringing one side of the pack to zero, whether through your body or a dropped spanner, there won't be any significant current flow as the other end of your pack does not (or should not) be completing a circuit.

It does still mean that you need to decide if you are going to ground or not and then design your circuit protection, for both life and equipment, accordingly.


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## samborambo (Aug 27, 2008)

vwdevotee said:


> I think I get it now. _I was thinking in terms of potentials only and not the charge involved in bringing one side of the pack down to zero. But it sounds like the current needed to bring part of an isolated battery pack to zero is infintesimally small so it's a non-issue. Is that getting any closer to the right thinking?_


There's a parasitic capacitance between any isolated conductors. You're on the right track. The capacitance between the body of the car and an isolated battery pack might be measured in femtofarads. Ridiculously small. So the current required to discharge that capacitance (through your body maybe) is also very very small.

Woodsmith, good explanation. That was what I was trying to get at.

Sam.


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## vwdevotee (Mar 8, 2008)

Ok. That makes sense then. I just had a thought though. I know when I work on my ICE car that the first step is almost always to disconnect the battery (unless for whatever reason it's needed in the repair). Is that not the case with electric cars?


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## Woodsmith (Jun 5, 2008)

vwdevotee said:


> Ok. That makes sense then. I just had a thought though. I know when I work on my ICE car that the first step is almost always to disconnect the battery (unless for whatever reason it's needed in the repair). Is that not the case with electric cars?


I would say so.
Just on a matter of safety I would say disconnect the pack and any service batteries unless they are part of the vehicle being tested or checked as part of the repair.

Even if you were just changing the brake pads on the car it would make sense to make sure that some 'helpful' friend doesn't stomp on a 'live' gas pedal when you want the brake pumping a bit.

In my electrician days I learnt to personally check and disconnect every circuit I was going to work on by removing the fuse and taking it with me or padlocking the circuit breaker switch just to make sure it was safe to work on.
The one time I didn't do that was when I trusted my Dad to walk to one end of the factory to disconnect the supply and then to the other end to disconnect the machine. He said it was safe as he had seen both ends of the circuit so I started pulling the cables out of the PVC conduit. When I found the end of the cables the 240v blew me off the ladder! I had one hand holding onto the steel beam the conduit was fixed to and the other hand found the bare cable ends.
Dad pulled the wrong fuse and was so used to working on live cables that he didn't notice he was disconnecting live connections because _he_ was isolated from ground!
I didn't take it personally but did start double checking after him until he retired.

Another time I took the fuse with me during a house repair, the site builder put a spare back in because 'the power was off'! He had to buy me a new set of cable cutters and supply his own generator set after that.

There are too many 'helpful' people around to make it risky if you don't disconnect.


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## vwdevotee (Mar 8, 2008)

Wow, those are scary stories. You're a much bigger man than me though, because father or not and honest mistake or not I would have been PISSED.


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## jaspersk (Jun 26, 2008)

samborambo said:


> There's a parasitic capacitance between any isolated conductors. You're on the right track. The capacitance between the body of the car and an isolated battery pack might be measured in femtofarads. Ridiculously small. So the current required to discharge that capacitance (through your body maybe) is also very very small.
> 
> Woodsmith, good explanation. That was what I was trying to get at.
> 
> Sam.


I have a bigger capacitance than that on my system. I get a fairly unpleasant shock when I touch my ungrounded pack. It only lasts a fraction of a second though so it is not a direct fault. I ran my conductors in separate conduits so that could be adding to the capacitance. Doesn't sound like that is typical for others though.

Sam, You clearly know what you are talking about regarding grounding and I agree with everything you have said. You need to know when you have a ground fault or your second fault is unprotected. I have a PLC installed in my car so my plan is to occasionally (when the car is started) check for a ground by looking at how much current flows through a resistor between chassis and the pack. It is not perfect but I think it is a good compromise between a floating and grounded pack. I minimize the risk of shock but still get an alarm when there is a continuous fault. I won't see an intermittent fault but that still seems like a good compromise which maximizes personal safety IMHO.


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## samborambo (Aug 27, 2008)

jaspersk said:


> I have a bigger capacitance than that on my system. I get a fairly unpleasant shock when I touch my ungrounded pack. It only lasts a fraction of a second though so it is not a direct fault. I ran my conductors in separate conduits so that could be adding to the capacitance. Doesn't sound like that is typical for others though.
> 
> Sam, You clearly know what you are talking about regarding grounding and I agree with everything you have said. You need to know when you have a ground fault or your second fault is unprotected. I have a PLC installed in my car so my plan is to occasionally (when the car is started) check for a ground by looking at how much current flows through a resistor between chassis and the pack. It is not perfect but I think it is a good compromise between a floating and grounded pack. I minimize the risk of shock but still get an alarm when there is a continuous fault. I won't see an intermittent fault but that still seems like a good compromise which maximizes personal safety IMHO.


Yup, a monitored earthing resistor is a good idea. Choose a resistor that will still limit lethal current from the maximum pack voltage (if earthing at one end of the pack). Note that it won't get rid of a high capacitance problem necessarily. What's needed is to monitor any voltage developed across said resistor. I suggest feeding that voltage into an active (opamp based) low pass filter to filter out any transient induced voltages, then schmitt trigger at an appropriate level, then into the "set" pin of an RS flip-flop and have a reset button on the "reset" pin. The output of the flip-flop will be your earth fault indication and contactor trip input to your PLC.

Sam.


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## Tesseract (Sep 27, 2008)

jaspersk said:


> I have a bigger capacitance than that on my system. ...


Most mains-powered devices, especially if they have or are a switching power supply, require noise-suppression capacitors from either line terminal to earth/ground (called "Y capacitors"). These are frequently in the 1500pF to 0.015uF range (1000pF to 0.1uF is allowable) and even 1500pF is enough to give you a brief surprise if charged up to 150V or so.

Anyway, this is relevant because a lot of people have found that a commercial 12V/13.8V output switching power supply makes a halfway decent "DC-DC converter" for keeping the accessory battery charged, especially if the output (as most are) is adjustable over a limited range. Hence, you now have capacitance from both sides of the battery pack to the vehicle chassis.




jaspersk said:


> You need to know when you have a ground fault or your second fault is unprotected. I have a PLC installed in my car so my plan is to occasionally (when the car is started) check for a ground by looking at how much current flows through a resistor between chassis and the pack....


That is about the only way to do it - but keep in mind that if your ground fault happens to occur at the same terminal as the one you are monitoring at startup then you won't see it, and that charging/discharging the aforementioned Y-caps, if present anywhere in the system, needs to be accounted for by the ground fault detector. (EDIT: do not leave the resistor from pack to ground in permanently; use a relay to physically disconnect the monitoring resistance and only test briefly and occasionally for ground faults otherwise the resistor becomes the cause of a ground fault, not just the means of detecting it!)

Unless you are using flooded-cell batteries (Ni-Cd or Pb), I wouldn't bother making something like this. If you are really paranoid about the pack grounding out then occasionally connect a 1M resistor from a couple points in the pack to the vehicle chassis and measure the voltage across it with a DMM - if it's close to zero then no worries.


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## major (Apr 4, 2008)

samborambo said:


> You have no idea whether the person giving you this information is qualified or experienced to do so.


Hey all,

This is good advice. So why not refer to some accepted standards, like: 

*UL 583 UL Standard for Safety Electric-Battery-Powered Industrial Trucks *

You'd want your electric car as safe as a forklift, wouldn't you?

Keep your high voltage isolated 

major

Edit: There is also:

*SAE J1673* *Revision / Edition: 96 Chg: Date: 07/00/96 * HIGH VOLTAGE AUTOMOTIVE WIRING ASSEMBLY DESIGN </SPAN> 

*Document Abstract*

1. Scope--This SAE Recommended Practice covers the design and application of primary on-board wiring distribution system harnesses to road vehicles. This document applies to any wiring system which contains one or more circuits operating between 50 V DC or AC RMS and 600 V DC or AC RMS excluding automotive ignition cable.


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