# Isolating the battery pack, yes or no?



## Huub3 (Aug 16, 2009)

CORRECTIVE REMARK:

It turns out I had a wrong recollection of the NCOP regulations. The correct text from the jan 1st 2011 version (v2.0) is:

_2.9 Hazardous Voltage Isolation
Any HAZV traction battery system must be isolated from the chassis of the vehicle, and also
from any auxiliary ELV components and wiring. Isolation must be designed such that there is a
leakage current of less than 20 mA between any part of the HAZV system and either the chassis
or ELV components in the vehicle, measured when the vehicle is at rest.
This requirement means that both the HAZV battery pack positive, and the HAZV battery pack
negative, are to be floating relative to the chassis during normal operation, and both are to be
treated as HAZV components.
A ground fault detection circuit or device may be used to identify that either the battery pack
positive or battery pack negative have come into contact with the chassis or ELV part of the
vehicle, and flag this as a fault to the driver or service technician.
_
Link to this document (with lot's of other interesting stuff in it) is:
http://www.infrastructure.gov.au/ro...COP14_Guidelines_Electric_Drive_01Jan2011.pdf

This is a very clear message, which together with the remarks from respected DIY members in the thread below, mean I want to make it very explicit that NOT ISOLATING THE BATTERY PACK IS A WRONG IDEA!

END OF CORRECTION 

All,

from a discussion in the "siemens motor advice" thread a topic popped up on isolation of the battery pack.

Some background:
I am planning a 600 V pack (using industrial ACIM and industrial (Hitachi) VFD). 600 VDC is comparable to 400 VAC three phase, and thus can be connected to the DC bus of the VFD without the controller noticing anything strange.

However, almost all useful appliances (DC-DC, heaters, etc.) are available for much lower voltages, most of the time max 300 V, same goes for most readily available chargers.

So a solution would be to tap the middle of the pack, and make it a split 2 x 300 V pack for all appliances apart from the VFD. Certainly contactors are needed to make the transition between charging and driving safely, and some other measures, but that is feasible to my opinion.

With this comes a question on whether this tap is usefully connected to vehicle "ground"/chassis.

Please be aware that this is not an AC/600V only question, also for DC, or for lower voltages, one of the terminals, or the pack middle, might be connected to vehicle chassis.

!! THE FOLLOWING IS NOT CORRECT, SEE REMARK ABOVE !!
[_It seems there are several opinions about this, and solutions, and official regulations are also not consistent as far as I can see_.]

What is the opinion on this in the forum, what experiences do we have with keeping the pack isolated, vs. grounding one side of it.

I am very much interested in learning real experience and data backed solutions, not so much opinions without solid backing, as I feel this issue might be very much safety related, and I would hate knowing that through wrong advice we had harmed one of our fellow-EV-ers.

Best regards, and looking forward to an interesting discussion,


Huub


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## Ziggythewiz (May 16, 2010)

I've never heard anyone recommend grounding the HV pack...I know some have done it but the majority of us would agree that it's a bad idea.

There are many conditions that can cause you to lose isolation, such as non-isolated gauges or chargers, worn cable insulation, or accidental connections. Isolating the HV pack from the chassis requires two such faults to kill yourself or the car, but intentionally using the chassis as a HV ground means you're one slip from making/being an arc welder.


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## PTCruisin (Nov 19, 2009)

All production EVs have an isolated traction pack. There is a very good reason for that. All DIY EVs shoul have an isolated traction pack, and an isolated DC-to-DC converter.


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## frodus (Apr 12, 2008)

Where have you ever heard to hook your Traction pack negative to chassis ground?

For all of the industrial and vehicle power sources I've seen, the general practice has been to isolate your HV pack from anything connected to the chassis (including other DC-DC converters, chargers, accessories). 

What do you have that needs a chassis ground? It's likely that the 12V system uses the chassis as ground, but that is the ONLY thing that should be "grounded" to the chassis. The 12V system should share no common ground with the Traction pack. If you use an Aux battery, you're fine, but if you use a DC-DC, make sure it's isolated.

Also, *splitting the pack is a bad idea*, as you'll have an unbalanced top pack and bottom pack because they discharge at different rates due to differing load. DO NOT connect unbalanced packs together to charge without some sort of regulation of current. The current will surge from the higher SOC pack into the lower SOC pack if you just connect with a contactor..... and it will likely weld the contacts together.


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## Tesseract (Sep 27, 2008)

Huub3 said:


> It seems there are several opinions about this, and solutions, and official regulations are also not consistent as far as I can see.


I don't recall anyone saying it's okay to ground the traction battery at any point in the string. Official regulations - where they exist - generally require a relatively high minimum resistance between the traction battery and vehicle frame (the ECE R-100 spec is 100 ohms per volt).

To sum it up in one sentence: try to keep the parts of the EV at traction battery potential as isolated as possible from the vehicle frame.


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## Huub3 (Aug 16, 2009)

All,

clear message. Will look into the regulations part, as I might even have misread that piece, and will correct my initial message if this is the case. Want to be crystal clear on this.

Frodus --> thanks on the remark regarding misbalanced packs. My intention is to have the load symmetrical over both half-packs. This would mean two DC-DC, two heaters etc., and two BMS strings with each own charger per half-pack.

Regards,


Huub


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## Siwastaja (Aug 1, 2012)

Huub3 said:


> Frodus --> thanks on the remark regarding misbalanced packs. My intention is to have the load symmetrical over both half-packs. This would mean two DC-DC, two heaters etc., and two BMS strings with each own charger per half-pack.


It is so difficult to have perfectly symmetrical loads that you need a powerful balancing BMS that balances the pack every time it is charged. With any asymmetry, even a shunt top balancer BMS might have hard time, depending on the amount of imbalance and the shunting current the BMS can support. Redistributive BMS might be good in this case.

Lithium has the very good side of having little or no internal unbalancing mechanisms. Do not ruin that feature...

Note that any heater has variable resistance depending on temperature, not only the PTC type. Hence, two heaters are more like guaranteed NOT to be in balance.


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## frodus (Apr 12, 2008)

Huub3 said:


> All,
> 
> clear message. Will look into the regulations part, as I might even have misread that piece, and will correct my initial message if this is the case. Want to be crystal clear on this.
> 
> ...


 
Why not just get the right equipment for the job? Get a DC-DC that will handle your pack voltage and convert down to a useable voltage for those devices, rather than overcomplicate things. They do have higher voltage DC-DC's, you may not find them on the normal EV sites, but they're out there.

What you could do is convert down to something like 100VDC (or whatever voltage the Heaters operate) as a main BUS voltage. Run the heaters off that. Then use a secondary converter to go from 100VDC down to 12VDC for your accessories/12V system. This way, all the current into/out of the pack is the same as it is for each cell, and everything keeps balanced as much as possible.

I'd still recommend a Full BMS system for a large pack like that. For short series strings, it may be ok to go sans BMS, but the longer that string, the easier it is for a battery to become unbalanced without noticing it.


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## rwaudio (May 22, 2008)

Ignoring battery imbalances etc. With the center of your pack grounded you have to be extremely careful never to touch the car and a cell at the same time. 
(unless it's those few cells in the middle, those are safe)


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## Huub3 (Aug 16, 2009)

All,

corrected my first message to reflect the right understanding of NSOP and your well-respected remarks above.

Thanks a lot,

Huub


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## frodus (Apr 12, 2008)

Thanks for updating us and giving the regulation to reference.... things like that are very helpful to others researching the same question.


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## Sunking (Aug 10, 2009)

The reason for floating the HVDC battery is really very simple safety and operation. Contrary to popular belief grounded system are dangerous and unreliable. If referenced to the frame greatly increases your chances of being electrocuted, and unnecessary outages/failures.

This is the reason industrial applications use a Delta 3-phase power (vs 3-Phase Wye) because they cannot afford to take unnecessary outages from ground faults. 

What I am really curious about does the EV industry require a Ground Fault Detection and/or do you DIY's install GFD? If not is should be a requirement.


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## nat_ster (Oct 19, 2012)

Good information in this thread. Thanks to all who contributed.

Nat


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## dtbaker (Jan 5, 2008)

I'm curious about ground detection now.... what would be the simplest/cheapest way to detect and indicate to driver that either + or - high-voltage develops a ground to chassis (which would need to be found and fixed!)


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## Huub3 (Aug 16, 2009)

Travis,

thanks for the good discussion here. As this topic is potentially dangerous, I thought it is better to include the correction in the starting message, to be very clear on the dangers and proper advice.

BTW: now also included link to the NCOP document, which also has many other good advices in it: http://www.infrastructure.gov.au/ro...COP14_Guidelines_Electric_Drive_01Jan2011.pdf

Best regards,


Huub



frodus said:


> Thanks for updating us and giving the regulation to reference.... things like that are very helpful to others researching the same question.


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## Sunking (Aug 10, 2009)

dtbaker said:


> I'm curious about ground detection now.... what would be the simplest/cheapest way to detect and indicate to driver that either + or - high-voltage develops a ground to chassis (which would need to be found and fixed!)


Two lights connected in series from pos to neg of the battery, and the common point between the two connected to the frame. 

Normally both are dimly lit. If one polarity goes to ground, one light goes dark because it is shorted out, and the other gets 4X brighter. That is a WWII GFD circuit. In other words a real simple voltage comparator using the frame as a reference point.


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## rwaudio (May 22, 2008)

Sunking said:


> Two lights connected in series from pos to neg of the battery, and the common point between the two connected to the frame.
> 
> Normally both are dimly lit. If one polarity goes to ground, one light goes dark because it is shorted out, and the other gets 4X brighter. That is a WWII GFD circuit. In other words a real simple voltage comparator using the frame as a reference point.


Correct me if I'm wrong, but by doing that you just created a ground fault, and now if you touch the pack and frame you can get zapped??

Your voltage divider puts the frame at 1/2 pack voltage, this might not be so bad with a 120ish volt system, but a 300v pack is gonna get you.


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## Elithion (Oct 6, 2009)

Anything other than a 12 V battery should float. 

Yes, people who use a non-isolated motor driver (such as Curtis and Alltrax) are SOL, and have to ground the traction pack. But, boy, do they regret it the first time they slip with a tool and short B+ to chassis!



Huub3 said:


> So a solution would be to tap the middle of the pack,


I believe that there is only way to use a split pack with just a single BMS (with 2 chargers, one load). Other than that, if there is current in or out of the split tap, you need 2 BMS.


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## frodus (Apr 12, 2008)

Curtis 1236/1238 or Alltrax users don't have to ground the controller, you just have to keep all the controls for the controller and the pack isolated from anything else. Most people that use the Curtis 1236/1238 or Alltrax never connect the 12V auxiliary to the controller ground (B-).

It's true that you'll be running isolated (aux lighting, BMS power, etc) connections near others that may be non-isolated (throttle, KSI, etc), but they're not ever connected to eachother cirectly in any system I've seen..... or if they are, opto-isolators are used.

The new Curtis 1238?? (due out soon) has Isolated IO/Communications.


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## dtbaker (Jan 5, 2008)

frodus said:


> ... you just have to keep all the controls for the controller and the pack isolated from anything else.



yeah, yeah... I was just looking for feedback on whether it is practical to try and catch the case where HV insulation rubs thru somewhere, or a loose connection jiggles into contact w/ chassis, or a LV wire comes loose and falls across HV terminal somewhere.


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## Sunking (Aug 10, 2009)

rwaudio said:


> Correct me if I'm wrong, but by doing that you just created a ground fault, and now if you touch the pack and frame you can get zapped??


No it does not cause a ground fault, and no you will not get zapped. The system is floating and not referenced to the frame.

You can do it with 2 LED's and 2 Resistors. So for example if the pack is 200 volts you would limit the current to something about 1/4 full brightness of the LED's. Let's say 10 ma. You would use 2 50 K resistors.


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