# Revision to "Poltergeist - Voltage Drop"



## gregdugas (Nov 14, 2012)

I recently replaced the OEM GEL's on my 2005 GEM with Lifepo4 cells and a mini-BMS system - right before leaving out-of-town for 3 months. On my return, I discovered the cells had discharged to such a low level as to render them unsalvageable.

In proceeding forward to troubleshoot the problem, I energized the GEM's electrical system with only a single cell having 3.333V charge. In noting that a voltage reading while one meter lead was connected to the + terminal of the cell and the other one to the GEM chassis resulted in a voltage drop to .807V I assumed that either a short drained the cells or a bad connection. 

A reading taken at the Power Signal Distribution Module (PSDM) where the cell terminal wires directly connect was 3.333V indicating no problem with the cell wires. The same voltage reading resulted from measurements taken at the main contactor, main contactor coil, and the motor controller indicating no shorts in any of the drive components. 

I then individually isolated each electrical connection to every power device by disconnecting it from the system. I accomplished these by removing a fuse and noting if the voltage measurement at the chassis increased to 3.333V. If nothing changed I then proceeded to the next one until all fuses were removed. There were no resulting changes in the voltage drop. 

I then repeated the same procedure and disconnected each one of the 6 wiring harnesses connected to the PSDM with no change in the voltage drop [from 3.333V to .807V.]

My next step using the same process was to individually disconnect each remaining wire of the electrical system that was connected to the DeltaQ charger, main contactor, and motor controller system. Again no change in the voltage drop.

_* Any suggestions on how to proceed with further troubleshooting or a new approach would be GREATLY appreciated!! *_


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## major (Apr 4, 2008)

gregdugas said:


> I recently replaced the OEM GEL's on my 2005 GEM with Lifepo4 cells and a mini-BMS system - right before leaving out-of-town for 3 months. On my return, I discovered the cells had discharged to such a low level as to render them unsalvageable.
> 
> In proceeding forward to troubleshoot the problem, I energized the GEM's electrical system with only a single cell having 3.333V charge. In noting that a voltage reading while one meter lead was connected to the + terminal of the cell and the other one to the GEM chassis resulted in a voltage drop to .807V I assumed that either a short drained the cells or a bad connection.
> 
> ...


That is a big improvement over the previous post. But it is unclear what is connected to the cell. And what does the cell voltage read from cell positive to cell negative when you read .807V from cell positive to chassis? What does it read from cell negative to chassis?

How long did you have the vehicle before you installed the LiFePO4? Did it function normally? Did you ever have a battery drain problem with the prior set of batteries? Did you test the vehicle after you installed the LiFePO4? 

I am not familiar with the mini BMS, but I do know of several other BMS systems which have been responsible for complete battery drain when left connected and unused for months at a time. It might be the case here.

Reading a potential from battery (positive or negative) to chassis is not unusual. In fact, you will always read some voltage. The real deal is to determine the leakage from the battery circuit to chassis. This is done by measuring the current to chassis. So put a 2000 Ohm resistor* from battery positive to chassis and read the voltage across the resistor. Use Ohm's Law to figure the leakage current. Do the same for battery negative to chassis. Report back.

BTW, this is called a ground, not a short.

Regards,

major

*It can be any resistor you have on hand: 100 to 10,000 Ohms.


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## PStechPaul (May 1, 2012)

Almost any further troubleshooting will require some potentially dangerous testing of the high voltage power section. So if you are at all unsure about what you are doing, or have any questions about why the voltmeter might give readings as it does, I would recommend and urge you to have the system checked out by a qualified EV expert. 

But if you "sign the liability waiver", then here are some ideas. You can connect each of the BMS monitor leads in turn to the single cell, and use a milliammeter in series or a resistor and a voltmeter as suggested. Assuming the cells are 100 Ah and they discharged to 100% DOD, in 3 months (2160 hrs), you will be looking for a current in the order of 100/2160 = 46 mA, or 46V/1000ohm. This is highly unlikely, and if the entire pack was drained, then it cannot be one faulty BMS module. But this will at least give you some useful information.

It seems more likely that a parasitic (or unintentional) load existed across the battery pack. Some systems have a DC-DC converter that keeps the accessory battery charged from the traction pack, and if this were left on during your absence, it would totally explain it. Assuming nothing has changed over that three month period, you should be able to connect a power source in place of the battery pack and measure the current draw. 

If you have a 200V pack, you will need a similar voltage, but it does not need to be capable of any more than 100 mA, and actually could be much less. A simple method would be to charge up a large capacitor, something like 1000 uF 400 VDC rated, to the pack voltage, and just a bridge rectifier across the AC line would give you about 170V, which might be close enough to your pack voltage. Or you might be able to use your charger to charge the capacitor to the exact voltage needed. In this case, when you connect it from your pack (+) to pack (-), observing safety precautions and polarity, you can measure the total pack voltage and see if it slowly drops. If there is a 46 mA load, it would be equivalent to a resistor of 170/46 or about 3.7 kOhms. This would have a time constant of 100 * 3.7 or 370 mSec, so within a couple of seconds the voltage would drop to just a few volts. 

It seems that it would be advisable to disconnect the battery pack if the vehicle is to be left for more than a few days without connection to a charger. A good design would check the pack voltage and disconnect it from the DC-DC and any other load if it dropped below a safe level. If you have a schematic of the system it would help tremendously. 

And you might also consider the possibility that someone may have used the vehicle and depleted the pack while you were gone.


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## major (Apr 4, 2008)

PStechPaul said:


> Almost any further troubleshooting will require some potentially dangerous testing of the high voltage power section.


 He's testing with 3.3 Volts.



PStechPaul said:


> So if you are at all unsure about what you are doing, or have any questions about why the voltmeter might give readings as it does, I would recommend and urge you to have the system checked out by a qualified EV expert.


He apparently installed the set LiFePO4 in the car and this is DIY.



PStechPaul said:


> But if you "sign the liability waiver", then here are some ideas. You can connect each of the BMS monitor leads in turn to the single cell, and use a milliammeter in series or a resistor and a voltmeter as suggested. Assuming the cells are 100 Ah and they discharged to 100% DOD, in 3 months (2160 hrs), you will be looking for a current in the order of 100/2160 = 46 mA, or 46V/1000ohm. This is highly unlikely, and if the entire pack was drained, then it cannot be one faulty BMS module. But this will at least give you some useful information.
> 
> It seems more likely that a parasitic (or unintentional) load existed across the battery pack. Some systems have a DC-DC converter that keeps the accessory battery charged from the traction pack, and if this were left on during your absence, it would totally explain it. Assuming nothing has changed over that three month period, you should be able to connect a power source in place of the battery pack and measure the current draw.
> 
> If you have a 200V pack, you will need a similar voltage, but it does not need to be capable of any more than 100 mA, and actually could be much less.


 The Gem uses a 72 Volt system. You can click on the poster's name and choose previous posts from the pull-down menu to get some history, like this:
http://www.diyelectriccar.com/forums/showthread.php?t=89243


PStechPaul said:


> A simple method would be to charge up a large capacitor, something like 1000 uF 400 VDC rated, to the pack voltage, and just a bridge rectifier across the AC line would give you about 170V, which might be close enough to your pack voltage.


You start off having him sign a waiver and seek an expert to use 3 Volts DC and now you tell him to play with 170V off the AC line  That's nuts, IMO.



PStechPaul said:


> Or you might be able to use your charger to charge the capacitor to the exact voltage needed. In this case, when you connect it from your pack (+) to pack (-), observing safety precautions and polarity, you can measure the total pack voltage and see if it slowly drops. If there is a 46 mA load, it would be equivalent to a resistor of 170/46 or about 3.7 kOhms. This would have a time constant of 100 * 3.7 or 370 mSec, so within a couple of seconds the voltage would drop to just a few volts.
> 
> It seems that it would be advisable to disconnect the battery pack if the vehicle is to be left for more than a few days without connection to a charger. A good design would check the pack voltage and disconnect it from the DC-DC and any other load if it dropped below a safe level. If you have a schematic of the system it would help tremendously.
> 
> And you might also consider the possibility that someone may have used the vehicle and depleted the pack while you were gone.


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## PStechPaul (May 1, 2012)

The reason I recommended using a higher voltage, _for further troubleshooting,_ similar to the battery pack is that there could be a non-linear load or insulation breakdown that would not be detected with 3.3V. The DC-DC converter is a likely suspect, and it would stop drawing current at some point well above the 3.3V level. It is also possible that it could draw increasing current as the voltage of the pack gets lower, which is typical of regulated switching supplies with a constant load. And another possibility would be a load on the 12V auxiliary system, or a defective battery.


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## gregdugas (Nov 14, 2012)

I think I may have located the Poltergeist??

The GEM's DeltaQ charger has a sensor which upon reading a battery temperature exceeding 120F shuts itself off. The sensor consists of a ring connector and two leads. The ring connector is attached to the battery's negative terminal. One lead is connected to the charger and the other to ground.

In my ignorance, I connected the 2nd lead instead to the -72V terminal of the Power Signal Distribution Module thinking it was equivalent to ground. I've now come to understand the difference between 0V and -72V. 

Once I connected the 2nd lead to the chassis, the voltage measurement between the POS cell terminal and the chassis changed from .807V to 3.318. This compared to 3.326 measuring between the POS and NEG cell terminals. 

Am I safe in assuming the differential of 8 mV will not be the source of a parasitic drain?

Am I also correct in assuming that the initial connection to -72V rather then ground was the cause of my $3K lost in unsalvageable LifePo4 batteries??? If so - and for any of you with patience and a penchant for instruction, I'd love to understand why. If not, I'm back to the drawing board!

Thanks for everyone's input. I did learn something - albeit at an high rate of tuition.


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## major (Apr 4, 2008)

gregdugas said:


> I think I may have located the Poltergeist??
> 
> The GEM's DeltaQ charger has a sensor which upon reading a battery temperature exceeding 120F shuts itself off. The sensor consists of a ring connector and two leads. The ring connector is attached to the battery's negative terminal. One lead is connected to the charger and the other to ground.
> 
> ...


The battery, charger, thermal sensor and all 72V circuitry needs to be isolated. In other words, NOT connected to ground. Ground is the chassis. You explained the proper connection for the sensor here: 


gregdugas said:


> Travis:
> The main wire coming from the Delta Q consists of 4 leads: White, Black, Green & Red.
> The White wire is connected to 1 of 2 wires forming a part of the temp sensor. The temp sensor on the GEM is a ring terminal located on the negative terminal of Batt 1. Its 2nd wire is connected to the "shunt resistor" lug of the GEM PSDM.
> The Red wire connects to the 72V lug on the rear of the PSDM;
> ...


Furthermore, with isolated systems, a unintentional single point ground can be tolerated. In other words, I doubt that mistakenly connecting a thermal sensor lead to the chassis would have been responsible for the drain of the batteries.


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## gregdugas (Nov 14, 2012)

Once the 2nd wire from the battery thermal sensor is connected to the -72V shunt resistor of the PDSM - which is the correct connection, what is the conclusion when in doing so the voltage reading between the POS terminal and the chassis drops from 3.318 to .609?

Where else should I be looking for a bad connection?


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## major (Apr 4, 2008)

gregdugas said:


> Once the 2nd wire from the battery thermal sensor is connected to the -72V shunt resistor of the PDSM - which is the correct connection, what is the conclusion when in doing so the voltage reading between the POS terminal and the chassis drops from 3.318 to .609?
> 
> Where else should I be looking for a bad connection?


I am not convinced you have a "bad" connection. You will almost always read a phantom voltage from battery to ground (chassis). What is the leakage? 



major said:


> Reading a potential from battery (positive or negative) to chassis is not unusual. In fact, you will always read some voltage. The real deal is to determine the leakage from the battery circuit to chassis. This is done by measuring the current to chassis. So put a 2000 Ohm resistor* from battery positive to chassis and read the voltage across the resistor. Use Ohm's Law to figure the leakage current. Do the same for battery negative to chassis. Report back.
> 
> BTW, this is called a ground, not a short.
> 
> ...


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## gregdugas (Nov 14, 2012)

1. DMM Connection: Red lead to POS terminal of cell & Blk lead to NEG terminal= *voltage reading of 3.326*.

2. After changing Blk lead connection to GEM chassis:
a) Temp Sensor Connection w/ 2nd lead connected to PSDM (-72V) lug = *voltage reading of .609V*
b) Temp Sensor Connection w/ 2nd lead connected to chassis= *voltage reading of 3.318V*

3. Why such a significant amount of change in voltage from .609 to 3.318 by changing connection point of temp sensor from (-72V) lug to chassis?

[NOTE: Prior to changeover to LifePo4, voltage remained the same whether measured from POS/NEG terminals of full battery pack of ~72V OR measured after changing DMM lead from NEG terminal to GEM chassis.]


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## major (Apr 4, 2008)

gregdugas said:


> 1. DMM Connection: Red lead to POS terminal of cell & Blk lead to NEG terminal= *voltage reading of 3.326*.
> 
> 2. After changing Blk lead connection to GEM chassis:
> a) Temp Sensor Connection w/ 2nd lead connected to PSDM (-72V) lug = *voltage reading of .609V*
> ...


Where are the voltmeter leads connected for the measurements in 2a and 2b?


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## gregdugas (Nov 14, 2012)

One DMM lead is connected to the POS cell terminals and the other to the chassis and stay that way for both readings. The only connection that changes is the 2nd wire of the sensor from the -72v lug to grounding on the chassis.


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## major (Apr 4, 2008)

gregdugas said:


> One DMM lead is connected to the POS cell terminals and the other to the chassis and stay that way for both readings. The only connection that changes is the 2nd wire of the sensor from the -72v lug to grounding on the chassis.


Thank you.

In 2a you are reading a phantom voltage from battery positive to chassis.

In 2b you have grounded the battery negative to the chassis thru the thermal sensor and are now reading battery voltage from battery positive to chassis. (minus .008V drop somewhere in the chassis which is insignificant)

You never want any connection to the chassis from the 72V system. Or from the thermal sensor.


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## kennybobby (Aug 10, 2012)

Still no measurement of the leakage current...? That would be the proof in the pudding.


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## PStechPaul (May 1, 2012)

A more definitive test would be at full pack voltage. I think you could replace the pack with a large capacitor (1000 uF or so) and use the charger to get the normal voltage. There should be very little current once the capacitor is charged to the maximum, so if it continues to draw current it indicates a load. And if the charger is removed or shuts off, the capacitor should remain charged for quite a while (several minutes). Is a schematic of the GEM available?

I found an owner's manual but no schematic:
http://cdn.polarisindustries.com/GEM/MY2014/catalogs/2012-Owners-Manual.pdf

Here is one for a 48V version that might be a good starting point:










And here is another one:

http://www.manualguide1000.com/view-gem-car-battery-wiring-diagrams


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## PStechPaul (May 1, 2012)

Here are some more detailed schematics of the GEM:










Note that there is a fuse mid-pack as well as a master switch which must be turned off whenever the vehicle is stored without the charger connected and operating.










Note that the DC-DC converter is connected and operational unless the master switch or main fuse are disconnected. This keeps the 12V accessory battery and various control circuits operating even when the key switch is turned off.










Note that the main contactor is operated from the keyswitch, but it only disconnects the motor controller.

Here are the schematics for the motor controller, although it is not likely to be the problem, unless the main contactor is welded on.


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