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Isolation problem with battery pack

3458 Views 14 Replies 4 Participants Last post by  marc426
Hi Everyone,

We're having some sort of a problem with an EV we're working on.
A bit of context :
the frame of the car is of the tubular kind, not unlike this one :

The battery pack is 106S divided in two 56S parts on each side of the frame (each side of the transmission tunnel).

So far we've isolated the whole traction chain from the frame : battery pack / inverter / motor.

The thing is that : when we probe from the battery + to the frame, the multimeter shows around 300V and that decreases to almost 0V with only the voltmeter impedance.
Now when you go around and do the same thing between the battery - and the frame, same thing happens, 300V decreasing to 0 and then you can go to the other side and start again...

Basically when you discharge one side, the other side is "charged" and so on and so on...

We've looked a bit at the literature and the accepted practice would seem to have a several Mohms resistor between the battery + and the frame and the same thing between battery - and frame...
So far, we've tried to isolate everything from the frame so we're a bit reluctant to connect it now... Even though through a Mohm resistor...

Is that an accepted practice?

And in any case, do you have any idea where that "capacitive" behaviour comes from and whether it's something to worry about or not?

Thanks for your feedback!

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How is the motor isolated from the frame? also the inverter?

The inverter chassis may be used as an earth ground and there is likely some AC filtering capacitors and possibly bleed-off resistor inside the inverter.
Decreases to zero and starts at pack voltage after testing the other side? Sounds like you may measuring the stray capacitance between the battery pack and the frame with a very high impedance digital multimeter. If the voltage drops to zero the actual leakage current must be very low.

Try this; at the end of the pack you want to measure first connect a 1 Meg ohm resistor (1,000,000 ohm) between that pack terminal and the vehicle frame. Then, measure the millivolts across that resistor. The number of millivolts you measure is the number of milliamps of leakage current. By putting the resistor in different spots and measuring the millivolts you can find the area of the leak, the voltage will be lowest near the leakage point.
Hi Kenny and evfun.

The motor is isolated through a non-conductive coupling and a peek flange.
The inverter through rubber mountings.

Evfun, regarding your mohm resistor method, wouldnt it be the higher voltage where the current is the highest?
And yes I think we're measuring the stray capacitance battery / frame, I'm just wondering whether this is an acceptable "value" / behaviour or not...

Thanks for your help!

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Do you have a schematic of your electrical system? Do you have contactors on both the + and - lines from the battery to the inverter?

What kind of cells do you have and are they in a metal enclosure?

i would put a 300 kOhm resistor between the battery (-) and frame to attempt to measure any leakage current, then 1 mV = 1mA, or an even lower value, like 30k or 3k, to get a higher signal that you could even measure.
Hi Kenny,

We have two contactors but both on the + side of the battery.
The cells are 18650 and they are in a copper "enclosure"

Thanks for your help.

Please feel free to describe in detail your entire electrical system. And be very careful about touching anything until you resolve this high voltage stuff.

i assume 2 contactors on the (+) line means a parallel path where one is for a pre-charge resistor and the other carries the main power?

And so is the (-) terminal of the pack connected directly to the (-) terminal of the controller/inverter? Is the chassis of the inverter touching the frame, possibly thru mounting screws?

And what inverter and what motor is being used? we need all the details to determine is there is a sneak current path somewhere in your system or wiring, etc...

What is the cell voltage--are they all charged up to 4.1 or what value? What is the pack voltage as measured from (+) to the (-) and the cables are disconnected from anything, i.e. the open circuit voltage? What is the OCV when cables are connected into the system?

What is meant by a copper enclosure--can any exposed metal surface of a cell possibly touch the copper enclosure? Is this a commercial pack or a home-made assembly of harvested laptop cells? What is the integrity and history of the cell wrappers or heat shrink material? Do any cell wrappers directly contact the "copper" enclosure?

Is the enclosure isolated from the frame? If so how is it isolated?

Is the pack really only a serial string of 106 18650 cells, or are there a number of parallel 18650 cells to make up one Cell, and the pack is 106S Cells? i can't imagine a car running very far on 106s1p 18650 cells.
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Hi Kenny,

The contactors are actually both on the power + (two KILOVACS EV200AANA), they're //ed since we plan to flow a lot of current through them.

Regarding the batteries, they are brand new 18650 cells with perfect untouched wrapper, each (commercial) module have been tested and passed.

The thing, is that even when we disconnect the battery (+ and - floating), we have the same "stray capacitance" behaviour.

About the inverter : we use an Emsiso em300 with an induction motor.
Sadly I cannot give much more details since this is not our EV.

Both inverters and battery packs are isolated through either rubber / isolating fasteners or PMMA sheets / non conductive material.


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Do you have a BMS connected?

Ive seen the same capacitive effect when the battery to chassis isolation is much greater than the impedance of the voltmeter Im using to measure.

If you have access to two voltmeters (appropriately rated!) then measure both sides of the battery string to chassis at the same time and verify that you see a 50% split of string voltage between the two meters.
Hi ABMcomp,

There is indeed an EMUS BMS connected.
We'll try to measure with two multimeters. I guess the same model would be preferable in order to have the same impedance...

Hi ABMcomp,

There is indeed an EMUS BMS connected.
We'll try to measure with two multimeters. I guess the same model would be preferable in order to have the same impedance...

Try measuring again without the BMS connected as well. It may have filter capacitance internal that youre seeing.
Hi everyone,

The problem is that disconnecting the BMS is quite the task at the moment (battery in the car, everything wired solid etc...). Let's say it's not an option...

Indeed with two multimeters, one between + and frame and the other one between - and frame, we find exactly half the battery voltage on each instrument.

What can we conclude of this result?

Thanks for your help,

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That sounds like the cross-over jumper between the two half-packs is somehow shorted to the frame. Inspect that jumper wire for nicks or cuts in the insulation, and the terminal connections for accidental contact to frame.
Its possible that there is a loss of isolation at the crossover point to chassis. I think it is equally possible that you're seeing an effect of balance capacitance in your BMS.

If the BMS can not be disconnected from the battery, can it be disconnected from the chassis?

Do you have a megaohm or greater resistor? You can try kennybobby's recommendation earlier to bridge one end of your string to chassis and measure leakage current. Capacitance will show a change in current v time as the cap charges or discharges. A loss of isolation will show as a constant current across your resistor.
We've checked and the crossover is definitely not leaking to the frame.
We've overbuilt a lot of the stuff in order to have peace of mind so the crossover is made using HOV7 95mm² cable and both connections to the half packs are using cable glands.

We'll see about the BMS ground connection, that's definitely a good test.

The weird thing is that we've already tried the leakage current test with a 10ohm resistor but didn't see any voltage.
Then again, the 10ohms might discharge it too quickly to see anything but it's also comforting in the sense that it's capacitance and not leakage current...
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