[Azimath]

Hello,
I've got a battery pack made of 2 groups of Chevy Bolt modules (of the hopefully less incendiary US built variant). I've got them all in place in the vehicle, and while connecting them up noticed that one module seems to produce quite a bit more stray voltage than the other. My Fluke 115 promising >10M input impedance usually pulls a single module from 3V to chassis down to <1V to chassis in relatively short order. After making a few connections between the modules, I'm seeing terminal to chassis voltages around 100V on the terminal furthest from the module I'm worried about.
I have an Orion BMS 2 which offers continuous isolation measurement, but haven't connected it to anything since my main connections aren't all in place yet. Does the described meter reading seem acceptable? I'm thinking I'll try the meter with a 1M resistor in parallel tomorrow, is there a better test I can do with stuff that might be lying around? The pack seems clear of foreign conductive debris, is there anything else I should check for?

Thanks,
-Nick

 

[pdove]

Hello,
I've got a battery pack made of 2 groups of Chevy Bolt modules (of the hopefully less incendiary US built variant). I've got them all in place in the vehicle, and while connecting them up noticed that one module seems to produce quite a bit more stray voltage than the other. My Fluke 115 promising >10M input impedance usually pulls a single module from 3V to chassis down to <1V to chassis in relatively short order. After making a few connections between the modules, I'm seeing terminal to chassis voltages around 100V on the terminal furthest from the module I'm worried about.
I have an Orion BMS 2 which offers continuous isolation measurement, but haven't connected it to anything since my main connections aren't all in place yet. Does the described meter reading seem acceptable? I'm thinking I'll try the meter with a 1M resistor in parallel tomorrow, is there a better test I can do with stuff that might be lying around? The pack seems clear of foreign conductive debris, is there anything else I should check for?

Thanks,
-Nick

Could be ghost voltage produced by capacitive loading. Stray wires running in parallel but not connected can cause this and I suppose there could be other items not connected causing this. Hard to tell not seeing the system.

 

[Azimath]

Could be ghost voltage produced by capacitive loading. Stay wires running in parallel but not connected can cause this and I suppose there could be other items not connected causing this. Hard to tell not seeing the system.

No wires near the HV conductors on the affected pack section, picture attached. + terminal of 2nd module from the left reads close to 0 volts, the others read 3.5 volts/cell + or - to the chassis. More insulation of bus bars is planned, I'm just setting up to try spinning the hubs. Note the - side is disconnected at the fuse.

 

[tiger82]

We had similar experiences when we were building up our Bolt derived pack. It appears that there are stray surface currents on the mylar pouch cells. As you build up the pack by adding cells in series, the problem gets worse. We used mylar sheets between layers of our modules and thermally conductive pads (electrically isolating) between cells and our cooling plates. Voltage isolation on the finished pack was above the required 500 ohms/volt level, but was not great. If it is just one module that is worse than others, there may be metal chips, or other debris, between the cells and cell carriers. If a mylar pouch is leaking, then this will also show up as greatly reduced voltage isolation. Checking how fast the voltage decays with a meter is a good "quick" test for voltage isolation problems. If you need a procedure to find out the actual level, the NHTSA has a published procedure, or there is a Tesla Patent (Method for Determining Battery Pack Isolation Resistance....) that outlines a simple procedure.

 

[Azimath]

We had similar experiences when we were building up our Bolt derived pack. It appears that there are stray surface currents on the mylar pouch cells. As you build up the pack by adding cells in series, the problem gets worse. We used mylar sheets between layers of our modules and thermally conductive pads (electrically isolating) between cells and our cooling plates. Voltage isolation on the finished pack was above the required 500 ohms/volt level, but was not great. If it is just one module that is worse than others, there may be metal chips, or other debris, between the cells and cell carriers. If a mylar pouch is leaking, then this will also show up as greatly reduced voltage isolation. Checking how fast the voltage decays with a meter is a good "quick" test for voltage isolation problems. If you need a procedure to find out the actual level, the NHTSA has a published procedure, or there is a Tesla Patent (Method for Determining Battery Pack Isolation Resistance....) that outlines a simple procedure.

I measured my pack with the procedure outlined in the Tesla patent with a 1M resistor and got 74Megaohms which seems high, but given that the resistor pulled the positive terminal voltage down to 2V to chassis I think it's actually ok.