openinverter forum

I've been trying to figure out why my VeroBMS (SimpBMS) is throwing errors and not showing correct cell count. I'm suspecting bad slave boards, more to come on this as it may or not be related to the below.

I have a B250e pack with 7S Tesla modules. Car runs, drives and charges and currently sits at 328V. In troubleshooting, I have been looking at potential HV isolation problems as I read that these modules can be prone to issues. Here's what I found:

- No voltage between either HV+ and chassis ground, or HV- and chassis ground with battery terminals disconnected (Pack voltage currently 324V)

I have an isolator switch on the positive side of the battery pack. Negative side is connected directly to ISA shunt. For the following measurements, everything is powered down, i.e. all contactors open.

- With battery isolator switch closed, I can measure 230V between chassis ground and battery side of + contactor. Meter stays steady and does not dissipate. I can also measure 115V from midpoint of pack to chassis ground, regardless of the isolator switch position.

I have been carefully stripping things back to try and find the cause. What I see is a path through the HV sense wire on the ISA shunt which is connected to the inverter side of my open positive contactor making a path to ground inside the inverter. If I remove the HV sense wire from the contactor post, the voltage drops to zero. If I disconnect at inverter HV positive post, it also drops to zero.

I can also measure 230V directly between the HV sense wire from ISA shunt and battery positive (not sure if this is considered normal?)

At this point I'm a bit stumped on where to go next. Not sure if I have a problem going on with the inverter or something else. I do have a megaohm meter new in box, but I'm unsure yet how to use it and if it can damage electronics etc. Any suggestions?

Use the ohm meter on the batteries to start.

You are saying inverter being disconnected removes the HV to chassis, then the inverter is suspect.

The Mercedes B250e modules suck for isolation and are prone to leaking via the mounting feet.
Also the slave boards do die, had quite a few duds.

Thanks for the reply. From the Megger, with inverter connected I get 15MΩ between HV positive and chassis, & 6MΩ between negative and chassis. Tested at 500V. With inverter disconnected, battery only, I get around 1500MΩ from either terminal to chassis. Had to test at 1KV to get a reading. What does good (and bad) look like? From this, it seems to confirm battery is OK, but inverter has an isolation issue?

Just checking the obvious...when you say inverter disconnected HV is live? (Otherwise your just measuring the wire to the contactor). I went with 1000v for isolation test because insulators start to break down at that voltage (plastic conduit becomes noticeably more conductive at 1000v !! Probably overkill)... With the VW gte and BMW 330e packs with everything dry 200 mega ohm is easily achievable and 1 giga ohm can be achieved, anything under 100 megaohms indicates something wrong..
I had to assemble my pack in less than perfect conditions in spring so moisture and condensation was as issue an I was getting 160mega ohm at 1000v, now everything has dried out a bit I'm in the 300 mega region but I'm using welding cable in conduit (which is in the manufacture spec 600v, but wouldn't recommend it nowadays as double insulated cable can be brought very cheaply and getting binned in the next battery rebuild (upgrade)).

Yes, live, by that I mean I have HV inside my HVJB, but contactors open. Here is a picture, showing where I took the positive reading on the megaohm meter. Negative was to chassis ground. If I unplug the HV connector on left side, I get 1500MΩ reading. That cable runs directly to the LS600H inverter. When plugged in, reading drops to 15MΩ and I can toggle it back to 1500MΩ by disconnecting the voltage sense wire from ISA shunt. I have tried changing shunts, disconnected pre-charge circuit, and removed charger fuse to rule them out.

robertwa wrote: ↑Tue Jul 22, 2025 6:16 pm If I unplug the HV connector on left side, I get 1500MΩ reading. That cable runs directly to the LS600H inverter. When plugged in, reading drops to 15MΩ and I can toggle it back to 1500MΩ by disconnecting the voltage sense wire from ISA shunt. I have tried changing shunts, disconnected pre-charge circuit, and removed charger fuse to rule them out.

are you saying if the isa shunt is connected you get 15mega or just inverter?

are you using a 3d printed cover on the hv connection on the inverter?
i printed a cover of the fuses on mine and found that petg is very conductive at 1000v ( i just ditched the design,and remade it so there is no contact with anything hv - air is better insulator)

rstevens81 wrote: ↑Wed Jul 23, 2025 10:27 am are you saying if the isa shunt is connected you get 15mega or just inverter?

I need both ISA HV sense wire and inverter positive lead connected to complete the circuit.

I think I'm getting somewhere:

I'm using the LS600H inverter, where I tap into the bus bars is different than on GS450H: I have a GS450H inverter, and connected HV to do a compare, verifying both inverter chassis are grounded together. The problem goes away. Looking closer inside the LS600H 'peep holes' (they were originally covered with a grommet) There is something yellow poking around there that may have been contacting my brass bushings..... might have to disassemble inverter to see what it is:

That will do it a partial insulator (or whatever that is) in contact with an HV terminal wall cause isolation resistance to plumit

Looking at a picture in the OSTI report, I can see that yellow LV wire shielding crossing over the path of where my brass inverter post connects. Looks like I will need to tear in and see if there is enough slack in the cables to tie them out of the way