FEDEVEL
Platform forum

, 02-10-2025, 12:14 PM

A client recently approached me to look into the feasibility of retrofitting legacy batteries with a new versions of their BMS. The challenge is that they would like to retain the current looms/harnesses in the batteries which relies on a common ground connection and connect them to the new version of BMS which has total of 4 isolated GND sections.

The isolation on the BMS is achieved by means of digital isolators and isolated DC-DC power supplies. The reason for adding this isolation was due to EMI issues and component failure.

As the common ground on the legacy system was done by splicing together all the GND wires on the harness and then having this single wire connect to the PCB, I can see no way to achieve this goal without violating the isolation. I hoping I am missing something and that someone can advise on a way to achieve this.

Regards, Friedl.

QDrives , 02-10-2025, 07:48 PM

In part I would say the wrong reason to split Gnd. I guess it is the board you posted about earlier?
But more importantly, I cannot say without schematic of BMS and bigger system.

, 02-12-2025, 06:05 PM

This is a new project. Unfortunately this is all the information they were able to give me as the engineer that designed this board has since left the company and they now need to find a way to make it work with their legacy batteries.

QDrives , 02-12-2025, 08:40 PM

Do I have this correct:
- You have a 'new' BMS that has 4 separated Gnds
- The 'old' batteries have only one Gnd
- The BMS design was split in Gnd due to EMC and failures (of unknown cause)
- They want to connect the old batteries to the new BMS and do not know how to handle Gnd?

I can think of way to many reasons why it failed, why it would fail again and, at the same time, why it would **not** fail (again) if all Gnds are connected together.
Without schematics it is almost pointless to list them all.

, 02-13-2025, 06:46 PM

LMAO... You have everything correct yes.

, 02-13-2025, 06:50 PM

Here is my plan preliminary plan:

1. I will modify the new board (with the multiple GNDs) to accomodate the connectors of the old battery harnesses.
2. I will disconnect ALL but one GNDs from the old harness only using the battery GND as the "Input GND" ground to the board. This leaves three more GNDs that used to be on the harness corresponding with the 3 isolated GNDs on the new PCB
3. I will design a small harness that will connect the three different GNDs from the battery to the corresponding isolated GND sections on the PCB.

, 02-13-2025, 06:51 PM

From what I can see the only problem is really that the GNDs on the legacy system was tapped toggether on the harness for some reason. Not sure why the went this route as they had the pin s available on the connector anyway.