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USB signal interference

I’ve been testing the pathfinder BMS with various equipment, and found that the USB connection is vulnerable to electrical noise from a big cheap inverter. We noticed this problem in the past with the JBD BMSs as well. (This doesn’t affect Wifi or Bluetooth)

This interference causes the USB port to stop communication, and the BMS must be rebooted to get it going again. We are working an getting the firmware to automatically restart the USB connection, but it doesn’t work yet. Remember, all Pathfinder BMSs can get firmware upgrades via OTA or UF2, so everyone will be able get the firmware solution later.

Other than getting a better inverter, the best solution seems to be adding ferrite chokes to the USB cable. I found that 2 or 3 snap on ferrite chokes is enough to stabilize the connection. Wrapping the cable 3 times thru a large ferrite ring had the best results, almost completely eliminating the interference when measured on the PC side.

Our very noisy inverter. This thing is crap, but great for testing interference.
Ferrites added to the USB cable. The large ring gave the best results.

We will do some experiments with adding common mode chokes to the Pathfinder circuit board and possibly add them to the next revision.

Better quality inverters produce far less noise, so this isn’t an issue for most customers. For example in the past we found that a Renogy inverter produced the least noise out all the units tested in our shop.

Details and scope traces:

I measured the signal interference using an oscilloscope at the PC end of the USB cable. Not much to see here. Yellow and Blue are the signal lines, Data+ and Data-.

The Inverter produces pulses of RF interference, and between pulses, the USB signal looks fine.

Inverter off, no chokes.
Inverter on, no chokes.

Here, I’ve zoomed out the time scale and added the purple trace on the inverter’s DC terminals. The pulses are happening at 21kHz, which is the inverter’s switching frequency. On the left side you can see a USB communication packet, which occurred between pulses of interference.

The real problem happens when the pulse of interference causes ringing on the USB signal lines as seen here. USB signals are mostly differential, but both lines pulled low means EOP or end-of-packet. This time, the ringing interference pulled the blue trace low enough that the USB controller would read an EOP signal.

Here, I added 3 snap on chokes (left), or wrapped the cable thru a large ferrite ring choke (right). In both cases the interference is dampened enough to prevent spurious EOP signals, but the large ring has the best results by far.

USB cable with 3 snap-on chokes, inverter on.
USB cable with Big ring choke, inverter on.

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New Self-Test feature

We added a super handy self-test feature to the Pathfinder terminal boards.

They now have a TEST button and fault indicator lights for each cell.

This feature will catch many of the wiring mistakes that can destroy the BMS before the damage can happen.

This test can detect swapped balance wires and open circuit cell connections, both of which would damage the voltage sensing inputs on the BMS main board if the terminal board was plugged in before fixing the faults.

The new 16 cell self-testing boards are finished and ready to ship with pre-orders.

(All of the terminal boards will have this feature at no extra cost)

Example Usage:

In this example, all of the balance wire connections are correct, and only the green LED lights when pressing the TEST button. This assembly is ready to connect to the BMS.

In this example, one of the cell terminal bus bars has been left off. Pressing the button lights up a single red LED corresponding to the open connection. If the BMS was plugged in like this, the full battery voltage would be present between 2 cell voltage inputs that are not tolerant of the full battery voltage.

In these examples, 2 balance wires are swapped out of position. The red LEDs will light even without pressing the test button. This may be the most damaging type of wiring fault because it puts negative voltage across several of the cell voltage inputs, which will destroy them if the connector is plugged into the BMS.

In this example, multiple faults are present and the red LEDs do a fun little dance. Might want to start over if it does this…

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First batch is sold out!

Sorry, I should have made this post 2 weeks ago: The first production run of Pathfinder BMSs is sold out!

Orders after order number 9025792 (from March 16) will ship from the SECOND production run of Pathfinder BMSs, so they will have some additional lead time beyond the original April 30 estimate.

Pre-order now to save your place in line!

The orders keep coming in, so the lead time will continue to increase for new orders until we get caught up. (Ramping up manufacturing is hard!)

Big thanks to everyone who has placed a pre-order, we really appreciate your support and patience.

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Quick Start Guide (bottom cover)

Pre-sale Update:

Some of the boards for the first run of Pathfinder BMSs have been ordered and are currently in production. A big thanks to everyone who has put their orders in!

I just finished the quick start graphics for the bottom cover of the BMS, what do you think?

The full user manual will have more example drawings for other cell counts.

While working on revisions to the main boards, I decided to add a wiring test feature that will detect faults in the battery wiring before the BMS is even powered up. This should help prevent a lot of mistakes that we have seen in the past when wiring up DIY batteries.