Pint VESC FM BMS bypass (the proper way)

[Admin Note]: this post is misleading as this isn’t necessary on Pints, but something that should be done on XRs… see comments


Pint VESC FM BMS bypass (the proper way)

As we further develop our Pint/XR conversion to VESC it has been determined that the old way of bypassing the discharge path of the FM (Future Motion) BMS was less than ideal. The old way (bridging the positive post) was successful in bypassing the discharge path but unfortunately it also bypasses (disables) one of the most important safety features of the BMS. The old way disables the BMS ability to stop charging if 1 or more cell groups are overcharging (>4.3v). Normally the charger should stop charging once the desired voltage is reached (63 volts) but there may be instances wherein the BMS may need to intervene and stop charging (broken charger, faulty cell group preventing the pack to reach 63 volts leading).

The latest guides out there suggest pigtails that involve extra XT60 connectors inside the battery box. Unfortunately the Pint enclosure is so small that extra XT60 connectors will not fit.

I was finally able to achieve the proper way of bypassing and wanted to share images. I am sure there are other ways of doing this on the Pint but these are 2 ways.

Method one leaves the stock harness alone but involves folding the wires on itself. This was doable but in the end I did not like the idea of high amp wires folded in a tight space.

Method 2 involves cutting the stock harness to avoid the extra folded wires. In the end I went this way as I don’t foresee going back to the stock pint controller. I can always make an extension if that need arises.

Have you tested the bypass mod described in my video to see if it actually prevents the bms from ceasing charge operations.

Seems awfully complicated for a Pint setup… What’s wrong with reusing the existing Pint charge port and just bridging the negative like we normally do?

However, what you’re proposing seems to be the solution people need on XRs

I believe the issue with bridging the postive post to bypass the bms for discharge is that it also bypasses the ability of the bms to stop charging during overcharge situations which according to Mario is the primary function of a BMS with cell balancing as another function. Balancing was still possible with the old bypass as balancing is usually done thru the balance leads. The new way of bypassing the BMS does not alter the CHARGE path so should retain the BMS ability to shut off the flow of power during those rare bad bad situations.

I saw how they did the changes on the XR and implemented it on the Pint

Sorry cant find the link to your video

Right, but the point is that this is unnecessary for the Pint. So your post is quite misleading…

Hey folks! Sorry if this is a silly question as I’m not quite sure if I understood the comments and admin notes above correctly, so I just wanted to confirm this for my peace of mind. Is the following correct?

On the Pint, we can simply bridge the positive post and the BMS will still be able to shut off the flow of power during overcharge situations.

Thanks :pray:

Picture of the bridging method for Pint BMS from Steve Shoomaker on Facebook

You bridge the positive (RED) cable side.

(From Chi Battery’s Quart install youtube)