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[rgutt]

This is just a short in the dark, but I'm wondering if anyone here has intimate knowledge of how the Zero BMS detects battery isolation from the chassis. I'm familiar with how the theory of it work, but I don't know what resistive values Zero uses in their BMS and whether they actually execute the checks against the full battery voltage or some subset of it.

The reason for my inquiry is that I have had an analog pressure sensor on my rear brake hydraulic line for about the past 2 years. Several weeks ago, it started randomly activating on its own. This degenerated into activating near constantly and rhythmically. Upon investigation, I found that the issue was a floating voltage relative to battery ground present on the frame. This degenerated to the point that I can now measure exactly 1/2 battery voltage between the frame and battery negative keyed off. Upon keying on, the voltage between frame and battery negative becomes a square wave from 0 to 1/2 battery voltage on about 2/3 second cycle, strong suggesting something under a microprocessor control. Given how chassis isolation is measured, I suspect this circuit to be the culprit, but I cannot be 100% certain.

I am at least certain the issue lies within the BMS or its wiring because I have removed the front cover from the monolith and disconnected the ribbon cable at which point the floating voltage on the frame disappears. Obviously, I cannot power the bike at that point since the BMS has no power.

[Specter]

Thats typical of a ground detector itself fault, when it shows about half the system voltage.  Like a midpoint bleed  / isolation etc resistor or tank is leaking to ground.  Not enough to really pull any current and let the magic smoke out or blow fuses but enough to show the power on the  leg, looking for a place to get back.. ie a solid ground.

Another option is you have a shorted / cooked something somewhere that's low on both ends, plus and negative and it's pulling both sides down, but again, there's enough resistance to the whole thing, it's a corrosion short so not a lot of current can pass, so it pulls the power down enough to show a partial on the bus, but again, not enough to blow any fuses to say this or that is faulty.   Water in something is a very good way for this type of fault to occur just as an FYI.

If you isolate the BMS and it goes away, you pretty much found the culprit.  Sadly those i dont  think you can really fix parts on, unless you want to open it up and see if there is something obvious and then so a bit of soldering.

aaron