Sensorless Q128 cutting on high grade low speed - S09S

Hi all, as the title says, I have a bit of an issue with my new setup. It's my first time using the S09S sensorless controller. Motor is a Q128 36V and 201RPM winding on a (for now) 24V battery.

Top speed is a bit low as you would imagine, but the battery is capable of 7-800W in a pinch.

I also have an S-LCD5 display, on which I didn't change any setting at all.

The thing is, what seems to happen, if I hit a bit of a hill at low speed (5mph-ish) and full throttle the motor/controller just cuts out. The BMS doesn't cut - checked with live DC voltage readings.

Any pointers on where to start? The googling so far didn't surface many clues, but I have a sneaky suspicion it's the sensorless (because frankly it's the only new thing in this bike vs the half a dosen I built already)

Thanks for reading

The most common reason for this problem is voltage drop under load.

If the controller is meant for a higher voltage battery than it's being used with, then the battery will only "work" for part of it's voltage range. The controller's LVC (which will shutdown the motor whenever the battery is below that voltage) will be set for whatever the typical minimum voltage (empty) for the battery it was meant to be used with.

So if the battery sags a little bit on the hill from the higher load, whcih is normal, then the controller may shut off because that voltage is too low, while under less load it isn't.

The same thing will also happen as the battery discharges and gets closer to empty than full--it will be lower than the controller's LVC.

The battery itself doesn't shut off...but the controller does.

Thanks amberwolf, makes sense. The controller is meant for 36/48V.

Any way I could modify the LVC in the BMS Battery S09S controller? I have a lot of experience with Infineon-s and XPD, but not with this.

If you can find the voltage divider it uses for this, then probably. But remember that if the gate drive voltage drops too low, it may not turn the FETs on all the way, and then they'll be resistive and heat up and can blow up. Also, the LVPS (low voltage power supply) that converts battery voltage to 12v and 5v may have a minimum voltage input that it works properly at, so when battery is lower than that, it might give you unpredictable results (potentially damaging). Depends on how low you actually go vs what the designer considered.

Hi amberwolf, thanks for the reply. Here's my late feedback.

Turns out the S09S doesn't work below around 23V without mods. I have checked with a PSU, trimming down the voltage until I got a cut. Exactly why it cuts, I'm not sure yet. Could be software limits, or as you say, could be hardware. There is the open source firware for these controllers thoug, and using that I hope to see if the issue is actually a soft LVDC limit in the controller (by removing it and testing). I bought the ST serial gizmo, need to solder the JTAG pins to the controller and see where that takes this project.

I did sort the initial issue though, by using a S06S controller, which is meant for 24V. It's cheaper too.

I'll try to document my mods of the S09S when I get to it, cheers!