S18V20F12 and BMS Undervoltage Cut-off Problem

Hi I’m using the S18V20F12 step up / step down regulator in this configuration with this BMS and a 4S Li-ion battery:

When I discharge the battery to its cut off (12V) and remove the load I have 0V on the regulator outputs, 1.4V on the regulator inputs and 1.4V on the charger port. This means I cannot charge my battery again until I disconnect the regulator at which point the voltage returns to normal.

Is this behaviour expected? It is a problem because I have everything sealed in a single enclosure.


How are you measuring the 1.4V on the regulator’s VIN? I suspect something more complex than just DC 1.4V is going on. To be clear, the 12V cutoff you mentioned is from your BMS, correct?


Hi Claire,
I’m measuring the voltage with a multimeter across VIN and ground. And yes the 12V cut off is from the BMS.

I have removed the regulator and cycled the battery a few times with the BMS attached and it seems to be working normally. I have tried attaching again two different S18V20F12 regulators and had the same issue.

I’m assuming from your answer that this isn’t expected behaviour?

Best regards,

This is what I suspect is happening. When the battery discharges enough to drop below the BMS threshold, the BMS switches off, which disconnects the regulator and allows the battery to recover a bit. When its voltage goes back up, the BMS and regulator turn on again, which loads down the battery enough to trip the BMS again and repeat the cycle. (Even with no load connected, the regulator still draws a few milliamps, which might be enough to cause a noticeable voltage drop especially when the battery is nearly fully discharged.) Could you look at VIN of the regulator with an oscilloscope?