Temporary MOSFET Temperature Increase on JK BMS Without Load – Normal Behavior or Hidden Issue?

Q&A and troubleshooting
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1

Hello everyone,

I’ve noticed an interesting behavior with jk-bms:

  • The MOSFET temperature briefly rose to around 5°C, even though there was no load connected, and then it dropped back to normal ambient temperature (~5°C).
  • There were no other anomalies: current was zero, and cell voltages were stable.
  • This is happening periodically (week/s)
    jk bms MOS temperatue
    jk bms MOS temperatue537×296 19.7 KB

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My questions are:

  • Has anyone else observed this behavior on JK BMS units?
  • Is it possible that the BMS periodically tests the MOSFETs internally, even without load, to check their functionality?
  • Or is this just a normal fluctuation caused by internal electrical noise or sensor measurement quirks?

Any experiences, technical insights, or official information would be highly appreciated!
P.S. graph - at 9PM started to charge my car with 7.2kWh

Thanks in advance!

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You may want to add:

BMS Type
BMS Version
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Steps to reproduce this

3

Have a read of this article explaining how back to back MOSFETs work in a BMS when ATC or ATD are active at low current flows and the body diodes are used for the current path. Could this be what is happening in your case.

4

Both CH and DIS are always on and there’s no alarm on BMS. At that time - not balancing, no current consumption reported by HA or VRM and no variation in cell voltages (dropping). Afterwards everything going back to normal (I have 4 battery packs that I can data compare with). I can monitor each battery individually and there were no sharing currents between banks.

5

I’m not seeing this on my 2 JK inverter BMSes, the MOS temperature always seems to be related to the current.

6

Well guys, same hour but different day, mystery finally solved. Yesterday I was just checking remotely. Today I’m standing right next to the battery banks. And guess what… it’s exactly what it looks like…Verry funny.

IMG_8563
IMG_85633024×4032 3.48 MB

2 Likes
7

Thanks for letting us know, we all try to think of esoteric electrical or code causes and miss the external factor.

8

Haha. I’m still looking at your photo and…seriously. I have no idea. What am I missing?

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The sun on JK

10

Haha. I thought about that, but didn’t expect it to do that much. Nice.

My father used to tell us; if you can’t find it, then it is usually right in front of you.

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