Electronic – Question about an electronic component which could be a fuse

Fuses blow due to heat, and heat is due to current via \$I^2R\$. Therefore, what blows a fuse is the current.

However, a fuse needs to blow with a large enough gap that the voltage behind current cannot overcome the gap. A fuse with a 1mm gap isn't going to be able to interrupt a current with 10,000V behind it (it might make a massive, long-lived arc that eventually extinguishes at best, and at worst the 10,000V will cause sparks across the gap or melt and weld the gap.) That's what determines the voltage rating of the fuse. The voltage rating is irrelevant while the fuse is not blown.

High voltage fuses do have different features such as sand or other materials that melts or buries the contacts when the fuse blows to reduce arcing or explosions.

As you found, fuse blow times are inexact and vary. If you look at the datasheets of some fast-blow and slow-blow fuses, you will find charts of how long it takes for the fuse to blow at a rated current. At 100% of the rated current, the fuse could take a very long time to blow (could run for minutes on end). At 200% or 500% of the rated current, it blows much faster. Don't expect a fuse to behave ideally and instantly blow within 1ms as soon as you exceed the rated current.

I cannot explain why it took 1-2 seconds to blow at 1600% overload though.

Here are a few things that I would do to start, based on previous reverse-engineering experience. These are just to give some initial information:

• Look at the BMS PCB visually, don't limit yourself to measurements only

  I see there is some kind of gray plastic "wrap" holding the BMS to the battery in that photo. You would need to open this to reveal the BMS, remove the metal top plate (it's likely for protection and possible heatsinking of MOSFETs underneath) so you can see and view the PCB (which might have components on both sides). I know that leaves you with the job of replacing that "wrap" (or keeping that one BMS+battery as a lab-only, not-for-resale, unit - which I suggest is a good idea, as further disassembly may be needed).

  You might see a typical 8-pin IC connected to the external UART connections (perhaps via an opto-coupler) matching an RS-485 or RS-422 transceiver. That would immediately give you confidence that your idea about why the UART connections are labelled A & B, is indeed because they are a differential pair.

  MODBUS over RS-485 is used in some other BMS modules. If you have to make guesses without dismantling anything then, based on the A & B labels, I would certainly consider MODBUS over RS-485 as a possibility (default UART setting as a starting point is often 9600 bps, 8N1) and try using a PC RS-485 adapter (with A, B and Gnd connections to the BMS). Also remember that some devices use the opposite convention for which cable is A & B, so if unsuccessful, it is always worth swapping A & B cables at one end and trying again.

• Consider more reverse-engineering of the BMS

  If it has a UART, then it has an MCU or some other intelligence (I'll just say "MCU" for that device.). Its top marking might be removed, but you may be able to guess at least the device family. Again, the related circuitry between the MCU and external UART wires will give some clue about what type of interface that is.

  Also investigate how the MCU is powered, as that will tell you what is a valid maximum voltage at its inputs. Depending on what interface ICs you find between the MCU and the external red & yellow UART wires, this might be useful if you want to perform tests with pull-ups later, as you will know a valid voltage for the pull-up.

• Image search the BMS PCB

  Once you can see the whole PCB, image search that PCB online. You might find a seller of the same BMS who gives more information than you have at the moment. Also, seeing the whole PCB might reveal useful markings on the PCB silkscreen.

• PC software available?

  I have reverse-engineered some devices, where the manufacturer provided PC control software (but did not provide proper documentation for the interface and its commands) by intercepting, recording, and later analysing the data flow to and from the PC, when running their software.

• Reverse-engineer is like an onion - revealing one layer at a time

  Each new piece of information obtained from examination and tests, leads to other investigations. What you discover from those first steps can lead towards eventually learning what you wanted. But at the beginning, we don't know whether the journey will be successful. So until there are more details, I can't give you the answers you are seeking yet. However I hope those points are helpful.

You asked about the connector - the visible white latching connector does not seem to be related to the UART connection, but instead is typical of the connections between the BMS and the individual cells in the battery - it looks like 7 cell connections.

It's possible that other people here would see useful details in the Chinese documentation from the manufacturer, if you can repost the part which you said mentions a UART default configuration.

Finally just one point (which I know you don't want me to labour, but I'm concerned about this): If the manufacturer isn't interested enough in helping you with full details of the UART interface when there is a pending order, how interested will they be in helping you, in the event of you asking future questions or having problems, once they already have your money? I hope you see why I'm worried.