Pylontech BMS burnt component

Hi all, please help me identify (part number) for this ‘diode’, thanks

I’m going to guess it’s an SMBJ tranzorb protecting the CAN (or more accurately, indicating that your warranty is void). You can determine the voltage by testing one of the good ones. I’m assuming U345 is a can transceiver, I can almost make out “P2561” so likely an MCP2561.

If that tranzorb is done, you should also check U345, and whatever was on the other side of the CAN interface, you most likely also have to clean up the parts of PCB that became carbon.

Going with a very wild guess as I don’t have any idea what was in the rest of the system: Did one of the PV inputs become earthed? Perhaps a surge arrestor did it’s job and earthed one of the PV poles?


Thanks for the response and yes it’s MCP2561.
The other good ones are open both sides…no diode reading.
And yes, the tranzorb is done…I don’t know how to check the transceiver.
I’ll try to replace the tranzorb (someone suggests it’s a 400w TVS, yes?) and see. And is it bidirectional?
I have no history of the BMS (CMU), got it as junk and trying to fix…i am no expert either.

The tranzorb is most likely just to clamp the data lines if the voltage goes too high - so the circuit should be able to function if you remove it. To test the voltage, you have take them out and connect them to a current limited power supply. Something like 20mA should do. Then increase the voltage until it starts to clamp.

But all of this is just guesswork without reverse engineering the circuit and working out exactly what the purpose of the component is.


Thanks a lot
It turns out the PMU board is not working… swapped it out and the battery works. I’ll try using the CMU as it is but avoid using the CAN port.

Where are you located?
There’s got yo be someone in your neck of the woods who knows which end of a soldering iron to pick up…

I’ve seen that on old Seagate drives, and in that case it did indeed function correctly when we simply removed the component. Of course sans any protection.

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Through the magic/curse of the internet (i.e. you blow stuff up I am not responsible) the diodes (D370, D371, D417 and D418) should be originally manufactured by Polytronics.

The “AG” surface marking can either be uni-directional SMAJ or bi-directional SMBJ.

The lack of cathode band should point towards bi-directional (assuming the conformal coating and/or light angles do not hide the band in the pics). Being part of the communication circuit in the vicinity of hi/low/CAN/RS485 I assume (and the elders will smack me with a rolled up newspaper as required) a bi-directional diode would probably be expected).

To further distinguish between SMAJ vs SMBJ with the “AG marking” the distance between the PCB pads is possibly the one area that could help answer it.

Polytronics TVS series catalogue for options and measurements.

most likely candidate to me looks like SMBJ6.0CA (keep in mind I still get confused by which is the working end of a soldering iron).


Do Pylontech have an office in SA?

Thanks and i know how to solder, at least.

Thank you for your help :pray:

The reason I ask this question (to which there are no replies) is that I understand this product is highly respected on this forum.
This is good start since chances are there will be local support. The question could be forwarded to them and they might be able to help officially and possibly unofficially. Typically the support one can glean from a local representative is way better than an official website somewhere in the world.

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No, but they have partners in SA. All of them have local repair centres. Sadly they only accept returns fro installers and not the public.

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Ok then I would find the partner that has a strong relationship with the product and contact them to see if they have scrap PCBs (before they are sent to the recyclers)
I know this is a long shot but the OP could do with a board that is being thrown away (and believe me there is plenty of this stuff that is ditched)
Of course if the brand is anal and insists that all scrap is shredded then you have a problem. But you don’t know that until you ask…

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I have been to CNBM, seen alot of stuff in their junk container but they won’t give or sell to me. They want me to bring the battery in for repairs i.e. buy a replacement board plus labor fee.

For another case, I reached out to Revov to buy PACE BMS which is used in their batteries and a few others like Hubble, BSL, Greenrish…but they refuse to sell to me…

‘Partners’ & ‘distributors’ in SA have a highly restrictive business model

I have no idea about prices for replacement boards but a possible source to purchase without needing to hand in a broken board or one of your children as collateral. Looks like it is possible to purchase the board for own use as opposed to having them do a repair.

Maybe they have other boards

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Thanks, they don’t have one for us3000

Just a question … why do you need the exact same BMS?

As long as one can program the new BMS for the cells one has, it must work.

Cells store the power, the BMS is an add-on to protect the cells during use.

The case architecture…one for us3000c will fit in older us3000 case. Us3000 x2 boards (PMU and CMU), US3000C merged into 1 board…

Thank you all for the contributions and as mentioned before, the CMU is working in its current state…just doesn’t start automatically in sequence with the others.

@Village_Idiot, local electronics shop don’t have SMBJ6.0CA. The have SMBJ6.5CA, ‘AK’; should I try it?