Gotway MCM V3 not charging

[chopsywa]

Hi All

I am looking for some advice on my BMS

I will preface this post with a background. I am an electronics technician who has been out of the business for 25 years. I can still fix a lot of things, but I have lost my edge, particularly with MOSFET power circuits as these were just coming to the fore when I left the industry.

I purchased a new Gotway MCM V3 which was a DOA from the point of view that it would not charge. The excellent seller in Australia (urbanlab.com.au) organised a new pack to be sent from China, but due to flight restrictions this will take 40 odd days to arrive. I would like to point out too that the reseller offered to replace the entire wheel, but I was impatient and wanted to get riding. As the reseller said I could keep the old pack, I figured I could get it going while I wait for my new pack. What I did was to create a small harness with a male/female XT10 soldered back to back and then teed off a small connector to charge directly on the load side. This works fine and I have had many good rides. 

Today I decided to see if I can repair the BMS, so I cut into the heatshrink over the circuit board. I was hoping to find a dry joint, or something obvious, but no such luck. Essentially the issue is that the main power mosfet is open circuit. I am not sure if this is because it is being driven as such, or it is faulty, but it looks to be just an overvoltage protection MOSFET. I am not sure if it is a multi layer board, or even double sided in situ, but it looks like the negative charge lead goes to the mosfet and nowhere else. If this is the case, then the MOSFET is most definitely just there for protection. This would also mean that the balancing circuitry is active all the time and will shunt cells even when the unit is switched off and idle. I find this a bit disconcerting.  

Anyway, what I am asking is am I correct in my assumption about the role of the MOSFET and that the balancing circuitry is perpetually active? If that is the case, I will just leave the rig I made in place as the genuine charger won't over charge anyway and my pack is being balanced (assuming the rest of the circuitry is in fact OK). 

Cheers

Mark

[Keith]

@chopsywa, if the negative charging lead goes directly and only to that MOSFET then, yes its role is overvoltage and probably reversal protection on the charging circuit. It is fairly unusual for MOSFETS to fail open circuit, unless obviously burnt, short circuit tends to be more likely, so it may be an issue with the monitoring circuit that controls it. Are you getting the correct voltages from the battery, I.e. No faulty cells, etc? My thinking is that it is worth carefully checking that there isn't a real fault condition it is reacting correctly to like for example a failed cell. If all looks OK then shorting source to drain on that transistor would be no worst than what you are currently doing and would allow you to use the charging connector. Obviously it then becomes important to monitor charging voltage carefully as the protection is missing.

I would expect the balancing circuit to always be active, it has to protect the cells during regenerative braking for example, however most BMS balance circuits are quite crude and simply clamp each cell to 4.2V. As such the will have no effect once each cell is below that value.

[Chriull]

@chopsywa - i assume you already looked into 

As far as i know/read - gotway bms did not have reverse protection diodes nor overdischarge protection Mosfets. But there where imho no phtots from the  newest BMS versions from Gotway...

Another post with maybe some interesting details for you:

 

[chopsywa]

2 hours ago, Keith said:

I would expect the balancing circuit to always be active, it has to protect the cells during regenerative braking for example, however most BMS balance circuits are quite crude and simply clamp each cell to 4.2V. As such the will have no effect once each cell is below that value.

Thanks Keith. Yes cells are all good. They fully charge to 67.2V (4.2V per cell) which is the no load output voltage of my charger and I get a full discharge cycle from them. The gate on the Mosfet is at 0V, so I suspect as you said it is the overvoltage circuit that is failing. I am loathe to do much with the board as the voltages and potential currents are dangerous. It sounds like it is going to be fine the way I have it. This makes me very happy. Maybe I can buy a horizontal driver board and do the mod I have seen where I can put both battery packs in to make it into a 520WH unit.

Here's a picture I took while I had it open. I took this to get a good close up view of the board looking for dry joints, but you can see the transistor. It is the only power MOSFET on the entire board and as it is only in the charge line, I would suggest it is only for overvoltage.