Inmotion V8F battery repair (unicycle won't turn on)

[Ingvard]

Hello everyone! I wish you all a nice day and hope for your help.

I recently purchased a used Inmotion V8F unicycle. And almost immediately its battery stopped charging, that is, the unicycle did not respond to the connection of the charger.

I have some small experience in working with electronics, but I have not dealt with batteries and BMS boards before.

After disassembling the battery, it turned out that some elements on the BMS board were damaged and were replaced. In particular, the CRST045N10N power MOSFET was replaced because it had a short between all its legs (red ellipse). The group of parts circled by ellipses on the left side of the image was also replaced:

1. The resistor marked with a yellow ellipse had a rating of about 10 ohms. It burned out and was replaced with a 100 kΩ resistor.

2. The zener diode marked with a pink ellipse was replaced with the same one rated at 12 volts.

3. The bipolar transistors marked with blue and green ellipses have also been replaced with STN9260 and MMBT5551 respectively.

I assume that these parts are needed to generate the Vcc voltage. To the right there is a similar group of parts that are probably needed to generate the Vdd voltage.

After connecting the BMS board to the battery, test point Vcc showed 12V and test point D3.3V showed 3.3V relative to point B-. I also charged and balanced the batteries using another 5S BMS.

After installing the battery in the unicycle, there was a normal reaction to connecting the charger - the blue battery symbol on the wheel body lit up. The wheel turned on and worked fine, I even drove it for a bit.

But then it turned out that after some time after turning it off, it is impossible to turn it on with a button. Only after connecting the charger and waking up the main board of the unicycle, you can turn it into working mode. After turning off, the LEDs on the main board still light up for a short time, during which time it is possible to turn on. After the LEDs on the main board go out, it becomes impossible to turn on the unicycle with the button.

Please tell me where to look for the cause of such a malfunction? Thank you very much for any help!

[alcatraz]

Try shorting the mosfet perhaps. For testing purposes.

[alcatraz]

You can do it at the empty pads to the left, run a wire. (not the gate)

[Ingvard]

55 minutes ago, alcatraz said:

You can do it at the empty pads to the left, run a wire. (not the gate)

Do you really mean to short the drain and source pads?

[alcatraz]

Yes. It keeps the output enabled.

If it works normally after that you can analyze the gate or mosfet operation.

They shouldn't get hot, and the gate should signal the mosfets on.

I'm not an engineer but I've watched hundreds of hours of electronic repair videos. Whatever that's worth.. 

Edited September 30 by alcatraz

[Ingvard]

5 hours ago, alcatraz said:

Yes. It keeps the output enabled.

If it works normally after that you can analyze the gate or mosfet operation.

They shouldn't get hot, and the gate should signal the mosfets on.

I'm not an engineer but I've watched hundreds of hours of electronic repair videos. Whatever that's worth.. 

Thanks for the tip. I shorted the drain and drain of the power transistors. The unicycle can turn on and off, but the LEDs on the main board do not turn off. I assume that means it doesn't go into standby mode. I think the battery will discharge faster in this condition.

Regarding the transistors, for now, I can say that there is a little heating in the area where I changed the bipolar transistors. Since the BMS board is connected to the battery, the heat spot is significantly larger than the parts and I currently cannot determine which part is heating up.

Do you know the battery sleep and wakeup algorithm? What signals should be on the CHG, UD, ON, OFF, PGND contacts?

Here are the voltages measured at some test points relative to the B- line (battery not attached to unicycle):

Vcc 12 V
Vdd  0.3V
D3.3V 3.3V
A3.3V  0V
CO  9.7 V
DO  9.3V
GD  0V
CG  0.3V

[alcatraz]

Sorry.

Maybe solder an open end wire to that gate terminal and monitor the signal as you're powering down the wheel. Does it change? Is it zero? Broken trace, broken solder joint?

Here's an ugly compromise if you get tired of failsearching. Add a switch or relay to enable the output. Or even better. Add a low power switch to supply the gate voltage. Ensure it's the right gate voltage to ensure the mosfets stay cool. Heat = very bad.

Obviously, ride with plenty of gear for the first hundred kilometers to test the mods.

[alcatraz]

The datasheet to those fets specify a 4V threshold gate voltage. Channel N. 

[alcatraz]

You'd need at least 6V on Vgs (gate-source) to fully open the mosfets so for normal operation you should look for >6v for on, and <4v for off.

Maybe the chip that supplies the gate voltage loses some of that voltage on the way to the mosfets. Measure at the chip and compare with the mosfets. 

Or maybe the bms chip is missing its power input from the battery side, which is why it only powers on on the charger. Look for its datasheet.

Edited October 1 by alcatraz