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Discussion Starter · #1 ·
I'm trying to understand how the Ryobi 40V mower battery works....
These batteries have a locking problem. https://www.diyelectriccar.com/foru...0v-bms-troubleshooting-and-repair-197409.html
The BMS locks while all cell are good.
I orderer an aliexpress compatible battery.
After discuss, the seller sent me photos of the BMS




The controller seem to be a PIC16F1933
I started to note the schematic.


I wonder how the PIC manage to measure the voltage cells ?
I didn't note any other wires.
I scared to see the voltage cells not measured, unless the cells own specific protections directly.
The schematic is just approximate, just looking wire and component.
The transistor could be an other model and pinout.
I'm waiting the parcel to explore the battery.
Else, did you have an idea ?
 

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From the board picture it looks to me that U1 is a voltage regulator. The PIC may always be ON. PIC V+ is into pin 20, and ground is pin 8.

[edit: modified my description from previous version]
Then PIC pin 10 is driving the base of Q1 thru R1, and this turns ON Q2 thru R4, and the Pack voltage is dropped thru R5 R6 divider to create a base or gate voltage. There is a via between R5 and R6 that connects to the base/gates on all 10 NPN/ N-FETs (Q3-Q12). This allows the PIC to read the voltage of all the CELLS. Q1 is like a switch that turns ON the BMS for monitoring all the CLs.

To read the CL voltages for example, CL6+ is routed to R31 and R21, then through Q5 to PIC pin 2.

[edit: strike this]
there is a via near R33 which brings in the voltage from CL4+. i'm guessing that Q6 is a P-fet(?) and the gate gets connected to ground by Q1 to turn it ON and then passes the voltage to the PIC controller at pin x? some via under the chip.
[edit: p-fet with body diode didn't seem right]

Not sure if there are enough transistors left over to do balancing.

U3 is missing, board not populated? What was it supposed to do?
 

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Discussion Starter · #3 · (Edited)
Thank you very much Kenny,

With your analysis, i think i understood the PCB
The P mofset internal diodes allow to transmit the cell voltage to be mesured by the PIC.
R17+R27 and R7 make a voltage divider bridge. But the internal mosfet diode forward voltage must be compensated.
 

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Re: unknown BMS design (not recommended)

My first guess at p-fets is likely wrong. Now i think they must be N-type devices, either npn or n-fets. A 10:1 divider cuts the voltage from 40 down to 3.6, not 4V as shown, but i can't see well enough to read the values, just a guess.




i'm not a fan of this approach because they are trying to tune the resistor-pairs on Q3-Q12 to account for the different bias of the CLs above ground. Seems like someone tried to build a BMS with this concept that was not successful. If you have a bad Cell in the series string then it will throw off all the other Cells.

NOT RECOMMENDED FOR USE AS A BMS
 
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