i2c BMS for hobbyking lipos

The darlington array is common emitter, and the emitter is tied to same gnd as the mcu. So the base pins will be at the emitter voltage when mcu pins are LOW. The array sinks from the PNP transistors that do the actual cell drain.
 
you should be able to remove one of those opto couples, and maybe combine 2 of those dual opamp into one quad. you seem to be trading part count for price, you don't need a Darlington array to turn on the pnp's 6 2n2222's would cost 6c
 
You could also replace those darlington's on each cell with a standard PNP (I like SS8550 personally, can be found for pennies a piece). It will be cheaper and have a lower saturation Vce, so less heat dissipated.
 
the 8550 has a totally vertical turn on Ic vs Vbe. it is used as the shunt transistor for the older BMSs with high shunt currents before the pchannel FETs became so common.

i have seen no mention of how to protect from shorts on the output. that seems like it is the most important thing.

no analysis of failure modes either and how high the cell voltages can climb when the device is locked up and fails to function to stop overcharging.

seems like a big risk to build it for lipo without ever having tested it on more stable chemistries like lifepo4.
 
I have been looking at the AD8277 with built in resistors. More expensive but better CMRR then can be achieved with external resistors. Plus gets rid of 24 resistors on the board. Power is 200ua on the amp.

I'm going to get the cell monitoring and isolated communications working first. Then ill develop the charging/shunting portion.
 
Frequency is basically zero, so common mode rejection should not be a problem even with cheap op-amps. Remember that it's not necessary to sense voltage down to the mV, what you really need it to be able to tell when cells are close to their max, which can easily be done with "only" 10mV of resolution. Do what you please, but I would not remove a few pennies worth of resistors and cheap op-amps in favor of a substantially more expensive amp just to reduce part count.

Also, I would suggest calculating the current and voltage in each node of your op-amp circuits. I haven't done the math myself, but I have a feeling you're drawing more current through the 20k resistors than you might like.
 
Another idea to reduce power consumption is to add a opto-transistor to turn the power to the op-amps on only when you want to take a measurement. Power draw from the opto would be relatively high during the on state, but far lower when off, which would be the majority of the time.
 
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