Other Parts Discussed in Thread: TIDA-010030, EV2400
Hi
I'm a software engineer who likes playing with hardware. Pretty much everything I know about electronics is self-taught, and making lots of mistakes. One of my current projects is to build a set of LiFePO4 batteries and to date I have assembled the cells, together with a Battery Management System that controls the charge/discharge rates, as well as balancing the cells. I am now building a Battery Monitoring System that at a minimum will include a coulomb counter and information on individual cell voltages. Following a request for advice on the allaboutcircuits forum, a member replied and told me about the BQ76930 device. Since then I have attempted to read through the specifications and due to my lack of expertise, I still have a few questions left unanswered. I thought this forum would be the best option for getting device specific information...let's hope so!
My battery details...
8s8p configuration using 38120 LiFePO4 cells. There are 3 of these 24V batteries in parallel.
Battery charge/discharge management and cell balancing has already been implemented.
I also have implemented an 'overall' battery level (coulomb counting) circuit that monitors the total flow in/out of the 3 parallel batteries, but depending on complexity involved, I may decide to use the BQ76930 onboard coulomb counter so that each battery can be measured individually - particularly useful if I decide to split the configuration differently.
I will be feeding battery information (via a PIC MCU) to a graphical touch screen (Nextion), so will expect to be able to interrogate each battery pack (and hence, each bq76930) individually from the control MCU.
Having said all that, these are the questions I still have regarding the BQ76930.
- How to wire up my 8 cells to the device. I referred to the 'Cell Connections' table in the specification document but I found Cell 5 details confusing. This cell is wired to VC6 and VC5b but I don't see any short required between VC5 and VC5b - are these 2 pins connected internally or have I misunderstood the table? I've attached a schematic showing how I've interpreted the table - not sure whether the short I've added between VC5 and VC5b is necessary.
- In the 'Typical Applications' section, the schematic shows each cell connected through a resistor denoted as Rc (guessing this is a current limiting resistor?) and adjacent cell lines connected with a capacitor, denoted as Cc.
Are these required if the BQ76930 is to be used solely for
a) cell voltage measurement
b) temperature sensing via the 2 thermistors.
If they are needed what is their purpose, and what values should they have? - In a similar vein, there are many other components on the typical application diagram, for example, the CHG/DSG FETs, lots of capacitors and resistors on the RHS of the schematic. Which, if any of these, do I need to include in my basic circuit?
- The thermistor connections puzzle me. I was expecting there would be 2 outputs (there are - TS1 and TS2) and that these would be connected to ground (each via their respective thermistor). However, the typical application schematic shows them connected quite differently. TS1 appears to be powered by VC1 while TS2 is somehow connected to VC5x (which is described as being Thermistor #2 negative terminal). I'm struggling to understand the logic behind this set of connections.
- How do I configure the device? I see references to an evaluation board and software that is prohibitively expensive. Can I configure the device using i2c?
From what I've read in the specification document, this seems like a really powerful monitoring chip and should be very capable of carrying out the limited functions I require. What is tripping me up is the amount of detail in the documentation that I am not able to comprehend. Any help with simplifying this or explaining what are/are not essential in my implementation is appreciated.
Thanks.