Going deep into a BMS the bq20z655


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J_Mack58

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Joined
Apr 7, 2021
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145
It's either feast or famine, for almost 2 years my battery pack(s) had no BMS. It has a Daly on it now. Now about 3 weeks ago someone gave me 234
data storage UPS battery packs, that has 6, 18650 cells in them. Time to make a new battery pack. Normally its rip them apart, test cells and rebuild new pack
at the desired voltage. I'm older and lazy. What if I can build a battery with the desired voltage (48 volts) with the "as-is" packs? That's when I went deep. I destroyed a pack to find the brains of the BMS is a Texas Instruments bq20z655 chip.(pictured). Getting the datasheet on this chip It does what a BMS suppose to do plus it's "gas gauge" like feature that keeps track of all the power going in and out of the cells. COOL! What is really cool is we humans can communicate directly with this chip through its SM Bus. A simply three wire communication set up. (CLOCK,DATA and COM). The protocol is I2C or SMBUS. Both are the same to me and both Raspberry Pi and Arduino have I2C ports. I have several Raspberry pi's. I have not built a pack with these cells and may never will but now I'm writing code in python, (actually stole some code and modified it) Transferring the information from the BMS to a data server and displaying it on a webpage. What I'm doing now has absolutely nothing to do with a BMS or building a battery pack but I hope with the knowledge gained I will go from no BMS to designing my own BMS using this chip (because I mastered it) and creating a webpage GUI (Graphic user Interface) to monitor and program it. If anybody else reading this post is doing this sort of thing let's swap some knowledge (more like let me steal your knowledge on this subject because I have very little)
 

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J_Mack58

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Joined
Apr 7, 2021
Messages
145
I drained down cell 1 with a 1 ohm resistor, now I’m charging to see what action the BMS takes also to see if this thing does balancing.
 

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DG98

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Feb 4, 2021
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This is miles above my pay grade, but still terribly interesting to me -- I think we would all love to find BMS equipment (or components) that were as easily re-usable as battery cells. It'll just take guys like you doing the heavy lifting for the rest of us! Please keep the thread updated as you learn more, thanks!

Cheers, John
 

J_Mack58

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Joined
Apr 7, 2021
Messages
145
And just like that it's over before it got started. I learned I2C and SMBus communication . I learned more about the Raspberry Pi and I got me some Python programming in. But one thing I won't be getting is these battery packs to do what I want them to do which is work as a higher voltage and capacity with there individual BMS's monitoring the cells. The packs worked in series together, I series 6 of them for a 60 volt pack with only 2 Ah of capacity. Then i took just the cells from one pack and paralleled them to the cells on the BMS's board this would give me a 4ah pack. This is true except if you can't tell the BMS brain chip that the capacity is higher then it will only discharge the capacity it knows . The BQ20Z655 is a great chip but it has 3 modes Full Access, Unsealed and Sealed. If you "seal" the chip you have entered the mode of no return. What ever parameters that are programmed can't be changed. The Battery pack maker used the seal mode. There is another mode call "Ship" that I have been trying to get the chip in with no success. I joined the Texas Instrument group and may get a evaluation board of there newer BMS/Gas Gauge chip but as for this project I'm signing off.
 

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