Electronics > Projects, Designs, and Technical Stuff
Intelligent BMS (Battery Management System)
JohnnyBerg:
I started a new project, a intelligent BMS (Battery Management System). It is a max. 7S system, that can be stacked to any number of cells. The system is able to monitor any chemistry that is in the range 2.5 ~ 4.6 Volt
My objective is to enhance the live of the batteries. That is done by not charging them to the maximum voltage, and not discharging them to the minimum voltage. I aim for over 2000 cycles, instead of the usual 500 or less.
This is what it is supposed to do:
- Measure voltage of each cell, with a accuracy under +/- 0.1% with a resolution of < 1 mV. Max. voltage 4.6 Volt, min Voltage 2.5 Volt.
- Measure the in-going / out-going current, maximum 50A, with a accuracy under +/- 0.2% with a resolution of < 25mA.
- Cell balance, with a maximum current of 2A, fine tuned over PWM.
- Auto detect the configuration, anything from 2S to 7S.
- Accurate state of charge/discharge over Coulomb counting, in combination with monitoring cell voltage.
- Switch of the load with a maximum of 200A, under programmable conditions:
- over voltage
- under voltage
- over current
- and any combination of the above
- Quiescent current from each cell < 40 uA, except cell 1, maximum 300uA
- Bluetooth low energy connection in master and slave, to stack units. Over the Bluetooth interface, a Android or PC app can monitor the state of the BMS, like total voltage, condition of each cell, state of charge and so on. When the cells are charged/discharged with a variable load, the internal resistance of each cell can be calculated.
I plan to post regular updates on project progress. The PCB is being manufactured and assembled, in the meanwhile I start working on the software.
aiq25:
Interesting project.
Are you looking to use a battery management IC along with a microcontroller or do it all using a micro?
Siwastaja:
Sounds like a numerical overspecification. 0.1% voltage measurement accuracy is hard to reach reliably, or will be expensive, and won't deliver any benefit whatsoever regarding SoC estimation or cell life. Since li-ion capacity and lifetime is limited by the cells and specifically cell cost, build a lower-cost BMS and use the saved budget on more cells, to build a better performing battery (as a whole).
Another example, 2A balancing current requires careful thermal analysis, heatsinking design, will add clearly non-zero cost and weight, while provides no benefit whatsoever, unless your packs are truly massive (1000Ah+?). There is a reason why commercial BMS's tend to limit to around 0.2-0.5A, and why I designed mine to balance at 40mA, using an intelligent algorithm dividing the balancing time instead of brute-forcing it.
I would try to dedicate all efforts primarily to make they system as robust and fail-safe as possible (simplicity is one of the keys), and secondarily, study (by literature research, or own measurements) which mechanisms are important for cell aging, and which aren't. This depends on the actual cell, as well, so it's best to choose a few you "support" best, and study how they perform.
Example points regarding robustness:
Look for stuck-on balancing, possible heat generation and thermal fusing in worst case firmware/logic failure.
Example points regarding cell aging:
Tapering charging current before the CV voltage may easily increase the cell life by 10x (say, from 100 cycles to 1000 cycles), with only small impact on total charging time.
Similarly, adjusting charging current based on temperature, or heating/cooling battery properly, may easily increase the cell life by 10x (Say, from 100 cycles to 1000 cycles). And no, this may mean you need to heat the battery above room temp, instead of cooling, in certain conditions.
JohnnyBerg:
--- Quote from: aiq25 on November 08, 2019, 02:08:53 pm ---Interesting project.
Are you looking to use a battery management IC along with a microcontroller or do it all using a micro?
--- End quote ---
It is based on a a microcontroller.
JohnnyBerg:
--- Quote from: Siwastaja on November 08, 2019, 05:27:46 pm ---Sounds like a numerical overspecification. 0.1% voltage measurement accuracy is hard to reach reliably, or will be expensive, and won't deliver any benefit whatsoever regarding SoC estimation or cell life. Since li-ion capacity and lifetime is limited by the cells and specifically cell cost, build a lower-cost BMS and use the saved budget on more cells, to build a better performing battery (as a whole).
--- End quote ---
hmm .. limiting cell loading at 4.10V with a measuring accuracy of +/- 0.1% gives a range of a little more than 8 mV. So voltage is between 4.096 and 4.104 V
For me a compromise ;)
--- Quote ---Another example, 2A balancing current requires careful thermal analysis, heatsinking design, will add clearly non-zero cost and weight, while provides no benefit whatsoever, unless your packs are truly massive (1000Ah+?). There is a reason why commercial BMS's tend to limit to around 0.2-0.5A, and why I designed mine to balance at 40mA, using an intelligent algorithm dividing the balancing time instead of brute-forcing it.
--- End quote ---
Brute force seems a bad idea ;)
Cell balance current is max 2 A, and can be adjusted over PWM. Furthermore, the intelligent part can determine a algorithm for balancing, mixing thermal considerations versus time and number of cells to be balanced.
As you said, reducing the cell end loading voltage with 0.1V has a dramatic increase on the life time. So has discharging voltage, charge current and discharge current. So to achieve the actual life time enhancement, one must be able to do measurements with a certain accuracy.
Anyway, I start working on the firmware and app next week, after doing some basic tests in the last weeks.
I already played with Bluetooth Low Energy, which give some nice high end features. Besides monitoring and logging, one could think of firmware updates and in case of automotive and e-e-bike/e-motor/e-scooter operations a unlock code, to prevent usage in case of theft.
The Bluetooth module can operate in master or slave mode. The app connects with the master, and in a network of BMS-es als slaves report to the master, giving a opportunity to build many different configurations, in series or parallel.
I thought of a GPS module for tracking, and locking on distance. Perhaps in a next version ..
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