Electronics > Beginners
Learning Path for buiding my own BMS
redgear:
Hey there!
I am from a completely non-technical background. I decided to build my own e-bike and this month has been a lot of learning... I learnt many new things calculating power requirements, mechanical calculations, battery pack sizing, motor controllers, etc.
I am now in the stage of selecting a Battery Management System for my build. This area interests me so much that I plan to build one myself from scratch.
My BMS should have the following features:
-Overcharge Protection
-Overdischarge Protection
-Overvoltage
-Undervoltage
-Thermal Runaway
-Cell Balancing
-Charging (implement a modified CC-CV algorithm based on a research paper)
-SOC
-SOH
- Secondary protector fault detection
-Total voltage, voltages of individual cells, minimum and maximum cell voltage
-Current in or out of the battery
-Maximum charge current as a charge current limit (CCL)
-Maximum discharge current as a discharge current limit (DCL)
-Energy [kWh] delivered since last charge or charge cycle
-Internal impedance/resistance of a cell (to determine open circuit voltage)
-Charge [Ah] delivered or stored
-Total energy delivered since first use
-Total operating time since first use
-Total number of cycles
-Average Temperature, Temperature of each cell (switching on air cooling when temperature exceeds a value)
-Communicate all data to be nicely visualised on a smartphone using bluetooth
I looked up on the internet and understood that a Fuel Gauge can monitor these things. I decided to go with TI BQ78350-R1, with a TI BQ76930 AFE. Is it a right choice? Are there any others that I should look into? Open for suggestions. My battery pack is a 6S3P pack.
I think, these require a MCU to work and I haven't decided one yet. Open for suggestions.
I looked up on the reference designs but I am not able to understand anything. It is all too much for me RN.
Should I use a single cell gauge or a multi-cell gauge? Are there any advantages of one over the other?
My questions is how do I start? What all things I should learn before I can design a BMS myself? I have completed All about circuits Vol I book and have now started reading The Art of Electronics.
I can put in 6hr/day. How long would it take me to build circuits like that? What learning path should I follow?
Thanks a lot!
TL;DR: What should be the learning path for a beginner in electronics to be able to design Battery Management System from scratch?
ogden:
--- Quote from: redgear on July 24, 2019, 06:58:22 am ---I looked up on the reference designs but I am not able to understand anything. It is all too much for me RN.
--- End quote ---
If you are beginner who do not understand anything about simple reference designs of battery management IC's, you are guaranteed to fail building complex high current BMS on your own. I am afraid that you shall rethink your plans, split your project into steps of incremental complexity, plan more than one version of your bike, build first prototype using off-the-shelf components. *AFTER* first version of the bike is running, you possibly will understand more about electronics, have better chances of writing sensible specs and planning your build and maybe even succeed building something that does not melt of burst into flames.
Before you plan to build moon rocket - train your skills building paper planes first.
--- Quote ---TL;DR: What should be the learning path for a beginner in electronics to be able to design Battery Management System from scratch?
--- End quote ---
First rule: plan your builds according to your *current* knowledge. You can't pick project that is two orders of magnitude out of your skill/knowledge set and think that you will be able to succeed. Perhaps you shall start with BMS for pocket flashlight, toy car or something of similar scale.
redgear:
Thanks! Yep, That's what I am considering RN... Splitting the project into milestones and learning things one by one. I am also not sure if I should just jump in and start doing projects or learn some theory before that.
Siwastaja:
My personal stance:
Most li-ion management ICs do very poor job at making the designer's life easy. They have their own faults (or some are outright broken-by-design and dangerous in unobvious ways), so you need to understand them and babysit them.
I tend to prefer doing everything from scratch; at least then I know what's going on.
I have had to make one product recall (luckily only <5 units) due to a broken-by-design TI BQ li-ion charger IC with an internally faulty overvoltage protection. Inaccurate application note I followed contributed to the problem; this part was actually my fault, but I want to point out even I as an experienced designer fell into the trap of copying parts of the appnote; it happens to a beginner even more easily.
As a result, I'm never putting a BQ anything in any li-ion product I design, ever. The trust is completely gone. They tend not to have any safety certificates either, so no bonus for paperwork - you are still fully responsible to the same degree as in a completely home-brew system.
An integrated product would make everything easier, but then it would need to be a really proper integrated solution, not some TI-acquired BQ shit IC which fails and requires as much control code as a full-blown BMS control algorithm.
My personal learning path has been:
* Looked at overly complex redistributive balancing schemes, built prototypes
* Teardowns of failed commercial BMS units, recognized the usual issue: too much quiescent current, possibly because being stuck in the wrong state, killing the cells
* Designed an overly simplified distributed BMS system. Still a valid design
* Tried two TI BQ ICs in another product, trying to "reduce development time" since I had much more to do than just battery management. An utter failure. Removed all TI BQ ICs for production PCBs, redid from scratch.
Additional points:
--- Quote ----Overcharge Protection
...
-Overvoltage
-Overdischarge Protection
...
-Undervoltage
--- End quote ---
What's the difference?
--- Quote ----Thermal Runaway
--- End quote ---
I assume you mean temperature monitoring, and specifically the upper limit.
Do not forget to add lower limit for charging. Limit charging currents below 25degC linearly, until you reach zero current at about -5 degC. (Traditionally a step function of no charging below 0 degC, full charging above 0 degC is used. I don't recommend it, as full charging currents at +1 degC can be damaging to the cell lifetime.)
You can't stop thermal runaway if it happens, and it's almost never happening due to anything that can be prevented by BMS temperature monitoring. The only such scenario I can think about is totally failing the thermal design of the pack, or completely miscalculating the fuse rating. Don't forget the traditional fuse!
So thermal runaways happen due to internal cell failures, physical damage (puncture/deformation), severe overcharge (which you have already covered by voltage monitoring).
--- Quote ---Cell Balancing
--- End quote ---
Remember you need very little balancing, and not much muscle to do it. I have seen systems designed to shunt currents in excess of 1A, and fail at thermal design with stuck-on balancer. Don't do that. My system balanced at 40mA, and was still fully able to keep a repurposed 300Ah pack with damaged cells with increased self-discharge in balance.
--- Quote ---SOH
--- End quote ---
I haven't seen any formal, scientifically valid definition for this. I have a feeling it was a trendy buzzword in battery academic papers a decade ago.
--- Quote ----Maximum charge current as a charge current limit (CCL)
--- End quote ---
Remember to add cell temperature dependency to this. Charging does less damage at higher temperatures. Somewhere around 30-40 degC is optimum. I have measured greatly decreased cycle lives when cycled in a refrigerator (at +6 degC) at the rated current.
--- Quote ---Total number of cycles
--- End quote ---
Cycle counting can be difficult, as the cycles are of different length, and with different starting points.
A cycle between 60%->30%->60% is almost meaningless for the battery life. A cycle of 100%->70%->100% if a lot worse. A cycle of 100%->10%->100% is not that much worse anymore.
redgear:
--- Quote from: Siwastaja on July 24, 2019, 08:17:28 am ---An integrated product would make everything easier, but then it would need to be a really proper integrated solution, not some TI-acquired BQ shit IC which fails and requires as much control code as a full-blown BMS control algorithm.
--- End quote ---
Can you recommend any other alternative? I read good reviews about them and that's why I chose them. Thanks for letting me know.
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