MAXIMUM_AMPS
10 mW
- Joined
- Nov 13, 2019
- Messages
- 33
Hello folks, I am an embedded systems engineer. I design and program all kinds of electronics, as a hobby and a profession. Electronics from realtime control units that run constantly for years at a time, to "simple" electronics like BMS boards.
Battery Management Systems are safe and convenient when they actually work, but on the flip-side, can be outright dangerous when they aren't protecting and balancing every cell.
To get my point across more concisely, I will show the schematic for a common, cheap and dangerous li-ion BMS. This design is used in the infamous, fire-prone hoverboard batteries: View attachment 1
This is a very unsafe design for 3 reasons:
1. Each of the protection chips (DW01A) is separate, there is no cooperation. Meaning: one chip could fail, due to ESD, water droplet, etc. Then that cell group is completely unprotected, opening up the possibility that the cell group could become overcharged and explode unexpectedly! With a 2-wire monolithic design, if one part fails, due to ESD, etc, the output FETs are turned off PERMANENTLY. This is much safer, as charge can no longer flow in or out. Crisis averted, while also alerting the user of the failure.
2. For exactly this reason, the manufacturer states clearly that the DW01 is a 1S ONLY protection chip. Using them as a 4S, 10S, 14S, etc is NOT RECOMMENDED because of this safety hazard! That's what the "01" in DW01 stands for! Here is the datasheet confirming my claim: http://www.kexin.com.cn/pdf/DW01.PDF
3. The protection and balancing chips are separate, leading to a condition where the balance chip fails, and the cell group is no longer able to bleed overcharge anymore. This is less dangerous, but still a design flaw stemming from using a 1S protection chip incorrectly. Proper multi-cell chips will perform balancing as well as protection.
Here's an example of an e-bike BMS using the exact same design but extended to 10S. I've seen clones of this circuit expanded to 13S, 14S even 20S, and are as cheap and abundant as they are unsafe.
DO NOT risk them. Buy a proper, high quality 2-wire BMS with a monolithic multi-cell IC. They cost more, but are are 1000x safer and cheaper than a fire!
When working with electronics, ESPECIALLY a safety critical item like a BMS, be sure to practice proper ESD precautions! This can make the difference between a house fire and no house fire!
Battery Management Systems are safe and convenient when they actually work, but on the flip-side, can be outright dangerous when they aren't protecting and balancing every cell.
To get my point across more concisely, I will show the schematic for a common, cheap and dangerous li-ion BMS. This design is used in the infamous, fire-prone hoverboard batteries: View attachment 1
This is a very unsafe design for 3 reasons:
1. Each of the protection chips (DW01A) is separate, there is no cooperation. Meaning: one chip could fail, due to ESD, water droplet, etc. Then that cell group is completely unprotected, opening up the possibility that the cell group could become overcharged and explode unexpectedly! With a 2-wire monolithic design, if one part fails, due to ESD, etc, the output FETs are turned off PERMANENTLY. This is much safer, as charge can no longer flow in or out. Crisis averted, while also alerting the user of the failure.
2. For exactly this reason, the manufacturer states clearly that the DW01 is a 1S ONLY protection chip. Using them as a 4S, 10S, 14S, etc is NOT RECOMMENDED because of this safety hazard! That's what the "01" in DW01 stands for! Here is the datasheet confirming my claim: http://www.kexin.com.cn/pdf/DW01.PDF
3. The protection and balancing chips are separate, leading to a condition where the balance chip fails, and the cell group is no longer able to bleed overcharge anymore. This is less dangerous, but still a design flaw stemming from using a 1S protection chip incorrectly. Proper multi-cell chips will perform balancing as well as protection.
Here's an example of an e-bike BMS using the exact same design but extended to 10S. I've seen clones of this circuit expanded to 13S, 14S even 20S, and are as cheap and abundant as they are unsafe.
DO NOT risk them. Buy a proper, high quality 2-wire BMS with a monolithic multi-cell IC. They cost more, but are are 1000x safer and cheaper than a fire!
When working with electronics, ESPECIALLY a safety critical item like a BMS, be sure to practice proper ESD precautions! This can make the difference between a house fire and no house fire!