Tough BMS
Posted: Thu Aug 01, 2019 11:31 am
I have used another distributed BMS for a long time now. Surprisingly i had absolutely NO issues with it.
It was designed to dissipate energy from cell at 4V per cell. Not 4.2V but 4V since this way i am assured of longer cell life and from 4V to 4.2V it is like 30minutes of charging. During that time even the dummest charger starts to go to CV.
I used TL431 reference diodes to trigger PNP/NPN pair. NPN transistor is actually NFET capable of 5A continuous drain. However i noticed even 1A in closed space can be a fire hazard. So i use 0.5A drain. It works in my car for 4 years now. No events
Also i designed a LV alert into each module. I used Microchip TC54 voltage trigger to signal EMPTY at 3V per cell. While there is some power still in the cell at 3V it can drop to 2.5V in under a minute! I use optocooupler PC817 to signal events for a single cell. Currently i have this alarm set so it lights red LED on my dash. But i could just as easily connect BMS pin of Rev 2 inverter to put the car in "turtle mode".
There is also provision to use HV alert when a single cell reaches 4V BMS loop can slow the charger to 1A so that all cells align.
I tried to eliminate PNP transistor, but i found out its base hysteresis keeps TL431 from turning on too fast. This was designed explicidely to work with LiPo cells. It is a rather crude system with low parts count, but it is also EMI proof and reliable. I made 8S boards because i use cells in packs of 8.
You are free to use this system and further develop it if you like.
It was designed to dissipate energy from cell at 4V per cell. Not 4.2V but 4V since this way i am assured of longer cell life and from 4V to 4.2V it is like 30minutes of charging. During that time even the dummest charger starts to go to CV.
I used TL431 reference diodes to trigger PNP/NPN pair. NPN transistor is actually NFET capable of 5A continuous drain. However i noticed even 1A in closed space can be a fire hazard. So i use 0.5A drain. It works in my car for 4 years now. No events
Also i designed a LV alert into each module. I used Microchip TC54 voltage trigger to signal EMPTY at 3V per cell. While there is some power still in the cell at 3V it can drop to 2.5V in under a minute! I use optocooupler PC817 to signal events for a single cell. Currently i have this alarm set so it lights red LED on my dash. But i could just as easily connect BMS pin of Rev 2 inverter to put the car in "turtle mode".
There is also provision to use HV alert when a single cell reaches 4V BMS loop can slow the charger to 1A so that all cells align.
I tried to eliminate PNP transistor, but i found out its base hysteresis keeps TL431 from turning on too fast. This was designed explicidely to work with LiPo cells. It is a rather crude system with low parts count, but it is also EMI proof and reliable. I made 8S boards because i use cells in packs of 8.
You are free to use this system and further develop it if you like.