Other Parts Discussed in Thread: BQ76920, , TIDA-00792, BQ78350, BQ76200
Tool/software:
Dear Sir/Madam,
This is a 5S2P battery pack, 18V/5000mAh/90Wh. Single cell is 3.6V/2550mAh.
The BMS uses BQ78350-R1A and BQ76920.
There’s SMBUS communication with host MCU, so protection switching uses high side N-channel field-effect transistors to maintain low-side referenced communication even during protection, referring to TIDA-00792.
We have wake-up circuit from shutdown mode, referring to TIDA-00792 Page 7, 2.2.8 Sleep and Wake-up Circuit, Figure 5, PACK+ Boot Circuit.
Attached PDF is the schematics.
This is our setting:
Over charge voltage: 4.2V * 5 = 21V
Charge voltage: 20.815V
Over discharge voltage: 3V * 5 = 15V
Shutdown voltage: 2.8V * 5 = 14V
Permanent Fail voltage: 1.8V * 5 = 9V
We expect that, when the battery pack is stored quite long time, so the cell voltage is slowly discharged from 3V (over discharge to sleep) to 2.8V(sleep to shutdown), then to 1.8V (PF voltage), then to a quite low voltage, e.g. 0.3V (which we think it’s not safe to charge again), plugging in the power adapter, the battery pack cannot be charged, otherwise the battery pack may fire or explode.
However, we noticed that:
- The wake-up circuit can still wake up the BMS from shutdown mode and can charge 0.3V cell !!!
- Only when the cell voltage drops from 3V to 1.6V or lower very quickly, the cell cannot be charged again, but the BMS doesn’t enter shutdown mode.
Our questions:
- Above 1&2 is expected behaviour of this design or not?
- How to disable charging the cell lower than 1.8V even it’s slowly discharged to (but not quickly)? We don’t want to charge the battery pack with extremely low cell voltage, which may induce fire or explosion.
