4. Monitored parameters¶
This section describes the parameters monitored by foxBMS. The parameters are monitored periodically, using thediag
module. A counter is dedicated to each parameter. Each time an error condition is detected, the corresponding counter is increased, otherwise it is decreased. If the threshold configured for the parameter is exceeded by the counter, an error occured and the BMS takes an appropriate action. This mechanism avoids false error detections, e.g. in case of short peaks. As the parameters are monitored periodically via the periodic tasks, the counter threshold corresponds to the response time between error happening and error detection. A default response time of 100ms is selected as a trade-off between peak detection and safety. It must be adapted to the application. foxBMS provides three different error levels. The first error level is called Maximum operating limit (MOL), the second error level is called Recommended Safety Limit (RSL) and the third error level is called Maximum Safety Limit (MSL). The first two error levels indicate that a parameter is reaching the limits of the recommended operating area and counter measures should be initiated to prevent an unwanted opening of the contactors. A violation of a Maximum Safety Limit means the safety of the system and the persons cannot be guaranted anymore and leads to the opening of the contactors. The BMS state machine consequently switches to the Error State to prevent a further closing of the contactors. The Error State can only be left if the cause of the error has been removed and the Standby State is requested. After that the BMS is back in normal operational mode. All error flags are available on CAN messages, allowing the system to react accordingly to prevent hazardous situations.
4.1. Battery Cell¶
4.1.1. Parameter: Cell voltage¶
Error condition | Response time | Cause | Counter measures | Actions of the BMS |
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Cell voltage > overvoltage limit | 100 ms | Overcharging (derating error, charger defect) |
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Cell voltage < undervoltage limit | 100 ms | Deep-discharge (derating error, high standby/ leakage current) |
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4.1.2. Parameter: Cell temperature¶
Error condition | Response time | Cause | Counter measures | Actions of the BMS |
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Cell temperature > overtemperature limit charge | 100 ms |
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Cell temperature > overtemperature limit discharge | 100 ms |
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Cell temperature < undertemperature limit charge | 500 ms | Ambient temperature |
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Cell temperature < undertemperature limit discharge | 100 ms | Ambient temperature |
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4.1.3. Parameter: Cell current¶
Error condition | Response time | Cause | Counter measures | Actions of the BMS |
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Cell current > maximum charge current | 100 ms |
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Reduce charge current |
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Cell current > maximum discharge current | 100 ms |
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Reduce discharge current |
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4.2. System¶
4.2.1. Parameter: Battery system current¶
Error condition | Response time | Cause | Counter measures | Actions of the BMS |
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Current > precharge current limit | no response time | Precharge resistor defect | – |
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Contactors opened/closed under current | no response time | System failure detected | – | Set flag that contactors were operated under current (10A) |
4.2.2. Parameter: HV measurement¶
Error condition | Response time | Cause | Counter measures | Actions of the BMS |
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Voltage difference(Vbat - Vdc,link) > precharge voltage limit | No response time |
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Fuse tripped (Vbat - Vfuse) > limit | 100 ms | Fuse tripped | Replace fuse | Set warning flag that tripped fuse is detected (10000mV) |
Voltage difference (Vfuse - Vdc,link) < limit | 100 ms | Precharge contactor defect | Check contactor | Measurement only done when contactors are open. Set error flag that defect precharge contactor is detected (2000mV), switch to Error State |
4.3. Contactor feedback¶
Error condition | Response time | Cause | Counter measures | Actions of the BMS |
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Contactor feedback unequal to requested state | 100 ms |
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Check contactor | Set warning flag that measured contactor feedback is different from requested state. switch to Error State |
4.4. CAN communication¶
Error condition | Response time | Cause | Counter measures | Actions of the BMS |
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No CAN staterequest message (ID: 0x120) on CAN0 received | 1000 ms |
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Set warning flag if state request message (0x108) is not received in an interval of 100ms +/- 5ms. Switch to Error State. |
No CAN currentsensor messages on CAN0 received | 1000 ms |
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Set warning flag if for 200ms no new current sensor message received and switch to Error State |
4.5. Slave communication¶
Error condition | Response time | Cause | Counter measures | Actions of the BMS |
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Daisy-chain communication not working | 5 ms |
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Set warning flag that daisy-chain communication fails and switch to Error State |
4.6. Open wire check¶
Error condition | Response time | Cause | Counter measures | Actions of the BMS |
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Cell voltage open-wire check detects error | No response time | Wiring error/defect | Check voltage sense wiring | Set warning flag that open wire is detected and switch to Error State |
4.7. Master¶
4.7.1. Parameter: MCU die temperature¶
Error condition | Response time | Cause | Counter measures | Actions of the BMS |
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MCU die temperature > maximum junction temperature | 1000 ms | Ambient temperature | Reduce thermal stress | Set warning flag that maximum junction temperature is violated (T > 105 °C, T < -40 °C) |
4.7.2. Parameter: Coin cell voltage¶
Error condition | Response time | Cause | Counter measures | Actions of the BMS |
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Coin cell voltage < minimum cell voltage limit | 1000 ms | Coin cell discharged at end of life | Replace coin cell if voltage is below threshold | Set warning flag that minimum coin cell voltage threshold is violated to prevent loss of data (2200mV). Typical lifetime of a coin cell is 2-3 years. |
4.8. Software checks¶
4.8.1. Plausibility check¶
Error condition | Response time | Cause | Counter measures | Actions of the BMS |
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Measurement timestamps (I, V, T) not upated | 1000 ms |
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Set warning flag that measurement timestamps are not updated periodically (200ms) and switch to Error State |
Cell temperature < below measurement range | 100 ms | T-Sensor short circuit |
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Cell temperature > above measurement range | 100 ms | T-Sensor disconnected |
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Cell voltage > above measurement range | 300 ms |
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Cell voltage deviation from average cell voltage too large | 300 ms |
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Difference between pack voltage and LTC / current sensor measurement too high | 100 ms |
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Set measurement value invalid because Batterypack voltage and LTC / current sensor voltage is too high (3000 mV) |