Batteries are expensive and fragile. I'd like to solve both of these problems simultaneously.
The idea is a ruggedized battery module that can be used either stand-alone or stacked with additional modules in series and/or parallel. Each module is self-contained and includes over-current protection, under-voltage, over-voltage, over-temp, under-temp as well as passive cell balancing.
By utilizing the same module for multiple tasks the per-cycle cost can be decreased. Take an electric lawnmower as an example. Since a typical usage pattern would be for 6 months out of the year, and only 1 day per week the total yearly cycles would total only 24. In this application it would be impossible to utilize all ~1000 cycles the battery would be capable of before the age based degradation destroys the battery. In fact only a few percent of the batteries potential would be used.
Regarding the Battery Management we need following functions :
At cell group level (one group = 24 cells wired in //)
Voltage monitoring with UV and OV detection - 6 Volts full scale - 2 mV resolution
Temperature monitoring UT and OT detection - -40°C / + 85°C (-49 °F / +185°F) scale - 0.1 °C (0.2 °F) resolution.
Passive balancing - Power: To Be Defined
At stack level ( = complete stack of 14 groups in series)
Bidirectional current measuring of battery current with SOC calculation - +/- 200 A - 100 mA resolution
Unidirectional current measuring of charging current (MICHEL) - + 100 A - 50 mA resolution
Voltage measuring - 80 Volt FS - 20 mV resolution
Main CB control
Inverter control (MICHEL)
CAN bus Communication (MICHEL)
Serial bus for lighting control (MICHEL)
Long term storage of all battery parameters and important events
Embedded webserver for user interaction
Consumption during active mode:
During active mode, the BMS monitors the battery parameters, logs them and insures battery safety.
During active mode, the consumption of the BMS shall be less than 50 Wh per day.
It shall be possible to disable / enable the BMS without having to disconnect wires. When disabled, the BMS is in storage mode.
Consumption in storage mode
During storage mode, the BMS does not perform any measurement / logging of the battery parameters.
When storage mode, the consumption of the BMS shall be less than 1 Wh per day. During storage mode a 4 KWh battery looses less than 10 % of its capacity when left sitting uncharged during one year (To Be Confirmed).
The main concern regarding Lithium batteries is overcharging. Overcharging can lead to degasing and even fire. Overcharging is avoided thanks to the passive balancing feature of the BMS. If this feature fails unexpectedly, one or more cell groups may get overcharged without CB tripping or warning. This would lead to a dangerous situation.
Following unexpected events may lead to battery overcharging / overheating
Undetected erroneous cell group voltage measuring (voltage reading is too low).
Undetected erroneous cell group temperature measuring (temp reading is too low).
In order to mitigate these risks, the design shall include redundancy and/or built-in test mechanisms.
The cell voltage measuring inputs shall withstand 60 VDC during 1 minute without degradation
The cell temperature measuring inputs shall withstand 60 VDC during 1 minute without degradation
The current measuring inputs shall withstand 60 VDC during 1 minute without degradation
The BMS main board power supply input shall withstand 60 VDC during 1 minute without degradation
The BMS shall be fully functionnal between 0 °C and 70 °C (32°F / 160 °F)
The BMS shall be designed to function 24 hours/day - 365 days/year.
The BMS is intended to be used by JAMES and MICHEL in their personal applications. The BMS will be used and maintained by them during several years. Following design rules apply:
Use of obsolete or "hard to find" components or hardware shall be avoided
Use of components that can be found easily in Europe and in the USA is required
Minimise the complexity of wiring, testing and reduce mainenance effort
Include indicators and troubleshooting means that allow global health diagnostic "at a glance"
Software design shall be based on well known standards like C, HTML, CSS , PYTHON, LINUX
NOTE: the requirements specific to Jame's application and Michel's application of this project are tagged (JAMES) / (MICHEL) respectively.