Configurable relay and transistor output with programmable state: the relay can be programmed to react to low cell voltages either by energizing or by de-energizing the relay coil (fail-safe option) so the relay can be integrated in an emergency circuit.
The CVM has double isolation built in. Every VSU is isolated (in DC) from the main controller. As a consequence, a single isolation fault will have no adverse effect on the CVM or on the fuel-cell stack. The main controller is isolated towards the power supply and all outputs. So, even with a single fault, the stack will always remain isolated from other components in the system. Furthermore, if isolation components fail, they will do so in open circuit.
By default the CVM reports the average cell voltage and the current minimum and maximum cell voltage with the respective cell number in a high priority message.
Additionally, configurable data objects allow the user to switch on/off the transmission of detailed information containing the voltages of all cells. These messages have a lower priority.
All cell voltages are measured quasi simultaneously to avoid false alarms when the stack current changes rapidly.
All parameters are also user configurable by serial messages. These parameters include cell count, a node number to distinguish between CVMs on one bus and to determine serial message priority, idle time to limit serial traffic, two separate alarm thresholds with choice of operating mode and hour counter thresholds.
Data transmission is over isolated CAN (optionally : RS232, RS485). This allows deterministic messaging and adds to safety in the system in case of a failure, especially in a multi-stack installation.
The CAN protocol can coexist on a CANOpen network. The CAN ID mapping conforms to the CANOpen specification so the CVMs can be used on the same bus as other CANOpen nodes, e.g. an inverter. Also more than one CVM can be connected to the same system controller in case of multi-stack installations
Isolated digital outputs can be configured to operate in four different modes : normally open or closed, switch when a cell voltage drops below a fixed threshold or when it deviates from the average voltage. The fourth mode is for purge control.
An optional hydrogen purge control with configurable parameters relieves the main controller of the burden of controlling the purge valve as a function of cell voltage, time and current. This is particularly useful in the case of multi-stack installation where one purge valve per stack is used to limit hydrogen wasting. The CVMs will then coordinate their algorithms to avoid two valves opening simultaneously.
An connectivity check (unreleased feature) algorithm allows to check whether the individual cells and the CVM are properly connected.
The optional fixed cycle time can act as a watchdog for higher-level system controllers.
The standard input impedance of a cell voltage measurement is 10kΩ. This might be too low or too high for some applications. The input impedance can be set to 100kΩ (in production) or to 100Ω. The advantage of the very low impedance is that the fuel cells never reach high open circuit potentials and are all individually and equally “discharged” when the stack is stopped. The connectivity checker is not compatible with the 100Ω input impedance.
A LabView application is always supplied to help with commissioning of the CVM or the stack. This application can also serve as a starting point for programmers to develop the communication interface to the CVM on a PC or a PLC resulting in considerable time saving.
A stack ID can be stored in the CVM to enhance traceability.
Hour counters for open circuit operation and normal operation allow tracing back operating conditions.
An optional internal logger can store all measured data of many thousand hours of operation. This can eliminate the need for a PC connection in the installation. It allows easy logging in the field. The data is written onto an SD card that can be plugged into a standard card reader if one requires to retrieve the data e.g. after unexpected events.
An optionally available off-line application can be used to read and to analyze statistically and graphically the logged data describing cell and stack behaviour over time.