The DVC fuel cell module analyses the status of the different cells and activates an alarm in the case of a problem.
This system is based on miniaturised intelligent sensors (voltmeter type), which perform very precise measurements over a very extensive voltage range.
This module is developed in partnership with the Institut Vito (Vlaamse Instelling voor Technologisch Onderzoek).
Use: electric car, bus, electricity production plant (Solvay), etc.
CellSense FC: On-site Cell Voltage Monitoring
As a fuel cell manufacturer, system integrator or end user, you see individual cell voltage monitoring with mixed emotions:
- On one side it is comforting to know the stack and the balance of plant you are operating is doing well, and there is no better way to do this than with a CVM. Most likely you have designed for best performance, that is, highest possible cell voltages. So almost by definition, anything that goes wrong is reflected in a lower cell voltage somewhere in the system.
- On the other side you want to keep things simple to keep costs down. Additional hardware means additional chance of false alarms or bad contacts, additional maintenance etc.
We try to help you in making choice easier.
- We provide a stable CVM solution with proven reliability.
- We work on improving the benefits of a CVM, making it more performant and easier to integrate.
- We add functionality to CellSense so existing hardware can be replaced and no additional resources are required to connect CellSense.
- We reduce the inconveniences.
As time goes by “standard” solutions and technolgies will emerge in fuel cell stack integration. We will investigate these trends and continuously look for including maximum functionality in the CVM with minimum increase in hardware cost. This will make CellSense a “standard” component even in times where individual cells are all supposed to be identical.
Take a look at CellSense’s features to convince yourself. Of course, there will always be a price to pay but we can provide the CellSense solution at a price that is merely a fraction of the cost of the total fuel cell system. Economy of scale can bring this price down even further. This makes CellSense a great value-for-money CVM solution with a great perspective.
Learn more: www.cellsense.eu
What it is
Fuel cells are chemical reactors that transform hydrogen and oxygen to electric energy and heat. A single fuel cell produces its electricity at a voltage of less than one Volt. So, for practical applications fuel cells are connected or stacked in series to achieve high voltages. Generally, the gasses are supplied to the cells in a parallel arrangement.
A consequence of this setup is that the gas supply to the individual cells of a stack can differ while the current flowing through them is identical. This can result into large differences in cell voltages within one stack. When a cell produces current at a low voltage (compared to the other cells in the stack), this is a clear indication something is wrong in the operation of this cell.
Since the efficiency of a fuel cell is measured by the cell voltage, cells are always operated at the highest possible voltage. Consequently any deviation from this optimal situation, e.g., due to a failing component or control, will be translated to a lowering of cell voltages. Consequently, cell voltages indicate the correct and safe operation of a fuel cell stack.
The main problem, however, in measuring individual cell voltages is the presence of a common mode DC voltage that is different for every cell and changes in time. Of course solutions exist to cope with this, but they tend to be expensive. CellSense is a cell voltage monitoring device that can measure voltages of individual cells providing a solution to both problems. As a consequence it is well suited for monitoring commercial stacks and integration in control systems.
Many CVMs have been delivered and are in daily operation proving the reliability and robustness of the technology. Check out our references.
Technology used in CellSense is protected by a patent.
How it works
CellSense uses patent pending technology.
CellSense is built in a modular way. One component in this design is an analog to digital converter that converts the voltages of four adjacent cells to digital information. This is called a voltage scanning unit or VSU. Any number of VSUs are connected in parallel, one for every four cells in a stack. All these VSUs are powered over a common isolated power bus and they send and receive data over a common isolated data bus. The other component in the design is the main controller. Its task is to read the data generated by the VSUs, to treat this data and to generate messages for a higher level controller in the fuel cell installation.
There are several advantages in using multiple VSUs:
- Very modular design, very few unused channels
- High common mode voltage rejection
- Measurements are performed quasi simultaneously (within less than 0.5 msec)
- Very low component count to achieve low cost
- Great flexibility for PCB lay-out in case of custom design
Here is a block diagram of the CVM:
As pointed out, any deviation from optimal operation results in cell voltages dropping low. This means that malfunctions or unsafe situations can be detected by a CVM making it a effective and valuable component in safety, risk and operability assessments.
Possible advantages of the CVM are:
- Safety device to comply with risk assessment
- Lower gas stoichiometry (lower λ)
- Higher availability, better quality of electric power
- Less chance of damage
- Increased life expectancy
- Fast diagnosis in case of damage
- A complete history of on-site stack’s life
A detailed description of all features can be found in the user manual.
- 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.
- CAN bus specifications
- CANOpen specifications
- National Instruments LabVIEW
- National Instruments CAN Support
- Ground loop prevention
We can meet your CVM needs in two ways:
- A standard CellSense solution is available with hardware for 48, 96 or 144 cells. This way we support stacks with any number of cells up to 144.
We can build custom CellSense systems upon request with hardware for any number of cells and with a form factor defined by the customer in order to fit the CVM in tight spaces or close to the stack.
The table below shows the specifications for the standard system of the second generation of CVMs. Even in the standard system some options are available. In the second half of 2008 production of a second generation CVM will be started. This table will be updated when appropriate.The new or changed features of the second generation CVM are also listed.
|power requirement||< 30 mW / cell|
|power supply||18..32 V DC||yes|
|number of cells (hardware)||48, 96, 144||yes|
|conversion accuracy||10 mV|
|conversion rate||1000 cells / sec||yes|
|cell voltage range||-0.1 .. 1.1 V|
|measurement method||quasi simultaneous|
|cell voltage input impedance||10 kΩ||yes|
|Interface||CAN, 500kbps, 11 bit ID||yes|
|relay outputs||1 change over|
|relay contact rating||30VDC, 250VAC, 8A|
|open collector outputs||1 isolated|
|open collector output rating||50VDC, 150mA|
|isolation voltage (power)||1500 V|
|isolation voltage (CAN)||2500 V|
|isolation voltage (relay)||8000 V|
|isolation voltage (OC output)||2500 V|
|ambient temperature||-20..80 °C||yes|
|weight (excl. housing)||< 10 g / cell|
|housing||160 x 80 x 120||yes|
|cell connection interface||D25 connector per 24 cells||yes|
|internal data logger||2GB SD card||yes|
|These specifications are subject to change without notice|
CellSense is certified to CE. The certificate containing detailed information can be downloaded. Note that LVD certification is done for all CVMs, EMC certifications apply to the CVM built into a suitable housing.
The cell voltages are constant but the values as measured by the CVM fluctuate in a regular pattern.
The probable cause is a ground loop. The CVM is very well isolated on different levels but only in DC. Ground loops can exist in AC only. Also AC currents may flow through the stack if it is connected to equipment that is connected to the grid, e.g., an inverter, oscilloscope or electronic load. Often the problem can be remedied by adding an isolation transformer in one or more grid connections.
Can any spare inputs be used for other purposes than for cell voltage measurements?
Yes, but a number of issues should be considered. How will the additional sensors be powered and how will this relate to the isolation ? How will you set the stacks cell count ? If it includes the additional sensors, the minimum voltage will be calculated using also the sensor output. Will the sensor output range be OK ? If you need additional analog inputs, contact us.
Can the data traffic generated by the CVM on the CAN bus be limited?
Yes it can. There are two ways to do this : the cycle time can be modified or the transmission of individual cell voltages can be switched off.
Can CVM messages be given a lower priority on a CAN bus with vital traffic on it?
Yes. The approach to this is to figure out which node on your CAN bus transmits messages with the highest priority. If it is a CANOpen node, it should have a very low node number, e.g. zero. It will then produce message with CAN ID 180(hex). Assigning node number one to a CVM will have it transmit messages with ID 181(hex) which is a lower priority than ID 180(hex). Refer to the CAN specifications for detailed information.
Can I connect the CVM’s main controller to a known potential?
No you can’t, all connections to the CVM’s main controller are isolated. However, if you are working with high potentials (several hundred cells in series), this might be safer than isolating only. Contact us for assistance if you are in this situation.
Which interface should I choose?
The CVM comes standard with a CAN interface for a reason. Compared to RS232 or conventional RS485 the bandwidth of a CAN bus is much larger and it works over longer distances. Also it implements message prioritization and collision detection in the hardware making it a real time system. RS232 supports only point-to-point connections so only one CVM can be connected to one port. Also for the software development on the side of the main controller, CAN has its advantages as it is message oriented. RS232 or RS485 are character oriented communication standards which means programmers must implement the character to message algorithms themselves.
A system is of 4 stacks of about 100 cells each. All those stacks will be connected in serial configuration which means a maximum voltage of more than 400V between the last cell and the ground. Does the cellsense technology comply with that requirement in terms of insulation, because the sensing side can withstand only up to 300V?
Yes, it can comply to this requirement in several ways : multiple systems can be combined (e.g. one per stack). This would be a logical way to go as this would provide you with a serperate set of data per stack (e.g. minimum, maximum, average voltage, temperature). A second way is to connect the center point of the stacks (+ terminal of stack 2) to the CVM controller yielding + and – 200V common mode voltages. This may not be acceptable if you require isolation at this level. A third way is to replace certain electronic components with components with a higher dielectric rating. This is probably the most expensive way.
We might connect this system to the grid without galvanic insulation. Therefore, does CellSense FC withstand with a 2500Veff 50Hz dielectric test for 60 seconds between one sensing point and the ground?
All outputs of the CVM comply to this spec. The power supply does not (1500VDC) but, assuming you would use the same isolated power supply for the CVM and your FC controller, this would solve this limitation.