BATTERY MONITORING SYSTEM

Abstract: An electronically controlled system for monitoring a battery bank, which has associated with each battery or small group of batteries or cells a measurement module which among other things measures the cell voltage and cell temperature, digitally encodes and serially transmits these values via an isolated bus to a central processing unit (CPU) which stores and analyses the values and provides an indication of the status of each battery and produces alarms when appropriate. Each measuring module will be kept in close physical proximity to the batteries it measures such that the temperature measurement requires no extra outboard circuitry and electrical isolation between the monitoring system and the battery voltage is provided as close to each battery cell as possible. A number of measurement modules may be connected to common buses and each central processing unit may receive inputs from more than one bus.

Description: BATTERY MONITORING SYSTEM TECHNICAL FIELD This invention relates to a system for monitoring a bank of batteries. BACKGROUND ART Batteries, or more often banks of batteries, are often used as standby power supplies. When the mains are removed or are not present such standby batteries will take the load. Batteries may also be used for backup and starting a backup generator. Batteries have failure modes and reliability problems. This means they must be continually monitored in these applications. Monitoring will determine the capacity and the health of a battery or battery bank. This information provides an indication as to how long a battery system will function when the mains are out. A battery may be monitored by measuring the voltage while under charge. Several failure modes can be detected. More can be learnt about the state of a battery or battery bank by discharging the battery or battery bank with a constant current or constant power load. By discharging a bank the capacity of the bank can be determined and most other battery failure modes will be detected. Battery voltage and temperature indicate the health of the battery under discharge conditions. By discharging a bank according to the battery manufacturer specifications a determination can be made whether batteries are still within specification. An example of a known battery monitoring system is disclosed in US Patent 4484140. In that patent a digital device monitors the voltage across each battery sequentially by using relays to switch one cell at a time across a common two line bus which forms the input to an analogue-to-digital converter. An isolation amplifier is used at the input of the analogue-to-digital converter. Such systems have the disadvantage that the bus will be long with any sizeable bank of batteries and will not be isolated. It will be appreciated that the DC potential between either bus line to ground may be very high. Further, such a system does not measure battery temperature. DISCLOSURE OF INVENTION It is an object of the present invention to provide a battery monitoring system which goes some way towards overcoming the abovementioned disadvantages. Accordingly in one aspect the present invention consists in a battery measunng - module for a system for monitoring a plurality of batteries wherein said measuring module is associated with and measures parameters of one or more of a small number of physically proximate batteries, said measuring module comprising: at least one analogue input port which in use is connected to at least one battery, an analogue-to-digital converter connected to said at least one input port which produces digital values proportional to the voltage on said at least one input port, a device which stores said digital values, and a digital output port, said device presenting said digital values in serial form at the digital output port. In a second aspect the invention consists in a monitoring system for a bank of batteries comprising: a digital central processing unit which is capable of storing and processing digital parameters for each battery in the bank, said central processing unit having one or more serial input-output ports, one or more data buses, each connected to a respective serial input-output port on said central processing unit, and, connected to and in association with one or more of a small number of physically proximate batteries in said bank, a measuring module which may be polled by said central processing unit, said measuring module including: at least one analogue input port which in use is connected to at least one battery, an analogue-to-digital converter connected to said at least one input port which produces digital values proportional to the voltage on said at least one input port, a device which stores said digital values, and a digital input-output port, said device presenting said digital values in serial form at the digital input-output port upon receipt of a polling signal at said port, said digital input-output port of each measuring module being connected to said data bus or one of said data buses. BRIEF DESCRIPTION OF THE DRAWINGS The preferred form of the invention will now be described with reference to the accompanying drawings in which; Figure 1 is a block diagram of a battery monitoring system according to the present invention, Figure 2 is a block circuit diagram of a measuring module for a single battery, Figure 3 is a waveform diagram illustrating the operation of the battery measurement module, Figure 4 is a block diagram of a measuring module for a plurality of batteries with the voltages input through a multiplexer, and Figure 5 is a block diagram of a measuring module for a plurality of batteries with the voltages input through a resistive divider network. BEST MODE FOR CARRYING OUT THE INVENTION An overview of a battery monitoring system according to the present invention is shown in Figure 1. A battery bank 1, which may contain a number of batteries, has associated with a small group, preferably four, of batteries or cells a measurement module 2 which among other things measures the cell voltage and cell temperature, converts the values into digital form and supplies these values in serial form via a bus 3 to a central processing unit (CPU) 4 which stores and analyses the values and provides an indication of the status of each battery and produces alarms when appropriate. A number of measurement modules 2 may be connected to common buses and each central processing unit 4 may receive inputs from more than one bus. The communication protocol from each measurement module 2 to each central processing unit 4 will be described later. Central processing units may be networked by an RS485 serial interface and may optionally supply data to a personal computer 6 for further analysis via an RS232 serial interface 7. In more detail the monitoring system works as follows. The measurement module 2 connected to each battery measures the battery voltage. The measurement module also measures a reference voltage inside the module along with the battery temperature. It waits for a request from the CPU 4 to transmit this information on the isolated measurement module serial bus 3. The CPU 4 converts the values that the measurement module transmits to a calibrated voltage. The calibration constants for each connected measurement module are stored at the CPU. There is no theoretical limit on the number of measurement modules that can be connected to a measurement module bus but the number connected does affect the sample speed. For practical reasons the present system has a maximum of 10 modules per bus delivering 5 samples per second. Operation of Measurement Module In the first embodiment each measurement module 2 is connected to a single battery 8 and may be external to the battery or integrated with it as shown in Figure 2. The battery voltage is attenuated 9 by a voltage divider and supplied to the analogue multiplexer 10. The internal reference voltage 13 and the output of temperature sensor 14 are also inputs to the analogue multiplexer 10 and thus are also converted by the analogue-to-digital converter. Each signal is then converted to a time proportional to the voltage by an analogue- to-digital converter 11 and micro-controller 12. Figure 3 shows the timing diagram of the single slope analogue-to-digital conversion process implemented by the analogue- to-digital converter and micro-controller combination. These digital values are held by the micro-controller 12 and will be transmitted via the bus 3 upon command from the CPU 4. The measurement module derives its power from the battery it is measuring and where the nominal battery voltage being monitored is too low for this purpose, the measuring module will include a step-up converter. The micro-controller serial output port is isolated from the serial bus 3 by an optical isolation stage 15. Thus, in addition to sensing battery voltage and temperature and converting these values into digital form, the measurement module 2 provides isolation at the battery itself ensuring that the serial bus 3 is isolated with respect to ground. Further, micro-controller 12 in conjunction with CPU 4 provides data transfer from each measurement module by serial transmission. CPU 4 polls the microcontroller 12 in each measurement module to cause transfer of the stored voltage and temperature values to CPU4. A single set of values may be stored by micro-controller 12 or alternatively many sets of values may be stored in which case the polling frequency may be reduced. Each measurement module 2 has an 8 bit address. To initially configure the measurement bus 2 the CPU 4 polls each address from 0 to 254, then compiles a table of valid addresses. This takes around 20 seconds. These addresses are then used for future communication. Replacing a measurement module simply requires the CPU 4 to re-scan all addresses on a measurement bus port. At 5760 baud this will take less than 6 seconds. If the CPU 4 does not receive a response to a poll during normal operation, it will wait 10 seconds then run a complete address poll. The measurement module with the new address should inherit the configuration from the missing module. Data is transferred most significant byte first. Asynchronous protocol follows RS232 format (but not voltage levels) at 5760 baud, 8 data bits, even parity and 1 stop bit. Voltages above 2.5V are treated as high and below IV as low. The measurement modules must use timing to synchronise to the measurement bus protocol as there is no delimiter support in the measurement bus protocol. After receiving the address byte, the measurement module looks for the start condition of the second byte. If this has not occurred within 4 bit times then the measurement module starts looking for another address byte. The CPU 4 must send messages with inter-packet delays of greater than 8 bit times. The delay between transmitted bytes in the same packet should not exceed 3 bit times. In a second embodiment each measurement module is connected to a small group of batteries 16, preferably four as shown in Figure 4. An analogue multiplexer 17 connects each battery 16, as instructed by the micro controller 12, to a signal conditioning module 18 which provides attenuation to a voltage suitable for an analogue-to-digital converter 19, and low pass filtering for noise reduction. An analogue temperature sensor 20 is attached to the case of each battery 16 to indicate exterior battery temperature. Both the output of each temperature sensor 20 and the output of the signal conditioning module 18 are connected to the inputs of the analogue-to-digital converter 19. The analogue-to-digital converter 19 used may be of the commercially available variety, and its output is connected to the input of the micro controller 12, which will store and transfer the data to the CPU 4. In this embodiment a fault in a connection between adjacent batteries 21 can be detected by analysing the voltage drop between the terminals of those adjacent batteries. In a third embodiment each measurement module is connected to a small group of batteries 16, preferably four as shown in Figure 5. A signal conditioning module 22 provides attenuation of all the voltages from the group of batteries 16 to a voltage suitable for an analogue-to-digital converter 19, and low pass filtering for noise reduction. An analogue temperature sensor 20 is attached to the case of each battery 16 to indicate exterior battery temperature. Both the output of each temperature sensor 20 and the outputs of the signal conditioning module 22 are connected to the inputs of the analogue-to-digital converter 19. A range of reference voltages 23 are required to compensate for the different voltage that each battery is referenced at. These reference voltages 23 are connected to another analogue multiplexer 24 which is connected to the input of the analogue-to-digital converter 19. The micro controller 12 will decide which of the reference voltages 23 to switch in based upon the overall voltage. The analogue- to-digital converter 19 used may be of the commercial available variety, and its output is connected to the input of the micro controller 12, which will store and transfer the data to the CPU 4. This embodiment has the advantage of speed over the second embodiment. However the second embodiment is more accurate, using the full resolution of the analogue to digital converter 19 over each battery, whereas for the third embodiment the analogue-to-digital converter 19 resolution spans over the overall voltage. The present invention provides a battery monitoring system which has many advantages over conventional systems. Isolation provided near each battery ensures that no battery potential is connected to the central processing unit. Further, only one data cable pair is required for every ten (or in the case of the second and third embodiment up to forty) batteries and each measurement module can accommodate batteries from between 2 to 12 volts. For large battery banks the central processing units may be provided with additional serial ports to allow the monitoring of further multiples often (or forty) batteries. Because the measuring module is intended to be either located physically to the battery or integrated within the battery case, battery temperature may be measured without the need for separate outboard circuitry.

Claims: CLAIMS 1. A battery measuring module for a system for monitoring a plurality ot batteπe-, wherein said measuring module is associated with and measures parameters of one or more of a small number of physically proximate batteries, said measuring module comprising: at least one analogue input port which in use is connected to at least one battery, an analogue-to-digital converter connected to said at least one input port which produces digital values proportional to the voltage on said at least one input port, a device which stores said digital values, and a digital output port, said device presenting said digital values in serial form at the digital output port. 2. A battery measuring module as claimed in claim 1 wherein a voltage isolator stage is connected in cascade with said digital output port to ensure that the voltage potential at the output port does not exceed the digital signal voltage. 3. A battery measuring module as claimed in either claim 1 or claim 2 wherein said device is a microprocessor and said microprocessor together with switching devices comprises said analogue-to-digital converter. 4. A battery measuring module as claimed in any one of claims 1 to 3 wherein said measuring module is associated with a single battery. 5. A battery measuring module as claimed in claim 4 wherein said measurement module is fixed in close physical and thermal proximity to said single battery. 6. A battery measuring module as claimed in any of claims 1 to 5 wherein at least one temperature sensor is associated with a respective battery, said temperature sensor supplying a temperature dependent voltage to a said input port and said temperature dependent voltage is converted by said analogue-to-digital converter to a digital value which is also stored and serialised by said device. 7. A battery measuring module as claimed in any of claims 1 to 6 wherein said device has a digital input which input controls the serialisation of the stored digital values at said digital output. 8. A monitoring system for a bank of batteries comprising: a digital central processing unit which is capable of storing and processing digital parameters for each battery in the bank, said central processing unit having one or more serial input-output ports, one or more data buses, each connected to a respective serial input-output port on said central processing unit, and, connected to and in association with one or more of a small number of physically proximate batteries in said bank, a measuring module which may be polled by said central processing unit, said measuring module including: at least one analogue input port which in use is connected to at least one battery, an analogue-to-digital converter connected to said at least one input port which produces digital values proportional to the voltage on said at least one input port, a device which stores said digital values, and a digital input-output port, said device presenting said digital values in serial form at the digital input-output port upon receipt of a polling signal at said port, said digital input-output port of each measuring module being connected to said data bus or one of said data buses. 9. A monitoring system as claimed in claim 8 wherein a voltage isolator stage is connected in cascade with said digital output port to ensure that the voltage potential at the output port does not exceed the digital signal voltage. 10. A monitoring system as claimed in either claim 8 or claim 9 wherein said device is a microprocessor and said microprocessor together with switching devices comprises said analogue-to-digital converter. 11. A monitoring system as claimed in any one of claims 8 to 10 wherein said measuring module is associated with a single battery. 12. A monitoring system as claimed in any of claims 7 to 9 wherein said measurement system is fixed in close physical and thermal proximity to said battery. 13. A monitoring system as claimed in any of claims 7 to 10 wherein a second input port and a temperature sensor are provided, said temperature sensor supplying a temperature dependent voltage to said second input port and said temperature dependent voltage is converted by said analogue-to-digital converter to a digital value which is also stored and serialised by said device. 14. A monitoring system as claimed in any of claims 7 to 11 wherein said device has a digital input which input controls the serialisation of the stored digital values at said digital output.

Inventors: GUY JONATHAN ANDREW, HECTORS ANTONIUS CORNELIS, GUY, JONATHAN, ANDREW, HECTORS, ANTONIUS, CORNELIS
Applicants: BATTERY TECHNOLOGY LIMITED, GUY JONATHAN ANDREW, HECTORS ANTONIUS CORNELIS, GUY, JONATHAN, ANDREW, HECTORS, ANTONIUS, CORNELIS
Application year: 1998
Main classification: H02H7
IPC codes: G01R31, H02J7