Battery Management and Diagnostics

What is the difference between a Battery Management System (BMS) and the BMI Voltage Management System (VMS)?

A BMS is an electronic circuit board used in conjunction with a lithium ion battery which serves to prevent over charge, over discharge and it provides a balancing function for the cells in a battery pack. By doing this it ensures the longest life and best performance from the battery. For certain chemistries of lithium ion batteries the BMS provides a vital safety function since with over charge, over discharge and cell imbalance are conditions where certain more hazardous chemistries of lithium ion batteries could possibly catch on fire (or even explode under certain circumstances). This discussion is primarily aimed at the only safe chemistry of lithium ion i.e., Lithium Iron Phosphate (LiFePO4). BMI only manufacture safe LiFePO4 batteries. A BMS is especially important where multiple cells are used to construct a much larger battery pack. The BMS manages cell balance during charging and/or discharging as well as having the ability to disconnect the battery from the load when the batteries charge has been fully exhausted. This is very important since LiFePO4 cells can be severely (or even permanently) damaged if excessively over-discharged. Likewise a BMS should disconnect the battery from a charging source when the battery has received a complete charge so as to not become excessively overcharged since this can also lead to a reduction in the life of the battery.

 A VMS and BMS both manage and provide the cell balancing function in a battery pack. This is where the similarity ends.

A VMS differs fundamentally from a BMS in that it will not disconnect a battery automatically from external devices connected to it during over-charge or over discharge conditions. It will only provide a signal output or some other visual or audible warning when approaching these conditions. This signal output is used by electric vehicle (EV) manufacturers to interface with their own external control systems in different ways. For example the signal generated by the VMS when the battery becomes fully discharged might be used to drive an external relay or contactor via suitable switching transistors. The relay contacts then disconnect the battery from its load thereby preventing further discharge which could lead to battery damage and thus maintaining long battery life. The signal outputs from the VMS can be used in many different ways and are only limited by the imagination of the EV designer.

 

The BMI VMS is built into the HPS series of battery packs. We do however supply the VMS in kit form for those experienced battery constructors who would like to assemble their own custom battery. The complete VMS kit includes two circuit boards, interconnecting ribbon cables, temperature sensing thermistor and wiring loom to connect the VMS board to the individual cell terminals. There is no soldering required since all wiring and sensors are terminated with connectors and simply plug into the main VMS circuit board. The second circuit board is optional since it is required only for advanced applications but is useful to have due to the warnings it provides by indicator LED's and warning buzzer. This board is a display board and has two DB9 data connectors fitted so that batteries can be monitored on a computer via an RS-232 data link. A battery data converter and software is available which interfaces the VMS with a PC. Each VMS board connects to and monitors four cells and several VMS boards can be linked together "daisy chain" fashion to accommodate any number of cells. So for example if a 36V battery was to be assembled this would require twelve cells and three VMS boards.

 

Please enquire if you require additional information about the BMI VMS system.

         LiFePO4 Battery Balancers

Lithium Batteries Australia supplies the world's highest quality battery balancer for those who do not want to use a BMS and would prefer to balance their cells via an external balancer rather than a BMS. This balancer is the highest quality unit on the market and is very fast and accurate in its balancing ability due to its high 1.5 amps shunt current while charging. These are a very high quality unit made in an extruded aluminium case. They are made in Europe exclusively for LBA by our european partners.

Balancers are available in all the common battery voltages of 24V, 36V, and 48V. Custom units can be supplied for any quantity of cells upon special order. Please enquire if you have a special request.