offGrid Systems website
Project duration: ~about 1 year
Projectvorm: Fixed price
Period: 2015-2016
Activities: Idea, proof of concept, design electronics and firmware, prototyping, multiple improvements, production, support & warranty
Technologies: STM32 Cortex M3, CAN-bus, C, high-current, LiFepo4 charging, Active Lipo balancing, DipTrace, EmBits, powerelements, BMS
Parts: STM32F105, SN65HVD234, MAX15062, LTC3300, BQ76930, LM3404, LTC4364

Description:

On another page we have described the development of an off-grid energy system monitoring and management solution. One of the key components of such system is the energy storage facility. This can be accomplished by means of old-school unmanaged and heavy lead-acid batteries, or with modern light-weight fully managed Lithium-Ion battery packs, or another lithium flavor such as lithium-polymer (LiPo), lithium-iron-phosphate (LiFePo4/LFP). This follow-up project is about the development of the battery management system (BMS) that resides in these new battery packs.

8-Cell LiFePO4 BMS

Specifications:

Optional display module
  • Operating voltage 20-30VDC (nominal ∼25.6V)
  • 72Ah LiFePO4 cells in 8S1P or 8S2P configurations
  • 200A Discharge and 100A charge paths, individual switchable, short-circuit proof
  • Cell voltage monitors, current measurement and charge accumulator
  • State of Charge operating region 10% to 95%
  • 8-Channel 10Amp active balancers for individual charging or discharging of cells
  • Statistics-based balancing and advanced OCV-based end/top balancing
  • Heater output for cold climate operation (charge < 0°C, discharge < -20°C)
  • Load analysis
  • Cell impedance analysis
  • Battery statistics for delivered energy, saved energy, cell voltages and charge cycles
  • Event logging
  • CAN-based OGS control® bus connects up to 48 batteries with the OGS control® Master
  • Optional front panel graphic display

The multilayer board is affixed to a heat spreader plate, and is equipped with the highest-rated power mosfets available and Würth press-fit power elements. This enables the complete solution to reside on a single PCB with barely any off-board components (except the low-side shunt resistor and the high-current inductive disconnect clamping diode on the discharge contact).

Power switching:

Using an innovative approach to increasing the switching speed of the power mosfets, we can handle inrush and short-circuit currents in excess of 600A using only two paralleled semiconductors. First level short-circuit detection is handled by abusing a PCB trace as sense resistor, while the second and third level of protection rely on the much more accurate external shunt resistor.

Active balancing:

Lithium-based batteries require balancing of their cells. Often this is done using passive methods, which simply clamp the cell voltages to a certain value, and dissipate the excess energy as heat. Either their balancing current is very low, or they use big power resistors. Active balancing uses bi-directional flyback DC/DC converters to convert the excess energy to a higher voltage which is fed back into the battery. Also, these can charge individual cells.

Array with 72Ah LiFePo packs

Application:

The controller is used to build custom hot-pluggable 72Ah or 144Ah LFP battery backs for the OGS control® based off-grid energy system. Multiple of these packs can be combined to create solutions with very large storage capacity. Currently, the design allows for 48 parallelled packs for a total installed capacity of almost 7kAh (~170kWh). Of course that be expanded to greater numbers with the addition of OGS CAN-bus repeater devices.

Scalability:

Using the same algorithms and electronics build with similar parts, we can build battery management systems with other configurations up to 15 cells. And by changing some parts, we can scale to any number of cells. Contact us to discuss your requirements.

Testing:

One challenge with this kind of power electronics is the testing of the various functions. For this purpose, a Li-Ion cell emulator was developed to simulate individual cells with sufficient charge and discharge capabilities. Furthermore, a high current load-switch testing module is designed, which allows passing an adjustable current in excess of the short-circuit capabilities of the device under test.

Agentschap NL
Gerealiseerd onder het innovatieprogramma WBSO

ARM Cortex M3

'OGS control' is a registered trademark of Reijneveld Machinebouw B.V.