Overcoming Challenges in Battery Storage Systems with High-Isolation DC/DC Converters

Modern energy storage systems using Li-Ion batteries provide superior performance but each cell must be monitored closely to secure proper charging and avoid unbalanced cells, over charging, deep discharge or critical temperatures. This requires control circuits with individual, isolated supply voltages for each cell or cell array. For battery arrays working with nominal voltages up to 48V this supply voltage can be generated with standard, industrial DC/DC converter modules offering isolation barriers of 500V or 1600V.

But for higher power systems connected to AC mains and providing energy storage capacities of hundreds of kWh or even several MWh, the voltages across the battery arrays can reach to 600 – 800V and solutions with much higher isolation barriers are needed.

This customer was looking for a solution compliant to overvoltage category III requirements and 4kV isolation barriers. Each battery monitoring circuit needed a 30W converter and with hundreds of converters connected in parallel, the total leakage current across the converters was another challenge.

Low Isolation Capacitance Strengthens Noise Immunity

P-DUKE has families of medical converters with 5kV isolation barriers, even higher than the required 4kV. For this project the MPD30-24S12 was selected which generates the necessary 12V/30W supply voltages from the common 24Vdc bus.

Typical industrial 30W DC/DC converters can have an input to output capacitance of up to 1500pF. When like in this case up to 200 converters are in parallel, the total capacitance between the battery cells (connect to the AC grid via chargers) and the 24V bus can reach 300nF, resulting in AC leakage currents of over 20mA. Currents above 10mA can already cause a severe electrical shock and accidents.
The MPD30-24S12 has an input to output capacitance of only 20pF, with 200 converters in parallel the total capacitance is only 4nF and leakage current is less than 1mA.
This very low capacitance also avoids having transients or any noise being coupled across the converters from the AC/battery side to sensitive communication signals of the central battery management system.

With this solution the 4kV isolation and the low leakage current requirements had been solved. For the communication within the complete installation, the central BMS controller uses a Wifi router, powered over the Ethernet from a 53V source with 25W. Depending on the final installation the supply voltage can be generated either from 230Vac mains or from the internal 24Vdc bus.
For 230Vac a converter from the TSD30 family can be used. It is also available as Din-Rail type version for easy mounting in the controller rack.

Successful Projects

Generating the 53V from the 24Vdc bus can be done by a converter form the RCD30 family with the output trimmed down from 54V to 53V. It is also available with an integrated heatsink for easy cooling.

Example 1:

BMS circuit

Solution-

Example 2:

54Vdc/25W for PoE WiFi router

Solution-

Large-scale Battery Storage Systems can Benefit from the Utilization of P-DUKE's products:

The MPD30 / MPD30W family is a series of 30W DC/DC converters with reinforced, 5000Vac isolation voltage meeting the requirements in medical applications. They are available with different input voltage ranges and provide single or dual outputs. The family is qualified for 2MOPP and has 8mm clearance and creepage distances. This makes them ideal not only for medical equipment but also for other applications requiring high isolation and very low input to output capacitance.

The TDD30 family is a ready to use AC/DC solution for Din Rail applications. Like the RCD30W family it offers different output voltages and mounting options for all kinds of applications.