Part 1 – brematronic and elektor: Special DC/DC Converter for Project “Universal Battery Charger”

Part 1 – brematronic and elektor: Special DC/DC Converter for Project “Universal Battery Charger”

Part 1:

Hello folks.
We’d like to introduce you to an exciting new project. This is about the development of a special DC/DC converter which is suited for the purposes of the Elektor Labs project called “Universal Battery Charger”. You can read more about this project under the following link:

The overall scope of the project “Universal Battery Charger”

In short, the goal of the project is to have a working universal multi-chemistry battery charger supporting rechargable NiCd, NiMh, Li-Ion, Li-Po, LiFePo and lead acid cells. Single cells as well as multiple-cell arrangements are to be supported up to 4S-configured packs (Li-Ion, Li-Po, LiFePo). Charging as well as controlled discharging shall be supported including balancing for multiple-cell arrangements. The complete battery charging system will be built in a modular way. It will consist of three major modules: 1) An ESP32 based control unit 2) An electronic load and balancer module 3) The DC/DC converter for charging single cells or cell packs.

The control unit serves three main purposes: 1) User interaction with the charging system by loading, editing and storing charge and discharge settings 2) Giving the user status information like state of charge (SOC), charging current, cell voltages, or cell temperatures, and the like 3) Providing a digital interface to the DC/DC converter to feed its control loop with the intended control mode (either current or voltage control) and with the digital set points for current and voltage. There will be two distinct ways of user interface: 1) Via a touch-sensitive Raspberry Pi TFT display module 2) Via PC by means of a web interface communicating with the ESP32 WiFi module.

The electronic load and balancer module will be responsible for two things: 1) Discharging the connected battery cells with the desired value of discharge current down to the cut-off voltage. For this purpose the electronic load will have its own control loop. 2) Balancing the individual cells both in charge and discharge mode when the balancing option is selected by the user.

The DC/DC converter will be the responsible module for charging single cells or cell packs. For this purpose it will have its own control loops – independent of the ESP32 control unit – regulating voltage or current at its output. It will feature a digital interface to the ESP32 control unit whose purpose is to only transmit the set values whereas the actual regulation is left to the DC/DC converter.

The charging system shall be powered by a regulated 12 V DC laboratory supply for convenient home usage. Alternatively, it shall be capable of being powered from the onboard lead acid battery of a car for mobile usage scenarios. 

Planned specs of the DC/DC converter

Now that you have an overview of the overall project “Universal Battery Charger”, it’s time to dive a bit more into the details and specifics of the DC/DC converter project. In the following you’ll find the planned specs of the DC/DC converter taylored for the intended multi-chemistry charging application.

Electrical specifications: 

  • Constant voltage mode with limiting current
  • Constant current mode with limiting voltage
  • Input voltage: 10.9-14.9 V (suiting 12V lead acid car batteries)
  • Output voltage: 0.8-24 V
  • Output voltage control accuracy: +/- 25 mV
  • Output voltage ripple: <= 10 mV
  • Output current: 0-2.5 A
  • Output current control accuracy: +/- 2.5 mA
  • Output current ripple: +/- 1mA
    Additional features: 
  • Input undervoltage lockout and overvoltage lockout
  • separate input PIN for converter enable/shutdown
  • separate output PIN to indicate fault conditionsDigital I²C interface and optional SPI or serial Modbus RTU interface @ 3.3 V supply voltage

Besides, the converter shall exhibit a reasonably high efficiency for mobile use with a 12 V battery as the source for input power. And of course, cost shall be kept moderate and the design ought to be based on readily available components and parts.

Challenges in the DC/DC converter development

There are several aspects that make the development of this DC/DC converter a challenging, yet exciting undertaking.

First of all, the converter needs to have buck-boost capability. Depending on the actual input supply voltage and the number and type of battery cells connected in series at the output, the converter has to be capable of operating in either buck or boost mode. Second, the converter must be capable of delivering very low output voltages when the charging process of a discharged single cell is started. The worst-case scenario here would be charging a fully discharged single NiCd cell with a cut-off voltage as low as 0.85 V. Third, the converter must be able to operate in constant voltage mode as well as in constant current mode depending on the actual position along the intended charging curve of the connected battery. Constant voltage mode must be possible over a certain range of charging currents. Constant current mode must be possible over a wide range of output voltages and down to small setpoint values of charging current, requiring a fairly accurate output current measuring that is not overly disturbed by the converter’s switching noise. And fourth, the battery charging application imposes tight limits on voltage and current control accuracy and ripple.

Since these are challenging requirements we are excited to cooperate with the Elektor Labs staff on this project and to design and develop this special DC/DC converter for Elektor and its readership.

Some closing remarks

With its special and challenging requirements this DC/DC converter development involves advanced knowledge of various voltage converter topologies, their advantages and disadvantages. And it takes in-depth knowledge and skills in converter control loop design, stability criteria and feedback controller layout. We’re excited to tackle the challenge. As long-term Elektor magazine readers we’re really looking forward to give something back to the Elektor community. We’ll take you on the journey along the way with various follow-up articles as the development process will evolve.  

We believe that upon completion the “Universal Battery Charger” project will offer you electronics aficionados out there a fairly versatile and handy charging solution. Plus, the DC/DC converter itself with its simple digital interface will offer the electronics hobbyist a practical, useful and versatile power supply solution.

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