RCinFLA
Advanced Member level 2
Many low cost BMS's for LiIon battery arrays use simple low current bleed resistors for cell balancing. For high AH batteries, 100 to 200 mA of balancing does not do much more then keep up with leakage current variation between cells.
There are a couple of low cost boost switchers modules that can be modified to provide higher balancing current. The modules most suited are MT3608 based which are capable of about 2 amps of cell balancing current and XR2981 based which are capable of over 5 amps of cell balancing current.
The main modification is to replace the boost module inductor with a small powdered iron toroid flyback transformer to provide primary to secondary isolation. Each cell gets its own modified boost module. Secondary side is rectified and fed to total battery stack voltage to recycle the power bled from individual cells. The turns ratio on secondary of added flyback transformer depends on battery array stack voltage.
The existing BMS balancing dump resistor control is tapped into to supply the added boost switcher enable control so the balancing setting on the original BMS remains in control of balancing.
The added boost switcher flyback tranformer primary side should operate with 12-15v boost to stay within voltage limits of the boost switcher and provide highest efficiency on boost switcher. The voltage feedback on the boost switcher only provides a safety function and should be set about 25% higher in DC voltage then the maximum primary side voltage when flyback seondary output side is loaded by the battery stack, or at least to ensure the switcher stays below it maximum switcher voltage rating. The voltage feedback control would normally never come into play. This provides safety limit for boost switcher if the secondary load is accidentally left unconnected which would result in overvoltage breakdown on boost switching module I.C. internal MOSFET.
The simplest feedback control can be based on boost IC internal max switcher MOSFET current sense. This is done by having a primary side flyback inductance so peak cycle charge current just trips the max peak MOSFET current internal I.C. sensing. This is about 4 amps for MT3608 and 12 amps for XR2981.
A more elaborate current sense feedback would be a sense resistor with op amp to parallel switcher voltage feedback with current sensed feedback like seen on many constant current - constant voltage switchers.
With the higher balancing current , separate wires from each boost module input supply to each cell should be used to avoid wire voltage drop on original BMS balancing wires that may corrupt the cell sensing voltage fed to the BMS.
For the XR2981, have to be a little watchful of maximum temperature on device and ensure there is sufficient heat sinking with PCB layout. It gets a little warm at 5-6 amps of cell balancing current.
There are a couple of low cost boost switchers modules that can be modified to provide higher balancing current. The modules most suited are MT3608 based which are capable of about 2 amps of cell balancing current and XR2981 based which are capable of over 5 amps of cell balancing current.
The main modification is to replace the boost module inductor with a small powdered iron toroid flyback transformer to provide primary to secondary isolation. Each cell gets its own modified boost module. Secondary side is rectified and fed to total battery stack voltage to recycle the power bled from individual cells. The turns ratio on secondary of added flyback transformer depends on battery array stack voltage.
The existing BMS balancing dump resistor control is tapped into to supply the added boost switcher enable control so the balancing setting on the original BMS remains in control of balancing.
The added boost switcher flyback tranformer primary side should operate with 12-15v boost to stay within voltage limits of the boost switcher and provide highest efficiency on boost switcher. The voltage feedback on the boost switcher only provides a safety function and should be set about 25% higher in DC voltage then the maximum primary side voltage when flyback seondary output side is loaded by the battery stack, or at least to ensure the switcher stays below it maximum switcher voltage rating. The voltage feedback control would normally never come into play. This provides safety limit for boost switcher if the secondary load is accidentally left unconnected which would result in overvoltage breakdown on boost switching module I.C. internal MOSFET.
The simplest feedback control can be based on boost IC internal max switcher MOSFET current sense. This is done by having a primary side flyback inductance so peak cycle charge current just trips the max peak MOSFET current internal I.C. sensing. This is about 4 amps for MT3608 and 12 amps for XR2981.
A more elaborate current sense feedback would be a sense resistor with op amp to parallel switcher voltage feedback with current sensed feedback like seen on many constant current - constant voltage switchers.
With the higher balancing current , separate wires from each boost module input supply to each cell should be used to avoid wire voltage drop on original BMS balancing wires that may corrupt the cell sensing voltage fed to the BMS.
For the XR2981, have to be a little watchful of maximum temperature on device and ensure there is sufficient heat sinking with PCB layout. It gets a little warm at 5-6 amps of cell balancing current.